BREEDING AND GENETICS
IN POTATO IMPROVEMENT
F. J. STEVENSON, Senior Geneticist, Cc. F. CLARK, Horticulturist, Division of Fruit and Vegetable Crops and Diseases, Bureau of Plant Industry
[ABSTRACT.] MILLIONS of dollars are spent each year in providing ways and means of protecting the potato crop from diseases, and these colossal efforts have done their part in providing the consumer with potatoes fit for consumption; but in spite of them, many millions of bushels are lost each year. The annual loss from late blight alone has averaged 9,000,000 bushels for the past 10 years. Thus disease resistance has been a major objective in the well coordinated national potato-breeding program now being conducted throughout the United States, in which the Department of Agriculture and all interested States closely cooperate. Already many varieties have been obtained resistant to late blight, to scab, and to two of the mosaic diseases. The results indicate that by using genetic principles as a working tool, it should be possible to solve many if not all of the disease problems of potato growers by combining resistance with superior qualities of economic importance, such as shallowness of eye, desirable shape, good cooking quality, high yield, and adaptability to local conditions.
When the Spaniards invaded South America they found a large number of varieties and species of potatoes under cultivation, the tubers of which were used as a common article of food by the natives. Where these varieties and species originated is not well known, but they seem to be native to the American Continent, since their relatives are still to be found growing wild on the elevated regions extending from the southwestern part of the United States to the southern part of South America, particularly on the higher altitudes in Bolivia and Peru and the coastal regions and nearby islands of southern Chili. All the species seem to require a cool climate, since they are found growing at high latitudes in regions near the Equator and none is known to occur under tropical conditions.
If the origin of the potato is uncertain, its introduction into Europe and North America is also shrouded in mystery. Many interesting legends have been written concerning this, but few reliable facts are available. It is not hard to believe that the Spanish sailors, on their return from their many trips to the New World, brought back potatoes and introduced them into Spain and Portugal. That these were the common potato known today and not the sweetpotato is proved definitely by a report of Clusius published in his Rariorum Plantarum Historia in 1601, giving an illustration and description of a plant sent him in 1588 by the Governor of Mons. The flowers were light purple and the original plant obtained by Clusius produced a fruit ball and two reddish tubers. From Spain and Portugal the potato was taken probably to Italy, from there early in the seventeenth century to Austria, then to Germany, from Germany to Switzerland, and finally to France. The legends surrounding the introduction into Ireland have a more romantic setting. They are centered around Drake, one of Queen Elizabeth’s pirates, who was encouraged by the Queen to plunder Spanish ships and Spanish possessions. It is supposed that on one of these trips Drake obtained potatoes in the West Indies for his ship’s stores and that some of these were carried to Ireland about 1586.
Little is known of the introduction of the potato into North America. It is generally believed that the English colonists of Virginia and Carolina obtained the potato from Spaniards or from other travelers. The most authentic report shows that potatoes were first grown in this country at Londonderry, N. H., in 1719, from stock brought from Ireland. It was for this reason, no doubt, that the potato was called the “Irish” potato. The name is still used, especially in the South, where it serves to distinguish the potato from the sweetpotato.
Most of the stories of the introduction of the potato into Europe and North America are no doubt legendary, as well as some of the tricks that were supposed to have been adopted to get people to use the tubers for food. The spectacular increase of the potato as a food crop, however, is not legendary, but is one of the miracles of agriculture. Although it is only a little over 300 years since the first introduction into Europe and about 200 years since the first importation into the United States from Ireland, the potato is now grown in almost every important agricultural country in the world. The crop for the United States and Canada for 1934-35 was nearly half a billion bushels, and the total for the world, not including the Union of Soviet Socialist Republics and China, for which no data are available, had reached the stupendous figure of over 6 billion bushels.
The potato belongs botanically to the section Tuberarium (50) of the genus Solanum, the members of which are, with few exceptions, tuber bearing. This section comprises not only the cultivated forms but many wild species. Since it is probable that undiscovered. species occur in regions not yet explored by botanists, the total number cannot be definitely stated, but it is believed to exceed 100.
Considerable confusion has existed in the past in regard to what has been considered the species Solanum tuberosum L., which has generally been understood to comprise all cultivated varieties of potato. These have recently been assigned by Bukasov (9) to 14 species. According to his classification, S. tuberosum includes the Chilean cultivated forms as well as the commercial varieties of North America and Europe, which he believes to be of Chilean origin. Most of the common cultivated varieties of the Andes region are placed in the species S. andigenum Juz. and Buk. The 12 remaining species are grown in small local areas in the Andean countries and more or less resemble the wild species in their general appearance. It is doubtful whether any of the cultivated varieties of Europe and North America as known today have been derived from them.
There is little information available regarding the source of the potato stock grown during the first 100 years after its introduction into the United States. It is believed that not many new varieties of importance were produced during that period. During the second century of potato culture in this country, however, there was great activity in the production of new varieties. Data reported on 228 of these varieties show that they originated in 21 States. New York and Vermont, however, produced 50 percent of the total number. Of the 160 varieties whose date of origin is known, 80 percent were produced during the 40-year period 1861 to 1900 and 48 percent during the two decades 1871 to 1890 (12).
The most important varieties produced during this 100-year period and = originators, as reported by Stuart (71), are given in the appendix.
Special mention should be made of C. E. Goodrich, as his work was the first to produce lasting results. He believed that the disastrous epidemics of late blight during the years 1843-47 were the result of a reduction in the vigor of the plants caused by long-continued propagation by vegetative means and that this vigor could be restored by growing plants from true seed. While he did not succeed in the control of late blight by this means, he may be considered to have laid the foundation of potato breeding in this country by furnishing material to be used by other breeders. The ancestry of 170 varieties can be traced back to Goodrich’s Garnet Chili, a seedling of the imported Rough Purple Chili. They include several of the well-known varieties of commerce, such as Beauty of Hebron, Burbank, Early Ohio, Early Rose, Green Mountain, Prolific, and Triumph.
The work of Pringle, as recorded by Stuart (71), is outstanding in that it represents what is believed to be the first systematic effort to obtain seed by controlled hybridization. This seed was not only used for the production of the varieties he introduced but was disseminated to others through his contract with a seedsman in New York to furnish hybridized seed at $1,000 a pound.
Many varieties of excellent quality and high yielding ability, which were the principal objectives, were produced during this period. The work, which made important contributions to the agriculture of this country, was carried on entirely by private agencies rather than by public institutions. Most of the men engaged in the early work were practical potato growers, except C. E. Goodrich, who was a clergyman, of Utica, N. Y., and E. S. Carman, who was editor of the Rural New Yorker
The commonly grown commercial varieties of potatoes differ from one another in earliness, tuber shape, adaptation, depth of eye, cooking quality, and yielding ability. In certain of these characters, such as yielding ability, some of these varieties have reached a high standard of excellence when grown under conditions to which they are adapted. These same varieties are, however, poor in other characters. Some are not adapted to a wide range of conditions; others have deep-eyed tubers, which cause waste in preparing the potatoes for cooking. All are susceptible to one or more of the common potato diseases, including the virus diseases, late blight, common scab, fusarium wilt, rhizoctonia, early blight, and blackleg. Control of these diseases requires a continual fight on the part of the potato grower and adds greatly to the cost of producing the crop.
The group of diseases caused by viruses are perhaps the most widespread and the most baffling. In this group are found mild mosaic, latent mosaic, leaf roll, spindle tuber, and yellow dwarf. These diseases occur in every potato-growing region of the United States, and it is probable that not a single field could be found entirely free from them. They are not new. Their effects have been observed by growers for many years, but for a long time it was thought that they were due to “running out” or “degeneracy”, brought about by growing potatoes year after year from the same tuber stock. It is only a few years since it was discovered that these troubles are due to virus diseases. It was soon observed that some varieties did not “run out” so quickly as others, or, as we now say, some varieties are more resistant to the attacks of certain viruses than others. Knowing that such differences must have a genetic basis, breeders are working to obtain resistance to these diseases in combination with other characters of economic importance.
Late blight, caused by Phytophthora infestans (Mont.) DBy., adds more to the cost of producing the potato crop than perhaps any of the other diseases. The losses from this disease alone have been more than 9 million bushels a year for a period of 10 years, according to estimates issued by the Division of Mycology and Disease Survey, Bureau of Plant Industry, United States Department of Agriculture. In some seasons and in certain localities the disease causes very little damage. At other times (as in 1927, 1928, 1932, and again in 1936) large losses are sustained by growers. The heaviest loss‘for any one year, nearly 31 million bushels, was reported in 1928. That year late blight was reported in 15 States, with the loss in New York estimated at approximately 13 million bushels. In 1932 the reduction of the crop was estimated at 9,230,000 bushels, the greater part of which, 9,058,000 bushels, was reported from Maine. Again in 1936 heavy losses occurred in Maine.
It is true that late blight can be controlled to a large extent by careful spraying with bordeaux mixture, but despite the fact that control measures are being practiced more generally than ever and that spray equipment has been improved and spray programs have been more faithfully carried out, large losses continue to occur, not only from reduction in yields but also from interference with marketing operations. Rot may develop on infected tubers in storage and in transit. Because of the uncertainty involved, the buyer is reluctant to purchase potatoes for storage purposes. There is considerable expense every time a carload is regraded at a terminal market, and this happens frequently in blight years. These losses all affect the grower.
Common scab, caused by Actinomyces scabies (Thax.) Gues., is another disease that takes a toll from the grower. The organism causing this disease lives over in the soil and is also carried on the tubers. Treatments have been recommended that will kill the organisms on the tuber, but no one has yet devised a method to fully protect the growing tubers from the soil-borne organisms.
