FIG. 9.—Cedar rust gall in the red cedar with gelatinous orange spore masses fully expanded by rain. Maryland, near Washington, D. C., April 27, 1926
APPLE Trees Attacked by Cedar Rust
The three native crab apples of the eastern half of the United States have been known for many years to be attacked by one of the true rust fungi which forms orange-yellow swollen spots on the leaves. This rust was classified and named in 1859 and has been well known to students of fungi since that time and has been found widely distributed from the Great Plains eastward.
The common Virginia red cedar, Juniperus virginiana, occurring mainly east of the Great Plains area, but also extending from New Brunswick to British Columbia on the north and to Texas, New Mexico, and Arizona on the south, has also long been known to be attacked east of the Great Plains by a fungus which forms little brown galls mostly varying from one-fourth of an inch to an inch or more in diameter. This was also given a name by the early mycologists and classified as belonging to another group of the rusts.
About 1865 the remarkable discovery was made in Europe that the common wheat-rust fungus and the cluster-cup fungus of the barberry were alternate generations of the same organism. The barberry fungus was found not to propagate on the barberry but could only reproduce when its spores were sown on suitable wheat plants on which it produced the common wheat rust. The term “heterocism ” was coined to designate this method of fungus propagation. The principle of heterocism opened up a new field in the study of the rusts and was followed by the discovery in Europe that some of the orange rusts on pomaceous fruits were alternate generations of rusts on the junipers. Between 1886 and 1888 a number of American species of orange rusts on the pome family were proved to be alternate forms of rusts on the junipers, including the common orange rust of native American crab apples and the cultivated apple and Siberian crab apple.
The cultivated apple introduced from Europe was grown in Virginia alongside the red cedars and perhaps crab apples for nearly 300 years without anything serious happening, and there was the same experience during the last hundred years in the upper Missisippi Valley. Occasionally an apple tree was found with a few spots of the orange rust on its leaves, and in Virginia the Pryor Red variety