Millions of dollars are spent each year in providing ways and means of protecting the crop from the attacks of these and other diseases, but comparatively little attention has been paid to obtaining varieties resistant to the attacks. The colossal efforts in the way of plant protection have done their part in providing the consumer with potatoes fit for human consumption, but in spite of these efforts millions of bushels of potatoes are lost each year. Results already obtained indicate that by using genetic principles as a working tool it should be possible to solve many if not all of these problems by producing new varieties in which resistance to various diseases is combined with other characters of economic importance, such as shallowness of eye, desirable shape, good cooking quality, and high yield.
In the production of such varieties the plant breeder must be familiar with the local problems of growers; he must have a knowledge of the existing varieties and the important economic characters of each; and he must be familiar with the botanical structure and behavior of the various parts of the growing plants. A knowledge of the modes of reproduction with the advantages and limitations of each is the first essential.
The potato plant is reproduced in two ways. In commercial practice it is grown from tubers, a method of vegetative or asexual reproduction. Plants can be grown from true seed, however, by sexual reproduction.
A number of mutations in the vegetative cells of potatoes have been studied and described, but they are too few and far between to be relied upon in a breeding program as the only source of variation. In many cases, too, the changes are of minor importance. Clark (13) has described several mutations that have occurred in the color of the skin and of the eyes. Salaman (57) classifies mutations of this sort according to whether they are due to the acquisition or to the loss of a character and whether they affect the tuber only or the whole plant. Mutations due to the loss of a character are by far the most common. There are a number of instances where the red tuber loses part of its color and becomes "splashed", or loses all of its color and becomes white. Purple tubers may become red, purple splashed, or white. Somewhat similar changes have been observed for flower color. Mutations due to the acquisition of a character, which might be called positive mutations, are much rarer. Examples are a red-tubered sport from a white-tubered variety, or a fully colored one from a partially colored variety. Mutations occurring in more than one character in the same individual are extremely rare. Mackelvie (38) reports such a case in which there was a white-tubered mutation combined with a different leaf shape from that of the parent variety. The leaflets of the mutant were narrower and more pointed. The yield was reduced also.
Figure 1.—Potato plants grown from true seed in the greenhouse. These plants have been transplanted from the germination pots to individual pots where they will be grown until they are transplanted in the field.
East (22) carried out some rather carefully controlled studies on the occurrence of somatic or vegetative mutations in the potato. In these studies each variety was started from a single hill. During the course of the study five permanent changes from pink to white tubers, two permanent changes from long to round tubers, and four instances of changes from shallow to deep eyes were observed. Selection for high nitrogen content gave negative results.
Clark (16) made a study of six commercial varieties to determine whether their origin could be accounted for by mutation. The methods employed were those reported by Asseyeva (1) for this purpose. They consisted of removing from the eyes of the seed pieces the outer layers of tissues, which, in skin-color mutations, are the mutating tissues. This allowed the sprouts from which the plants under test were grown to develop from the deeper layers, the original unchanged tissue. Clark found that four of the varieties studied were mutations in the outer layers of the tuber. Two were mutations from smooth skin to russet skin, the third was a mutation from colored to colorless skin, and the fourth from colorless to colored skin.
That the occurrence of such mutations has not been an important factor in the development of potato varieties is shown by Clark (12), who reported that of 380 varieties that have originated in the United States and have at one time or another been introduced to the commercial trade, 306, or 93.3 percent, were of seedling origin, and only 22, or 6.7 percent, were reported as so-called sports or mutants. Of the 22 varieties reported as sports, 4 are white-tubered from varieties with colored tubers, and 4 are late-maturing variations found in early varieties. The meager information regarding the other 14 furnishes no basis for determining whether they were actual mutations or mixtures carried in the seed. stock or volunteers that had persisted in the soil from some preceding crop.
Even if only a few varieties have arisen as the result of somatic mutations, they are still a source of variation that cannot be ignored entirely by the breeder.
Since it is quite impracticable for the plant breeder to make much improvement by selecting tubers of a variety with the hope of getting something new, he has to resort to the use of seed as a means of inducing variations.
As has been shown by Clark (12), most of the varieties that have originated in the United States were produced by growing plants from true seed (fig. 1). Salaman (55), discussing the production of new varieties from the same source in England, states: “It is by this method that practically every variety which has ever been raised since the introduction of the potato in 1588 has been attained.”
True seed is a product of sexual reproduction and is found in the fruit or ball, which is quite similar to a small tomato (fig. 2). Under certain conditions these fruits are produced in abundance on some varieties but are rarely if ever seen on others. They are the result of the maturing of the flower, and each of them may contain 200 seeds or more.
The flower of the potato is what is known to botanists as a complete flower with calyx, corolla, stamens, and pistil. There are usually five stamens surrounding the pistil. The process of pollination is very simple because of the simple structure of the flower parts. Pollen may be brought to the stigma of the pistil in several ways. In varieties in which the pistil is the same length as the stamens or shorter, the mature anthers may come in direct contact with the stigma. When the pistil is longer than the anthers, pollen may fall upon it when the flower droops over, as it frequently does at the end of the day. Insects may be responsible for a much greater amount of cross-pollination than is commonly supposed; in some localities bumblebees and honeybees are often seen visiting potato flowers. Pollination may be effected also by the manipulations of the plant breeder.
The technique of cross-pollinating potatoes by hand is comparatively simple, but since relatively few varieties produce viable pollen, the setting of seed is often very small in amount. If a variety producing viable pollen is used as the female parent, the flower must be emasculated, that is, the anthers must be removed. This must be done before the anthers are mature, or the flower will be self-pollinated. Generally speaking, the anthers should be removed before the pistil protrudes through the bud, or a day or two in advance of the opening of the flower. A pair of sharp-pointed forceps is the only instrument necessary. All the flowers in the cluster that are in the right stage of maturity are emasculated, all others are removed, and the inflorescence is enclosed in a 1-pound paper bag, which is securely tied on. If the variety used as the female parent does not produce viable pollen, it is not necessary to emasculate before pollinating.
In pollinating, flowers with anthers ready to open are obtained from the male parent. These are placed on the thumbnail and the anthers are tapped gently with the forceps. The stigma of a seed parent is then gently rubbed in the pollen on the thumbnail until it is completely covered (fig. 3). The treated flowers are covered with the paper bag again. It has been found best to enclose as much foliage with the flowers as possible, to protect them from injury and to supply moisture; otherwise the flowers may dry up and fall off. The failure or success of the fertilization can be determined within a week or 10 days (fig. 4). As soon as the seed balls develop, the paper bags are removed and replaced by a cheesecloth sack, which is tied securely to the vines.
Every breeding method has its advantages and limitations, and which one or which combination is used depends to a large extent on the problem involved in a particular case. The potato-breeder today uses (1) the introduction of new varieties or species, (2) selection of clonal lines, (3) varietal crossing, (4) sib mating (brother-sister mating), (5) backcrossing to parental lines, (6) selfing and recombining selfed lines, (7) outcrossing to unrelated lines, (8) strain building, and (9) crossing of different species.
Since a discussion of these steps and methods involves a number of rather technical points, it will be postponed until later in this article, after the work now being done in potato breeding in the United States has been outlined.
Because of the effect of environment on the fruitfulness (seed production) of potato plants, it is difficult, if not impossible, for many of the State experiment stations interested in some aspect of potato- breeding work to produce true seed. This would prevent the application of genetic principles to breeding problems and close the most promising avenue for their solution. Certain Northern States and other States with mountainous regions where potatoes can be grown at high elevations are especially favored, since true seed of many strains and varieties, as well as tubers of good quality, relatively free from disease, can be produced in such places. In potato breeding, then, more perhaps than in any other breeding project, it is necessary that the interested States cooperate in their attacks on the many problems involved.
Since problems and objectives in potato breeding often cut across State lines and involve large regions of the entire country, the breeding work has been organized as a national project with all the interested State experiment stations and the United States Department of Agriculture cooperating. A number of the cooperating stations are now carrying on complete breeding programs—that is, they are able to produce true seed and raise seedling progenies for genetic analysis of the material they are interested in, and at the same time to produce improved varieties. The States of Minnesota, North Dakota, Michigan, New York, Louisiana, and North Carolina and the Department working in Maine and Colorado are at present producing true seed. If any of these find they can use more seed than they are able to produce, a supplementary supply is sent from Maine, Minnesota, or Colorado, or from any other State that has a surplus. True seed is also sent to other States not able to produce their own, such as Iowa, and in 1936, Nebraska and Wisconsin. The last two States have recently undertaken complete potato-breeding programs.
A number of States that do not grow potato seedlings test the most promising seedling varieties produced by others. New introductions, parent material, and tubers representing seedling progenies are distributed to any State experiment station that can make use of them. The Department has been active in the production and distribution of such material and by common consent of the cooperators has been designated as the coordinator and clearing house for the project as a whole. A brief outline of the work carried on at the various stations under this project follows.
For a number of years the Department workers have been. carrying on an intensive program of potato breeding at Presque Isle, Maine, in cooperation with the Maine Agricultural Experiment Station. As a part of the national potato-breeding program they are now engaged in (1) the production of true seed for use at Presque Isle and for distribution to other cooperating: experiment stations; (2) distribution of single-tuber selections of various progenies to cooperating stations; (3) distribution of named and numbered seedlings for tests at other stations; (4) yield trials; (5) disease-resistance tests, including resistance to mild mosaic, latent mosaic, spindle tuber, leaf roll, late blight, and common scab; (6) genetic and cytological studies; and (7) the production of early- maturing varieties. In all this work higher market and cooking quality are being considered.
The work at the United States Horticultural Station, National Agricultural Research Center, Beltsville, Md., is interwoven with that carried on at Presque Isle and at other stations included in the national potato-breeding program. Two of the important aspects of the work at this station are the production of seedlings and the testing of seedling progenies and parents for resistance to virus diseases such as mild mosaic, latent mosaic, leaf roll, and spindle tuber. Tests are also made in the greenhouse for resistance to late blight and to the attacks of Fusarium eumartii Carp. and in the field for resistance to F. oxysporum Schl. Resistance to blackleg is being studied in certain progenies.
Iowa produces seedling progenies from seed furnished from Maine and Minnesota and tests the promising material produced by other cooperating States for its adaptability to Iowa conditions.
Louisiana produces some of the seed used in the production of seedling progenies. Other seed and material are supplied by Minnesota and the Department. Resistance to various diseases and adaptability of potato varieties to southern conditions are being emphasized.
The work in Michigan consists of growing seedling progenies, testing Department seedlings and those produced by other cooperating stations, increasing new varieties, carrying on disease-resistance studies with special reference to yellow dwarf and common scab, and producing early varieties with better market and table quality and varieties more suitable for muck soils.
In Minnesota securing resistance to virus diseases and to common scab in combination with early maturity is the main problem. A study of inbreeding and the recombinations of inbred lines is being carried on, and also a study of the cytological behavior of species hybrids.
In New York (Cornell University) resistance to late blight, using an immune wild species as the parent from which to obtain resistance, is one of the objects of the breeding work. Tests for resistance to streak and leaf roll are being conducted. A much enlarged program is just now getting under way, which will emphasize market quality, cooking quality, adaptability, and resistance to a number of diseases other than late blight.
In North Carolina the work consists of the production and testing of seedling progenies produced at Raleigh and the testing of the most promising material grown at other stations. The emphasis is being placed on earliness and on resistance to disease and to leafhopper injury.
North Dakota produces some true seed but is supplied for the most part from Presque Isle, Maine. Varieties as early as Irish Cobbler but smoother and higher yielding are being sought.
The Department workers at Greeley, Colo., are attacking the problems of resistance to wilt, to psyllid yellows, and to common scab in relation to other characters of economic importance.
In Nebraska potato-breeding work has only recently been started. Earliness, yield, quality, and resistance to diseases will be emphasized. To begin with, resistance to fusaria is being studied.
In Wisconsin a complete breeding program has recently been undertaken. Studies of the breeding behavior of resistance to common scab and virus diseases, as well as of other characters of commercial importance, are getting under way.
In Charleston, S. C., at the United States Regional Vegetable Breeding Laboratory, breeding work recently initiated will deal with the problems of potato production in the coastal region of the Southern States, which center around yield, earliness, and drought resistance.
In Pennsylvania, at Drifton, a test project for potato wart resistance is conducted by the State Department of Agriculture in cooperation with the United States Department of Agriculture. A number of the most promising named and numbered seedling varieties are tested for resistance to this disease each year in soil infested with organisms that cause the wart disease.
In Florida, at Hastings, the Florida Agricultural Experiment Station has begun the study of resistance to brown rot in the potato in cooperation with the Department. A few varieties and seedlings have already been tested, and the experiment will be greatly enlarged in 1937.
In Indiana, Purdue University has recently initiated a program of breeding for disease resistance as a part of the national potato-breeding program.
In addition to the special problems enumerated above, the following States are cooperating to determine the adaptability of the new named and numbered seedling varieties produced by the Department or by any of the State experiment stations: California, Connecticut, Florida, Indiana, Iowa, Kansas, Louisiana, Maryland, Massachusetts, Minnesota, Michigan, New Hampshire, New Jersey, New York, North Carolina, North Dakota, Ohio, Oregon, Rhode Island, South Dakota, Tennessee, Virginia, Washington, and Wisconsin.
The cooperative potato-breeding work has not stopped at national boundaries, for material has been exchanged with a number of workers in Canada and other foreign countries.
Progress has been made by the use of clonal selection in the State of New York. According to a recent unpublished report by J. R. Livermore, one of the cooperators in the national potato-breeding program, this method has been employed in connection with potato improvement work at Cornell University in cooperation with growers since 1905. Four varieties have been selected from the Smooth Rural (Rural New Yorker No. 2) and named. These are Heavyweight, No. 9, Pioneer Rural, and Toanco 4. A strain of Irish Cobbler has been named Pioneer Cobbler. Many other high-yielding clonal lines have also been selected from commercial varieties and are being produced by various growers. White-skinned bud sports from russet stock, the plant tops exhibiting heat resistance and the rugged vigor of the Russet Rural, have also been found. The Red Warba, one of the varieties recently introduced by the Minnesota Agricultural Experiment Station, occurred as a bud mutation from the Warba. This might be considered a clonal selection. By the method of careful selection within a variety or clon, not only are mutations uncovered but disease-free material is obtained, which in itself is justification for careful observation and selection.
F.A. Krantz, of the University of Minnesota, another of the cooperators in the national potato-breeding program, has been placing
greater emphasis than other breeders on the method of selection in
selfed lines and recombinations between them. The results have been
very similar to those found by the corn breeders, that is, selfed lines
become homozygous and are less vigorous than the parent stocks
from which they originated. Certain undesirable characters have
segregated and been eliminated, such as short internodes, prostrate
plants, simple-leaved types, chlorophyll deficiencies, and tuber abnormalities. Most important from the breeding standpoint, a number
of self-fertile lines carrying desirable characters have been selected.
In a brief unpublished summary of results to date, Krantz says:
Selection in self-fertilized lines has been effective in establishing pollen fertility
in combination with a wide variety of other characters. Used in combination with
outcrossing it has facilitated the development of a large body of seed-setting
material possessing selected germplasm of the common varieties.
The results show that it will be possible through this method gradually to combine the best germplasm in the potato into a useful form for breeding purposes. The difficulties involved are the absence of information on the best selection technique and the necessity of developing one suitable for effectively selecting and combining desired characters. Pollen sterility at the outset also presented a serious problem since selection was restricted to the seed setting individuals. The difficulties mentioned are associated with potato breeding in general and are not specific to any particular method. Information is gradually accumulating on methods and technique of selection in inbred lines. The difficulty of maintaining and continuing inbred lines has been greatly reduced through selection for high fertility. Comparisons have been made and are in progress of varietal crosses, single crosses, three-way crosses, and top crosses to determine the relative hybrid vigor that might be expected from the different types of combinations. The inbred lines are valuable for their concentration of desirable characters in combination with pollen fertility, early maturity, upright sturdy vines, resistance to certain virus diseases, fine cooking quality, and varying rest periods.
Most of the attempts to produce blight-immune varieties of commercial value by the use of species hybrids have resulted in failure. Recently, however, Donald Reddick, of Cornell University, also a cooperator in the national potato-breeding program, has been getting promising results from such hybrids. For a number of years it had been known that certain forms of Solanum demissum Lindl. were immune to late blight. In 1928 Reddick began using hybrids between these and cultivated varieties in an attempt to combine this immunity with characters of the cultivated forms. Two years later the Department sent Reddick and three men from the Division of Plant Exploration and Introduction, C. O. Erlanson, Paul Russell, and M. J. Souviron, on an expedition to Mexico to find, if possible, more sorts of potatoes, either wild or cultivated, that might be resistant to this disease. A large number of collections were made. Among these several varieties of S. demissum were found to be immune and to have at the same time some frost tolerance.
In commenting on the methods and some of the difficulties that arise in this work, Reddick says:
The object has been to get the blight immunity and frost tolerance of a wild plant into a plant of commercial value. Owing to sterility, incompatability, etc., the results thus far have been simply what could be got, not what it was planned to get. Efforts have been made from the beginning to determine the mode of inheritance of blight immunity and frost tolerance, but inability to obtain sets of seeds of the kind desired has prevented determining the mode. The original crosses. are interspecific and involve hybridizing 72-chromosome plants with those having 48 chromosomes. Practically all of the crosses have had to be 48 male to 72 female, because the reciprocal cannot be effected. The second generation of such crosses does not segregate but “reverts” to the wild type. Repeated backcrossing eliminates most of the wild characters but blight immunity is transmitted.
This work has been in progress since 1928. Some of the families are progenies resulting from an original species hybrid between an immune species and a cultivated variety that has been backcrossed to cultivated varieties four successive times. Fifty percent of the progeny of the fourth backcross are immune to the late blight disease. As a result of this work Reddick has at the present time about 500 families of plants immune to late blight. Possibly 50 or more of these are approaching the commercial ideal as to size, color, shallowness of eye, heat tolerance, and date of maturity. They are still to be tested for yield, quality, and adaptability to various localities. A few of these families will stand at least four degrees of frost.
The potato breeders and pathologists of the Department, using the methods included in strain building (p. 429), are making real progress, both from the standpoint of scientific knowledge and of practical results in breeding for resistance to virus diseases. One variety produced, U.S. D. A. seedling 41956, is highly resistant if not immune to the virus of latent mosaic. It has been exposed to the disease in the field by being grown near other varieties known to have this disease, and diseased stocks of other varieties have been grafted on the stems and on the tubers, but so far it has withstood every attack. A number of the progeny of a cross in which this variety was used as one of the parents show the same character, which indicates that immunity to latent mosaic is heritable. A number of other seedling varieties are resistant to this same virus in the field-exposure tests but contract the disease in tuber-graft tests, where it is expressed as top necrosis or death of the top of the plant. On the other hand, several attempts have been made to find even a single plant of Green Mountain free from this disease, but up to the present time such attempts have resulted in failure.
A large number of seedling varieties are highly resistant in the field tests to another virus disease, mild mosaic, perhaps the commonest of the “running out” diseases. These varieties have all contracted the disease, however, in the more severe tests of tuber grafting. In comparison with these resistant types, Green Mountain has been known to become 100-percent diseased with mild mosaic in the field exposure tests within a period of 3 years.
The fact that two virus diseases can be controlled by the production of resistant varieties gives hope that other virus diseases, such as spindle tuber and leaf roll, can be controlled in a similar manner. Comprehensive field-exposure and tuber-graft tests of a large number of seedlings and varieties are being made at the present time to determine whether any of them are resistant to either of these latter diseases. So far the results have not been encouraging. Although there have been a few escapes in the field-exposure tests, it is quite possible that none of the varieties so far tested is resistant to either of these diseases. Some of them may carry recessive factors for resistance, however, and many of them will be analyzed genetically to determine whether this is the case, and if it is, efforts will be made to combine the resistance with other characters of importance to the grower. In the case of leaf roll, a few recent introductions are reported to be resistant and will be used in future work. At the same time, the search in the United States and in foreign countries will be continued until types resistant to every virus disease are obtained or until all the possibilities for finding resistance are completely exhausted.
Breeding for resistance to late blight was begun at a comparatively early date. In 1870 Darwin attempted through the use of species hybrids to produce varieties resistant to Phytophthora, but was evidently not successful. According to Stuart (71), the first American breeder to attempt the control of late blight by the introduction and production of blight-resistant varieties was Chauncey Goodrich, of Utica, N. Y., whose work has already been mentioned. This work was based on a small quantity of South American potatoes that he received in 1851 through the American consul at Panama. A number of other breeders made valuable contributions during the latter part of the nineteenth century, but with the exception of the work of Goodrich, resistance to late blight seems not to have been emphasized in the United States until potato breeding was actively undertaken by the Department in 1910.
According to Clark et al. (18), the only disease resistance sought at the time was to the late blight fungus. This work had not progressed very far, however, when it became evident that the virus diseases had to be given the chief consideration, and it was not until the present national potato-breeding program was under way that emphasis could be placed once more on breeding for resistance to late blight. Reddick, working in cooperation with the Department, had already undertaken the solution of this problem, using species hybrids. The Federal work at Presque Isle, Maine, and Beltsville, Md., includes the genetic analysis of the cultivated varieties to determine the possible existence of blight-resistant factors.
In 1932, when late blight caused the loss of over 9 million bushels of potatoes in Maine, 700 seedlings, representing 4 different progenies, and about 100 Green Mountain checks, were grown at Presque Isle in test rows of from 20 to 30 hills each. This plot, about 1% acres in area, was not sprayed with bordeaux mixture but was sprayed in July with a single application of calcium arsenate to kill the Colorado potato beetle. Late blight infection was first observed on July 22. Conditions favorable for the spread of the disease prevailed during August, so that by the first week in September nearly all the seedlings ae all the Green Mountain checks were completely killed, both leaves and stems. A few seedlings had stems and about one-fifth of the leaves remained free from blight infection, and a still smaller group had only a few infected leaves. There were no seelings completely free from the disease. A number of the most resistant lines were found in a progeny of Katahdin, naturally fertilized. A few seedlings of the cross Chippewa X Katahdin, both of which are susceptible to blight, escaped with very little injury. From this test it was evident that there are different degrees of resistance; that resistant varieties can be obtained by inbreeding certain susceptible varieties and by crossing two susceptibles. The test showed also that lateness is not completely correlated with blight resistance or escape, since all the seedlings were comparatively late, but hundreds of them were killed by blight before they had time to mature.
Since 1932 a large number of varieties and seedlings have been tested for resistance, some of them at Presque Isle, Maine, and some in the greenhouse at Beltsville, Md., under conditions favoring heavy infection. At Presque Isle blight spores are sprayed on the plants under test on evenings preferably cool and damp. In the greenhouse steam is turned into the section in which the plants are being tested, to produce a high humidity, and the plants are then sprayed with spores of the fungus. Heavy epidemics are usually induced by these methods, and unless a variety is resistant, there is Tittle chance of escape. A number of introductions from Germany and elsewhere, as well as a comparatively large number of progenies, have been put through these tests during the last 4 years. The results obtained with some of these were reported by Stevenson et al. (67) and need not be repeated here. It Bathe, be said, however, that there are now available hundreds of varieties and seedlings showing varying degrees of resistance to late blight. A few produced from seed received from K. O. Miiller, Berlin- Dahlem, Germany, have completely escaped infection, even under epidemic conditions such as prevailed in the Presque Isle tests in 1936.
From the commercial standpoint one of the most promising selections up to the present time is from a cross of Chippewa < Katahdin. This variety came through the epidemic of 1932, unsprayed with bordeaux mixture throughout the growing season, with very slight injury. It has been included in the greenhouse tests at Beltsville and again in the field at Presque Isle in 1936 and produces a good crop, even when sprayed with blight spores. It has yielded slightly more than Green Mountain for an average of 5 years at Presque Isle and has good cooking quality when grown at Aroostook Farm. Other promising seedlings highly resistant to blight are from a cross between No Blight and Katahdin. No Blight is described by Bonde (6) under the name Foster Rustproof, and Katahdin has been described by Clark et al. (18). No Blight is quite resistant to late blight. Katahdin is susceptible, but it carries a factor or factors for resistance in a heterozygous condition, as is shown by the fact that blight- resistant seedlings have been found in a progeny of Katahdin selfed: Some of the blight-resistant seedlings from the cross No Blight x Katahdin are being tested for yield and other characters. Two years’ tests show them to be in about the same class as Green Mountain with respect to yield. Another cross from which a number of promising selections have been made had for parents Ekishirazu, a Japanese variety, and seedling 45349, the latter a seedling of Katahdin open-pollinated. Seedling 45349 was selected in 1932 and has shown a fair degree of resistance in a number of tests since that time.
Figure 5 shows the difference between the appearance of the vines of a resistant seedling on which very little late blight developed and the Green Mountain check, which was completely killed by the disease. Both of these were sprayed with blight spores to induce the epidemic.
Figure 5.—Two seedlings of a progeny segregating for resistance to late blight: No. 319 (left) practically free from late blight; no. 320 (center) completely killed by this disease. At right, susceptible Green Mountain check. August 21.
The tubers of a number of the seedling varieties are resistant to tuber rot caused by the late blight fungus. Reiner Bonde, of the Maine Agricultural Experiment Station, has tested a number of them by putting blight spores on the surface of the tubers and then placing them in a moist chamber in a temperature conducive to development of rot. A number of the seedling varieties remained free from rot except where the skin was broken. The Green Mountain check rotted completely in a very short time (fig. 6). Many varieties therefore have been produced within the last few years by hybridization and selection that are resistant enough to late blight to be grown successfully without being sprayed even in years when this disease occurs in epidemic proportions. Several of these are promising also from the standpoint of other characters of commercial importance.
Figure 6.—Resistance of tubers to late blight. Sound tubers, seedling no. 336-302; decayed tuber, Green Mountain. Both lots artificially inoculated with spores of the organism that causes late blight.
Another disease that takes its toll wherever potatoes are produced is the common scab caused by Actinomyces scabies. Scab is not so noticeable in its effects on yields as is late blight, but it affects the market quality and hence the value of the tubers.
The behavior of a comparatively large number of varieties under widely different conditions indicates that resistance to scab occurs in the potato in varying degrees. Complete immunity has not as yet been demonstrated. Concerning this point, Berkner (3) says that absolute immunity does not appear to exist, but that there are decided hereditary differences in the degrees of resistance and susceptibility.
The nature of resistance to scab has been studied by several investigators. The fact that the varieties that have shown the highest degrees of resistance possess a thick russet skin has led some to believe that resistance is dependent upon this type of skin. Histological studies of several varieties by Lutman (37) led him to conclude that thickness of skin determines the resistance of the tubers to scab and that color does not play an important role. Stuart (70) showed the fallacy of the prevailing conception that the russet type of skin is the basis of freedom from scab and pointed out that scab was abundant on the tubers of Cambridge Russet during a period of 6 years. In tests of a large number of seedlings, Darling, Leach, and Krantz (21) found a high degree of resistance in smooth and thin-skinned seedlings as well as in russet types. No correlation was shown to exist between color of tuber and scab resistance.
Some of the American commercial varieties, such as Russet Rural, Russet Burbank, and Mahr Russet, have been known for a number of years to be resistant to scab. A number of European varieties, Richter Jubel, Arnica, Hindenburg, Oststarke, Treff As, Rheingold, and Ostragis, are also resistant, as was reported by Schlumberger (62,63). These varieties, with the addition of a U.S. D. A. seedling, no. 44537, which is highly resistant to scab, have become the basis of breeding for resistance to this disease. At Presque Isle, Maine, a large number of other varieties also have been tested for the purpose of obtaining additional material. The first tests, 1930-33, were conducted on land that was known to produce scabby potatoes in previous years. The results were somewhat variable. In 1934-36 lime was applied, before planting, at the rate of about 1 ton per acre, to land that was known to produce some scabby potatoes in previous seasons.
In the tests five hills of the variety or seedling to be tested were planted in alternate hills with Green Mountain, which is susceptible to scab, Comparisons were made between the seedling or variety and the Green Mountain check. The material included in these tests consisted of varieties of North American, European, and Asiatic origin, a collection of South American varieties obtained by the MacMillan-Erlanson expedition, and a number of progenies resulting from selfing certain varieties, as well as progenies from crosses. Several of the European varieties, including Hindenburg, Richter Jubel, Ackersegen, Arnica, and Hindenburg x Centifolia No. 9, which had been introduced because of their resistance to this disease, proved to be highly resistant in the scab test plots. All of these produced less than 1 percent of the amount of scab found on the Green Mountain checks. None of them had enough scab lesions on the tubers to classify them as scabby potatoes, while the Green Mountain tubers could not have been sold for table stock.
Golden, a new variety recently produced by the Department, was found in the 1935 tests to have about one-fifth as much scab on the tubers as was found on the Green Mountain checks. A few North American varieties—Russet Rural, Russet Burbank, and Mahr Russet—were intermediate in their resistance. A relatively large number of crosses have been made in an effort to determine the breeding behavior of some of these varieties. Although the data will not permit the formulation of an exact genetic hypothesis, a number of facts have been brought out in these tests. The progeny of a cross between two scab-susceptible varieties, Columbia Russet and Katahdin, were all susceptible. Another cross between two resistant varieties, Ostragis and Hindenburg, produced a progeny in which all were resistant to scab. Other crosses between resistant varieties showed segregation for resistance in the first generation.
Figure 7.—Resistance to common scab. The second and third tubers, seedling no. 416-50, are highly resistant to this disease. The first and fourth tubers are from the Green Mountain check grown in the hills adjacent to the seedling variety.
A heavily russeted seedling variety, no. 44537, proved to be highly resistant to scab. This variety produces good pollen and is self- fertile. A progeny of the variety, selfed, segregated for resistance and russeting. A number of the russeted types and a few of the smooth white-skinned segregates were resistant to scab. This variety has been crossed with susceptible and resistant varieties. The progenies of certain combinations segregated for resistance and susceptibility. Figure 7 shows a resistant segregate in comparison with the Green Mountain check. One of the most promising progenies from the standpoint of both resistance and vigor resulted from a cross between Richter Jubel and seedling no. 44537. This progeny segregated for russet and smooth skin and for resistance and susceptibility to scab. A large number of both the russet and the smooth types were resistant to scab. A number of these were desirable from the standpoint of shape, depth of eye, color of tuber, and vigor, as well as scab resistance, and were selected for future work. It is too soon to predict their commercial possibilities, but some of them are the most promising scab-resistant seedling varieties produced so far.
The foregoing results show progress. A practical accomplishment has been the distribution within the last 5 years of six new varieties of potatoes. Others are on the way. Warba and Red Warba have been distributed by the University of Minnesota, and four varieties— Katahdin, Chippewa, Golden, and Houma—by the Department. Warba is an early variety, and from reports from different sections of the country it seems to be outstanding among the early varieties in yielding ability. Red Warba has not been tested thoroughly, but it is assumed to be similar to Warba in all its characters except color, since it originated from that variety as a vegetative mutation.
The four new varieties distributed by the Department in cooperation with the State experiment stations are all resistant to the virus disease mild mosaic, which causes so much running out of the Green Mountain variety in Maine. Chippewa is being increased as rapidly as the available supply of seed stock will permit. It is medium early, has high yielding ability, and is widely adapted. Golden is a yellow- fleshed variety, like most of the potato varieties used for food in Germany. There is a limited demand for this type in the United States, and Golden is being grown by 50 or 60 growers in Maine and by a few in the Upper Peninsula of Michigan. The variety is not widely adapted, but where it can be grown successfully it is prized because of its very high yields, good cooking quality, and scab resistance.
Houma was selected from a group of U. 8. D. A. seedlings grown at Houma, La., by J. C. Miller, of the University of Louisiana, because of its high quality and adaptation to the Houma potato-growing section of that State. The season just past showed this variety to be somewhat drought resistant, but not sufficiently so to withstand the severe drought in the Middle West.
Katahdin (fig. 8) was the first variety to be introduced by the Department, and for that reason it is the most widely known of the Department introductions. It has had its ups and downs but is still on the increase. In a recent bulletin Moore and Wheeler (40) say in part:
An outstanding characteristic of the Katahdin that should appeal to most Michigan growers is its ability to produce satisfactory yields of good type tubers even under heat and drought conditions. The Katahdin sets fewer tubers per hill than the Rurals and develops them earlier. This characteristic often enables it to surpass Rurals in yield of marketable potatoes, particularly when the season is unfavorable for the Rural varieties. The results of tests and the experiences of many growers confirm this statement.
Four seasons unfavorable for Rurals but in which Katahdins have been grown successfully have occurred in Michigan since 1930. This is no doubt responsible for the fact that most of the 500,000 bushels of Katahdins grown in Michigan in 1935 were kept for the 1936 planting, although it is not definitely known how the market will receive this variety. Preliminary tests have shown that the Katahdin as grown in Michigan is superior to the standard varieties in market quality and is equal at least to the Rurals in cooking quality. In the fall of 1935, 53 bushels of Katahdin were distributed in bushel lots to 53 hotels in Michigan, Ohio, Illinois, and Indiana.
Fourteen hotels had a few criticisms to make. Thirty-nine reported that they found the Katahdin an excellent potato for baking, boiling, and frying, and that they were well pleased with the color and texture of the cooked product. Many of the hotel chefs made favorable comments on the attractive appearance of the Katahdin and were well pleased with its smooth, thin skin and shallow eyes, which reduced waste in peeling.
These same characteristics, smooth shape and shallow eyes (fig. 9), combined with the resistance to mild mosaic, induced the representative of Argentina, looking for seed potatoes in the United States and Canada, to recommend the purchase of a large quantity of seed stock of the Katahdin variety for that country. He is said to have purchased a quantity of Chippewa, too, although the price of these was comparatively very high. He refused to consider the deeper eyed types such as Irish Cobbler and Triumph.
The potato-breeding project as it now stands and the results up to the present time demonstrate clearly the necessity and the advantages of cooperation in scientific enterprises. The project is dependent on the Division of Plant Exploration and Introduction, Bureau of Plant Industry, United States Department of Agriculture, for new material carrying new genes for resistance and other important characters. Within recent years varieties resistant to late blight, others resistant to scab, and still others that are said to be resistant to leaf roll have been obtained. The plant breeders and horticulturists make the crosses and grow the progenies. The plant pathologists create the epidemics and secure the disease data. The Bureau of Home Economics of the Department, assisted by the horticulturists, makes the cooking tests. The State experiment stations cooperate in testing the new seedlings produced either by the Department or by any other institutions to determine their range of adaptability. If any of the investigators find a seedling adapted to growing conditions in their State they take the responsibility of increasing the seed and seeing that it is distributed to growers. All these steps are necessary for the success of the breeding work, and all the organizations share the credit for its accomplishments.A summary of the results of breeding work up to the present time shows:
The introduction of new varieties or species may not appear to be a plant-breeding procedure, yet it is fundamental to a well-rounded breeding program. It is improbable that varieties will be found in foreign countries adapted to any of the potato-growing regions of the United States, but even if the new introductions cannot compete with the standard varieties they may carry genes for certain characters that will make them extremely valuable from the breeding standpoint. For example, a number of varieties have been introduced recently from Europe that are resistant to late blight, others that are resistant to common scab, and still others that are reported to be resistant to leaf roll. These are being used in crosses for the purpose of combining the genes for resistance with those for other characters of economic importance. In the last 30 years approximately 10,000 sorts, including commercial varieties, seedling varieties, and species, have been collected by the Department. Thousands of these have been discarded because they were considered of no value, but others have provided a wealth of material for the work of potato breeders.
Selection is the principal tool of the plant breeder regardless of the method employed. Selection of clons or strains from a variety has often been designated as a special method. This method is limited in its application, since its success depends on the value of the bud sports or mutations that occur in a variety or clonal line.
Varietal crossing has been universally employed by potato breeders. As a rule, varieties are heterozygous or mixed in their inheritance for most characters and they will segregate into different lines when they are self-pollinated. Because of pollen sterility not all varieties can be selfed, and the only way to use valuable genes in a self-sterile variety is to make it a female parent in crosses. If both parents are heterozygous for the characters under consideration, segregation will take place in the first generation. Not all possible recombinations of characters can be expected in the first generation, however, since one of the parents may be homozygous or pure for some dominant character, in which case all the plants of the first generation would resemble that parent in that particular character. As an example, in some instances the first generation of a late variety crossed with an early may be all late, no segregation into late and early lines appearing. There are certain advantages in using crosses between heterozygous material, and also certain disadvantages. One advantage is that segregation occurs in the first generation; and if, because of sterility, a second generation cannot be produced, it may be possible in many instances to obtain the desired recombination in the first generation. Another advantage is that combinations differing widely in vigor often occur when heterozygous parents are used, and individuals are often found more vigorous than either parent. The main disadvantage in the use of heterozygous lines as parents is the difficulty of obtaining exact genetic knowledge of the breeding material.
Sib mating, or sister-brother mating, is one system of inbreeding and is useful for several purposes. It helps to determine the degree of homozygosity or uniformity of inheritance of a particular line. Selfing each of the sibs would of course give the same information, but this cannot always be accomplished because of the self-sterility of many lines. In some cases, too, segregates from a cross between two sibs are more vigorous than either parent.
Backcrossing is employed to good advantage in potato breeding, but as most of the parent material is heterozygous, it is not so efficient as if homozygous lines were available. If the original parents are homozygous all the plants of the first generations have the same inheritance although they differ in appearance, and a backcross to any one of them gives essentially the same results as it would to any other. If parents heterozygous for certain characters are used, the first-generation hybrids may differ genetically among themselves, and therefore would give different results in backcrosses. More work must be done to accomplish the same results when heterozygous parents are used than would be necessary if vigorous fertile homozygous parents were available. But in any case backcrossing is a useful method, and it is employed very frequently to obtain certain combinations of characters.
Selfing is employed at least to some extent by all plant breeders. Selfing and the recombining of selfed lines is being used extensively by corn breeders, but potato breeders have depended to a large extent on other methods to obtain desirable recombinations. The self-sterility of many clonal lines and the loss of vigor brought about by intensive selfing are probably two of the reasons why potato breeders have not generally adopted this method. Another reason is that the potato as grown commercially is propagated from tubers, and the grower gets a comparatively uniform crop even if the variety is heterozygous for certain characters. This is not true with crops propagated from seed, as these must be genetically uniform in order that a uniform crop may be produced. For genetic studies it likewise is desirable to obtain homozygous potato lines that are fertile. From the commercial standpoint some of the most promising material is obtained by selfing one or two generations, selecting the best of these selfed lines and outcrossing to one of the best commercial varieties. This method has sometimes been referred to as top crossing.
Strain building is not a method in itself but is a system of breeding that makes use of all methods. The system can best be described by an example from the disease-resistance work now in progress at Presque Isle, Maine. A few blight-resistant varieties were obtained from foreign countries, and several seedling varieties produced by the Department were selected that were only slightly injured by the blight epidemic of 1932 in the breeding plots at Presque Isle. The most promising of these from the standpoint of blight resistance were crossed with varieties carrying genes for high yield, good shape, shallow eyes, resistance to mild mosaic, and other characters of economic importance. The progenies were tested for blight resistance, and the most promising seedlings were selected. Some of these were selfed, some of them sib-mated, some backcrossed to the resistant parent, and others outcrossed to unrelated blight-resistant varieties. The resulting progenies were again tested for blight reaction and selections made, taking into consideration resistance to blight, shape and color of tuber, depth of eye, and vigor. By such a combination of the methods of introduction, varietal crossing, sib mating, backcrossing, and selfing, it should be possible to get any desired recombination of the genes of the parent cultures. Some of the resulting new varieties thus produced should be as good as or better than the commercial varieties in yield, shape of tuber, etc., and superior to them in disease resistance.
The use of species hybrids is usually the last resort of the breeder interested in the production of superior horticultural varieties. It is true that in order to do the best work he must be acquainted with the wild relatives of the plant with which he is working, the characters they possess, and their behavior in crosses. Research with species and species hybrids, then, must be one aspect of any breeding project. It sometimes happens that a desirable character is not to be found in the cultivated varieties of a crop but is present in a related species, so that the desired combination of genes can only be obtained by making a cross between species. An illustration of this is found in breeding potatoes for resistance to late blight. At present, although a number of cultivated varieties are resistant to this disease, none has been found immune to its attacks. Some of the related species, however, do show immunity. Attempts have been made from time to time to combine this immunity with characters of commercial importance by the use of species hybrids.
By the use of all the available methods, if it were not for sterility, the number of new varieties that could be produced through recombinations would be limited only by the characters of the available parents and the number of offspring it is possible to grow.
Sterility, or lack of fruitfulness, which is very generally present in potato varieties, is the source of the greatest difficulty in sexual breeding, and in spite of much study of the condition it remains the greatest handicap of the potato breeder.
Salaman (51, 52) and Heribert-Nilsson (24) found sterility of the anthers to be dominant to fertility. At first Salaman believed sterility was due to a single gene, but later Salaman and Lesley (59) indicated a more complex manner of inheritance. Edzell Blue, a variety that produces viable pollen, was heterozygous on its female side for male sterility. Krantz (32) points out that two commercial varieties, Green Mountain and Early Ohio, have produced fertile seedlings in progeny grown from self-fertilized seed. The Green Mountain variety usually sets some seed under favorable growing conditions. The Early Ohio variety sets no seed. It apparently produces viable pollen under very favorable circumstances.
Kessler (27) described a number of morphological characters of the pollen of various varieties and their relation to its germinating power. Neither the shape in itself nor the amount of granulation was an indication of sterility. A much surer test was obtained by staining with carmine in hydrochloric acid. The sterile pollen remained unstained. The influence of culture media, air, humidity, temperature, and light was considered. Light had an adverse effect, as had also temperatures above 95° and below 46° F. Studies of pollen tube development after artificial pollination indicate that the pollen tube of a particular variety may reach the ovule successfully when applied to certain varieties but not when applied to others.
Clark (14) enumerates four different types of plant sterility—premature abscission or dropping of buds and flowers, lack of viable pollen, hybridity, and physiological incompatability between parents.
Premature abscission constitutes a very effective type of sterility, since it is obvious that fruit cannot be produced when buds fall before opening or flowers persist for only a few hours. If this is not very pronounced, so that a few flowers open and persist for a few days, they may, under favorable climatic conditions, produce fruit when pollinated with viable pollen. The anthers of such flowers produce little pollen, and this is rarely, if ever, viable.
Pollen sterility is very strongly manifested in the cultivated varieties of potatoes. This condition appears to be inherent in the species. Stout and Clark (68) studied the pollen of 170 commercial varieties and 513 seedling varieties, representing material from many parts of the world. They failed to find a single variety in which there was not a fairly large percentage of sterile pollen. Of seven wild species studied, only one, Solanum commersonii Dun., showed the presence of this type of sterility to any marked degree.
Salaman (51) demonstrated the hereditary nature of sterility in potato a number of years ago. He stated that the potato plant, which is normally bisexual, carries a dominant factor that inhibits pollen formation at the pollen mother cell stage or even earlier. In a later paper Salaman and Lesley (69) showed by reciprocal crosses that the greater portion, if not all, of the sterility is inherited through the female parent. The fact that the eggs are often viable under conditions lethal to the pollen is well known and makes possible the use of many varieties as seed parents that cannot be used as pollen parents.
Irregular chromosome behavior has been advanced by a number of investigators as one of the chief causes for hereditary sterility. Longley and Clark (36) presented a study of chromosome number and of meiotic behavior in tuber-bearing forms of Solanum. In 37 cultivated varieties grown in the United States there was found a somatic chromosome number of 48. The meiotic behavior of this group varied from regular in a few cases to extremely irregular in many of the varieties. Only the few varieties with a regular chromosome behavior produced an appreciable amount of viable pollen; varieties with an irregular chromosome behavior produced practically no pollen.
Genetic factors and chromosome behavior no doubt make the development of sterility possible, since varieties are inherently different in the degree to which they will bloom or set seed. But seed setting is also influenced by environmental factors to a marked degree. Some varieties will set seed under a wide range of conditions, while others have never been known to set seed even under favorable conditions. Stow (69) showed that environmental conditions even influenced the chromosome behavior. He stated that—
the abnormal division is neither connected with the hybrid nature of the plant nor the nutritive correlation within its body; but is rather due to the environmental conditions or certain special nature of the plant itself.
He considered that sterility was mainly the result of abnormal pollen mother cells, which he observed with exposure to high temperatures (25°-35° C.). “At lower temperatures (15°-20° C.), on the other hand, the reduction proceeded in a regular manner, producing normal pollen grains.”
That length of day has an influence on flowering and seed setting has been shown by Stevenson and Clark (66). In the experiment reported by them that was conducted in the greenhouse at the Arlington Experiment Farm, Arlington, Va., the application of artificial light for 5 hours to 20 potato varieties to supplement the daylight period stimulated vine growth and blossoming to a remarkable degree. The varieties used in this experiment were grown also in the field at Presque Isle, Maine, where 10 of them produced seed in varying degrees of abundance, while the remaining 10 produced no seed. In the greenhouse experiment at Arlington, 70 percent of the plants in the 10 more responsive varieties came to full bloom under the lights, while only 5 percent of the checks without lights bloomed. In the less responsive varieties only 20 percent of the plants under the lights bloomed, and no blossoms were produced by the check. No naturally fertilized seed was produced, but inbred seed was readily obtained under the lights by hand-pollinating self-fertile plants. The chromosome behavior of the plants grown under the light was much more regular than that of the same varieties grown without lights.
The effect of environment on blooming was shown by Stout and Clark (68). Halved tubers of 15 varieties were grown, one set at Presque Isle, and the corresponding halves at the New York Botanical Garden. All the varieties bloomed profusely at Presque Isle. In New York only 2 of these varieties bloomed well, 3 produced a few flowers, and 10 produced no flowers that opened. Conditions are favorable for seed setting nearly every season at Presque Isle. A few other places in the United States have been found favorable for seed production in potatoes. At Estes Park, Colo., which has an elevation of 7,500 feet, seed sets in most years quite readily. Another striking example of the effect of environment is found in the potato-breeding work in Minnesota. Seed cannot be produced on a large number of varieties and strains at University Farm, St. Paul, but many of the same varieties will produce seed at Duluth on Lake Superior, and even better at Castle Danger on the north shore of the lake.
Complete or partial sterility may result from hybridizing, though there seems to be no general rule regarding the behavior of hybrids with respect to sterility, as some combinations produce hybrids with a very high degree of fertility, while the progeny resulting from other crosses may be completely sterile. In a species cross between Solanum fendleri A. Gray and S. chacoense Bitt., produced by Clark (15), the first generation was completely sterile when selfed and when backcrossed with either parent. Bukasov (10) reports that hybrids of S. acaule Bitt. with S. andigenum and with S. tuberosum are sterile.
In some species of plants certain combinations of crosses within the species as well as selfed pollinations fail to produce seed even though there is no degeneracy in either pollen or egg cells. Other combinations in the same species may produce an abundance of fruit. This type of sterility has been referred to as physiological incompatibility.
It was not found to occur in the cultivated varieties of potatoes studied by Stout and Clark (68), but was reported by Clark (14) to be present in the wild species Solanum caldasii glabrescens Dun. and S. chacoense.
Definite genetic data are available for only a comparatively few characters of the potato. Such data are accumulating from year to year, however, and as the knowledge grows the solution of many breeding problems becomes less complex. A partial summary of potato characters, their genetic behavior, the name of the investigator making the report, and the literature citations are given in the appendix.
A study of the table shows the usual array of gene interactions— dominance and recessiveness, complementary genes (either 2 or 3), multiple genes, cumulative effects, and inhibiting genes.
In several instances a number of different ratios are reported for what is apparently the same character; but it must be remembered that characters that look alike may be genetically different and as a result will behave differently in inheritance.
From the material available it is seen that little is known concerning the genetic behavior of some of the most important characters of the potato, such as yield, cooking quality, and resistance to various diseases, but the breeding work is being centered around such characters at present, and it is believed that, while they may be rather complex in their genetic behavior, they will all follow the general rule that a genetic character is the end result of the interaction of genes and environment.
POTATO SPECIES, THEIR CHROMOSOME NUMBERS, AND SOME OF THEIR VALUABLE CHARACTERS
While it is necessary for the breeder to obtain a thorough knowledge of the cultivated varieties and their important economic characters, it is also important to know the related wild forms and species of potatoes. It is true that new genes and gene combinations are being brought to light in the cultivated varieties, but it is quite improbable that all the problems can be solved by the recombinations of genes available in this group.
Much work has already been done with the species of Solanum related to the potato, and a fund of valuable information is available concerning them. The species fall into five groups with respect to 2n chromosome numbers, those having 24, 36, 48, 60, and 72 in vegetative tissues, as reported by several investigators. According to Bukasov (9), cultivated varieties have been found in the first four groups. Crosses between species with different chromosome numbers are as a rule difficult to obtain. For example, crosses between species of the 24 group and the 48 group are very rare, although many attempts have been made to produce them. Certain species crosses have been reported, however, and as knowledge increases concerning the causes of incompatibility and sterility, it may be possible in the future to get hybrid combinations that at present seem quite impossible. A partial list of species of the Tuberarium section of Solanum, with their 2n chromosome numbers, is given in the appendix.
Characters that would be especially valuable if they could be combined with those of the best commercial varieties have been found in a number of species. Among these are resistance to drought, frost, potato wart, viruses, and late blight. The characters for short-day development and short rest period are found in some of the species also. The character for short-day development might be valuable in potato districts where the crop is grown during the winter months under conditions of short-day length. A list of the species known to possess valuable characters from the breeding standpoint is also given in the appendix.
|O. H. Alexander, Charlotte, Vt.||Charles Downing, Dakota Red, Everett, Garfield, Green Mountain, Reliance, Trophy, White Mountain|
|Martin Bovee, Northville, Mich||Bovee, Early Michigan, Pingree|
|Albert Bresee, Hubbardton, Vt||Early Rose, King of the Marlies, Peerless, Prolific|
|C. W. Brownell, Essex, Vt||Beauty, Best, Centennial, Early Telephone, Eureka, Superior, Winner, Vermont Beauty|
|Luther Burbank, Lancaster, Mass.||Burbank (Burbank’s seedling)|
|E.S. Carman||Carman No. 1, Carman No. 3, Rural New Yorker No. 2, Sir Walter Raleigh|
|E. L. Coy, Hebron, N. Y.||Early Beauty of Hebron, Late Beauty of Hebron, Early Puritan, Empire State, Late Rose, Noroton Beauty, Thorburn, Vaughan, White Elephant|
|Thomas Craine, Fort Atkinson, WI||June Eating, Keeper, Potentate|
|C. E. Goodrich, Utica, N. Y.||Calico, Cuzco, Early Goodrich, Garnet Chili, and several others which were short lived in the commercial trade|
|D. W. Heffron, Utica, N. Y||Chicago Market, Climax|
|C. G. Pringle, Charlotte, Vt||Alpha, Adirondack, Rubicund, Ruby, Snowflake|
|Arthur Rand, Shelburne, Vt||Champion, Delaware, Matchless, Improved Peachblow, Silver Skin|
|Alfred Reese||Early Ohio|
|G. T. Safford, North Bennington, VT||Gold Coin|
|F. B. Van Ornam, Lewis, Iowa||Extra Early (Burpee’s), Great Divide|
|Katahdin||Department of Agriculture||Fertility, resistance to mild mosaic, good shape, high yield.|
|Chippewa||Resistance to mild mosaic, good shape, high yield, early tuber development.|
|Golden||High yield, some resistance to scab.|
|Houma||High yield, some drought resistance, good shape, good cooking quality.|
|24642||Fertility, resistance to mild mosaic, some earliness.|
|41956||Immunity to latent mosaic and resistance to mild mosaic.|
|44488||Resistance to late blight, high yield.|
|44537||Fertility, resistance to scab.|
|45075||Fertility, earliness, good shape.|
|45208||High yield, good cooking quality.|
|45349||Resistance to late blight.|
|46125||High yield and cooking quality.|
|46923||Fertility and earliness.|
|182-7||Fertility, and resistance to late blight.|
|336-123||High yield and resistance to late blight.|
|336-144||Fertility and resistance to late blight.|
|336-302||Resistance to late blight.|
|3868-8||Department of Agriculture from German seed.|
|444-12||Department of Agriculture||Fertility and resistance to scab.|
|Green Mountain||United States||High yield, quality.|
|Rural New Yorker No. 2||Good tuber type, hardiness.|
|Russet Burbank||Good quality, resistance to scab.|
|Charles Downing||Good quality and tuber type.|
|Irish Cobbler||Earliness and wide adaptability.|
|Mahr Russet||Resistance to scab.|
|Arnica||Germany||Resistance to scab.|
|Richter Jubel||Fertility, resistance to scab.|
|Ostragis||Resistance to scab.|
|Ackersegen||Resistance to late blight.|
|Paisley's No. 1|
|Paisley's No. 2|
|Paisley's No. 3|
|Paisley's No. 4|
|Imperia||Resistance to leaf roll|
|130.5-24||North Carolina||High yield.|
|101-91||Michigan||Good tuber shape, high yield on muck.|
|ND No. 64||North Dakota||High yield, good shape.|
|ND No. 82||Earliness, good shape.|
|ND No. 86||Earliness, high yield, and good shape.|
|ND No. 87||Russet|
|F.B. 32-1||Colorado (Fort Collins)||High yield, good shape.|
|E. 32-7||Earliness, high yield, good shape.|
|E. 32-8||High yield, good shape.|
|252||Fertility, shallowness of eye.|
|C.S. 48||Colorado (Greeley)||Fertility.|
|C.S. 106||Resistance to fusarium.|
|Cross no.||Parentage||Superior characters|
|774||S 41956 X Katahdin||Resistance to latent and mild mosaics.|
|792||S 41956 X S 45075||Resistance to latent mosaic.|
|926||Katahdin x S 45075||Earliness, resistance to mild mosaic.|
|1028||Katahdin selfed||Resistance to mild mosaic.|
|295||Russet Rural x S 44537||Resistance to common scab.|
|528||Richter Jubel X S 44597|
|336||No Blight x Katahdin||Resistance to late blight and mild mosaic|
|618||S 45349 X Ekishirazu||Resistance to late blight.|
|690||S 45349 X Katahdin|
|500 hybrid families||Immunity from late blight.|
|Line no.||Parentage||Generations inbred (selfed)||Sublines||Superior characters|
|1||Peerless||4||13||2||Early maturity, high pollen fertility, resistance to scab.|
|3||Cobbler X Peerless||4||11||3||Early maturity, vine type.|
|4||Keeper X Silverskin (U.S. 14329)||9||182||7||Early maturity, good tuber shape, high pollen fertility, free blooming.|
|5||Peerless X Lookout Mountain||7||90||43||Early maturity, high pollen fertility, short stolons, resistance to scab.|
|9||Warba X Katahdin||1||0||1||Vine type, tuber quality, resistance to virus.|
|11||Inbred 41-17 X Inbred 4-9-1||7||150||34||Early maturity, good tuber shape, high pollen fertility, vine type, vigor and yield, tuber set, free blooming.|
|12||Snowflake||5||29||3||Good tuber shape, vigor and yield, tuber quality.|
|13||Burbank X Inbred 66-1||2||0||2||Good tuber shape, tuber quality.|
|15||Inbred 11-1-25 X Inbred 66-5||2||0||5||Early maturity, good tuber shape, high pollen fertility, vigor and yield, free blooming.|
|21||Inbred 49-1 X Inbred 4-9-1||3||16||3||Early maturity.|
|29||Inbred 41-1 X Inbred 4-25||3||30||2|
|39||Irish Cobbler||5||40||4||Early maturity, good tuber shape, vine type, tuber quality, short stolons, resistance to virus.|
|40||U.S. Seedling 38946||3||23||3||Good tuber shape, high pollen fertility, vine type, free blooming, resistance to virus.|
|41||Early Ohio||5||130||9||Early maturity, vine type, vigor and yield, tuber set.|
|66||Katahdin||2||0||13||Good tuber shape, tuber set, resistance to virus.|
|82||Inbred 11-1-25 X Inbred 21-2-2||3||0||17||Early maturity, good tuber shape, high pollen fertility, free blooming.|
|Character||Genes and interactions||Segregations||Investigators and reference numbers|
|Purple X white||Purple dominant to white||East (22)|
|Lilac X white||Lilac dominant to white||Fruwirth (23)|
|Blue violet X white||Blue violet dominant to white||3 blue-violet : 1 white||Müller (41)|
|Light blue (selfed)||Heterozygous||Monogenic||Heribert-Nilsson (24)|
|Light blue X white||1 colored : 1 white|
|Violet blue (selfed)||Multiple factors||Violet-blue, red, reddish-purple, dark and light blue and white|
|Red X white||2 complementary genes (D+R for red)||Salaman (53)|
|Purple X white||3 complementary genes (D+R+P for purple)|
|White X white||In some strains colored and white||(Salaman) Matsuura (39)|
|Light lilac (selfed)||Complementary||9 colored:7 white||Clark and Stevenson (18)|
|White (selfed)||Homozygous||All white||Salaman (52, 53)|
|Red (selfed)||2 complementary genes, R+D for red||9 red : 7 white|
|Dark purple X white||3 complementary genes, R+D+P for purple||13 purple: 12 red : 4 white (F1)|
|Dark purple (F1) selfed||73 purple : 24 red : 75 white (F2)|
|Light red X yellow||Heterozygous||1 red : 1 yellow (F1), various grades of red, many having blue or dark-blue striping, also plants with colored eyes||Heribert-Nilsson (24)|
|Yellow X yellow||||Yellow and 10 percent blue black, blue, red or pale red|
|White (S. edinense selfed)||G and H complementary, Y inhibitor||13 colored : 40 white||Müller (41)|
|White (S. edinense) X white (S. tuberosum)||21 colored : 40 white|
|Parti-colored||3 genes complementary; B, basic, D, diluting, M+D=parti-colored pattern||||Kelly (26)|
|Multiple allelomorphic series for uniformly colored parti-colored and white.||||Collins (20)|
|Red skin color||3 genes complementary, D+R=parti-colored, D+R+A=uniformly colored||||Krantz (33, 34)|
|Red cortical color||3 genes complementary, similar interaction as for skin color.|||
|Red color||Genes complementary, A+D=parti-colored, A+D+R=uniformly colored; linkage between A and R.||||Asseyeva (1)|
|Genes complementary, R+S+D=red skin color, R+S=red eye color||27 colored : 37 white, 9 red-eyed : 7 white-eyed.||Sirks (64)|
|Color||Genes complementary, D+R=red, D+R+P=purple, and Han incompletely dominant inhibitor.||||Black (4)|
|Red (selfed)||2 genes complementary||9 colored: 7 white||Hüber (25)|
|Purple (selfed)||Single gene, color dominant||1 dark purple: 2 light purple: 1 white||Krantz (31)|
|Colored (selfed)||Complementary, C+I= colored||||Müller (41)|
|Colored X white||C+I+ inhibitor Z=white||All white|
|Tuber flesh color:|
|Flesh color||Multiple genes||Black (4)|
|2 genes, 1 giving yellow in plant homozygous or heterozygous, other homozygous only||Hüber (25)|
|Red v. green||Single gene, red dominant||East (22)|
|Medium red (selfed)||Salaman (52)|
|Pigment||3 genes complementary, a fourth gene an inhibitor||Müller (41)|
|Colored internodes||3 genes for color||36 blue:9 red:19 white for young internodes; 12 blue: 3 red: 1 white for old internodes||Sirks (64)|
|Seedling and sprout color:|
|Seedling color||3 genes complementary||Seedling color associated with flower, stem, and flesh colors||Müller (41)|
|Seedling color (blue, red, green)||2 or 3 genes complementary||Sirks (64)|
|Sprouts, blue-purple v. red purple||Single gene blue-purple dominant, same as P for tuber color||Asseyeva (1)|
|Habit of growth:|
|Upright v. prostrate||At least 3 genes (multiple)||3 upright: 1 prostrate; 15 upright: 1 prostrate; 63 upright: 1 prostrate||Salaman and Lesley (58)|
|Round (selfed)||All round*||Salaman (52)|
|Oval (medium long) (selfed)||Long: oval: round*|
|Long (selfed)||All long*|
|Medium round (selfed)||Range from round to long||Clark and Stevenson (18)|
|Round (selfed)||Not all round||Heribert-Nilsson (24)|
|Tuber shape||Dependent on more than 1 gene||Black (4)|
|Three genes (multiple)||Huber (25)|
|At least 4 genes (multiple)||Bartosh (2)|
|Depth of eye:|
|Deep (selfed)||Deep incompletely dominant to shallow||All deep eyed||Salaman (52, 53, 54)|
|Medium deep (selfed)||Deep, medium, shallow|
|Shallow (selfed)||All shallow|
|Deep v. shallow||Shallow eye dominant||East (22)|
|Time of maturity:|
|Late X late||Wide range of variablity in F2||Krantz (31)|
|Time of maturity||Multiple genes||Krantz and Hutchins (35) & Müller (42)|
|Inheritance of cropping:|
|Heavy v. light||2 or more genes, heavy dominant||Salaman (56)|
|Immunity and resistance to potato wart:|
|Immune (selfed)||Homozygous||All immune||Salaman and Lesley (60)|
|Single gene immune dominant.||3 immune: 1 susceptible|
|2 genes duplicate||15 immune: 1 susceptible|
|2 genes complementary||9 immune : 7 susceptible|
|Susceptible X slightly resistant||F1, 29 percent resistant||Heribert-Nilsson (24)|
|Susceptibility v. resistance||Susceptibility dominant||Collins (19)|
|Resistance dominant||Orton and Weiss (45)|
|3 genes (multiple) with varying values and cumulative||Black (5)|
|Resistance to late blight:|
|Resistance v. susceptibility||At least 2 genes acting independently||Six types of segregation; no close linkage with commercial characters.||Müller (43)|
|Chromosome numbers and species||Investigators and reference nos.|
|Solanum ajanhuiri Juz. and Buk.||Kovalenko and Sidorov (29), Rybin (49), Vesselovskii (72)|
|S. aracc-papa Juz||Rybin (49)|
|S. boyacense Juz and Buk|
|S. brevidens Phil|
|S. bukasovii Juz||Rybin (48, 49)|
|S. caldasii Humb. and Bonl||De Vilmorin and Simonet (73, 74)|
|S. caldasii glabrescens Dun||Longley and Clark (36)|
|S. chacoense Bitt||Smith (65), Longley and Clark (36), Rybin (48), Oppenheimer (44)|
|S. cuencanum Juz. and Buk||Bukasov (10)|
|S. fernandezianum Phil||Rybin (49)|
|S. goniocalyx Juz. and Buk||Bukasov (9)|
|S. jamesii Torr||Smith (65), De Vilmorin and Simonet (73, 74), Longley and Clark (36)|
|S. kesselbrenneri Juz and Buk||Bukasov (10)|
|S. looserii Juz||Rybin (49)|
|S. phureja Juz. and Buk||Kovalenko and Sidorov (29), Rybin (49)|
|S. polyadentum Greenm||Longley and Clark (36)|
|S. rybinii Juz. and Buk||Rybin (49)|
|S. stenotomum Juz. and Buk||Rybin (49), Bukasov (10)|
|S. vavilovii||Bukasov (10)|
|S. cardiophyllum Lind||Rybin (49)|
|S. cardiophyllum f. coyoacanum Buk||Longley and Clark (36)|
|S. chaucha Juz. and Buk||Kovalenko and Sidorov (29), Rybin (49)|
|S. chocclo Buk and Lechn||Bukasov (9)|
|S. commersonii Dun||Longley and Clark (36)|
|S. coyoacanum Buk||Rybin (49)|
|S. juzepezukii Buk||Kovalenko and Sidorov (29), Pissarev (46), Rybin (49)|
|S. maglia Schlecht||Rybin (49)|
|S. mammilliferum Juz and Buk|
|S. medians Bitt||Rybin (48)|
|S. riobambense Juz. and Buk||Bukasov (10)|
|S. tenuifilamentum Juz. and Buk.||Rybin (49)|
|S. vallis-mexici Juz||Kovalenko and Sidorov (29), Rybin (49)|
|S. acaule Bitt.||Kovalenko and Sidorov (29), Rybin (49)|
|S. ajuscoense Buk|
|S. andigenum Juz. and Buk.||Rybin (49)|
|S. antipoviczii Buk||Kovalenko and Sidorov (29), Rybin (49)|
|S. colombianum trianae Bitt||Rybin (48)|
|S. edinense Berth. (some forms)||(Campin), Salaman (55)|
|S. fendleri A. Gray||Smith (65), Longley and Clark (36), Rybin (49)|
|S. leptostigma Juz||Rybin (49)|
|S. tuberosum L.||De Vilmorin and Simonet (73), Longley and Clark (36), Pissarev (46), Rybin (49)|
|S. curtilobum Juz. and Buk.||Bukasov (9), Kovalenko and Sidorov (29), Rybin (49), Vesselovskii (72)|
|S. edinense Berth. (some forms)||(Campin) Salaman (55), Rybin (48)|
|S. semidemissum Juz||Kovalenko and Sidorov (29), Rybin (49)|
|S. demissum Lindl||Smith (65), De Vilmorin and Simonet (74), Longley and Clark (36), Pissarev (46)|
|Characters and species||Investigators and reference nos.|
|Solanum acaule Bitt||Pissarev (46), Vesselovskii (72), Schick (67)|
|Solanum ajanhuiri Juz. and Buk.|
|Solanum andigenum Juz. and Buk.||Kovalenko (28), Bukasov (9), Vesselovskii (72)|
|Solanum bukasovii Juz||Bukasov and Lechnovitz (11), Pissarev (49)|
|Solanum commersonii Dun||Bukasov and Lechnovitz (11)|
|Solanum curtilobum Juz. and Buk.||Bukasov (8), Pissarev (46), Rasumov (47), Schick (61), Vesselovskii (72)|
|Solanum demissum Lindl||Bukasov (7), Kovalenko (28), Pissarev (46), Rasumov (47), Schick (61), Vesselovskii (72)|
|Solanum edinense Berth||Bukasov (7)|
|Solanum juzepezukii Buk||Bukasov (9), Pissarev (46), Rasumov (47)|
|Solanum millanii Buk. and Lechn.||Bukasov and Lechnovitz (11)|
|Solanum semidemissum Juz||Bukasov (7), Kovalenko (28), Vesselovskii (72)|
|Solanum medians Bitt||Bukasov (10)|
|Late blight resistance:|
|Solanum ajuscoense Buk||Kovalev (30)|
|Solanum antipoviczii Buk||Bukasov (10), Kovalev (30)|
|Solanum bulbocastanum Dun||Bukasov (10)|
|Solanum demissum Lindl||Bukasov (10), Kovalev (30)|
|Solanum henryi Buk. and Lechn.||Bukasov and Lechnovitz (11)|
|Solanum millanii Buk. and Lechn.|
|Solanum polyadenium Greenm.||Bukasov (10)|
|Solanum vallis-mexici Juz||Kovalev (30)|
|Solanum vorrucosum Schlecht||Bukasov (10)|
|Solanum rybinii Juz. and Buk||Bukasov (10)|
|Solanum phureja Juz. and Buk||Bukasov (8)|
|Solanum rybinii Juz. and Buk|
|Solanum acaule Bitt||Bukasov (9)|
|Solanum antilobum||Pissarev (46)|
|Solanum antipoviczii Buk||Bukasov (9)|
|Solanum bulbocastanum Dun|
|Solanum demissum Lindl|
|Solanum goniocalyx Juz. and Buk.|
|Solanum juzepczukii Buk|
|Solanum semidemissum Juz|
|Solanum squamulosum Mart. and Gal.|
|Solanum vallis-mexici Juz|
|Solanum verrucosum Schlecht|
|Short rest period:|
|Solanum boyacense Juz. and Buk.||Bukasov and Lechnovitz (11)|
|Solanum kesselbrenneri Juz. and Buk.|
|Solanum phureja Juz. and Buk|
|Solanum rybinii Juz. and Buk|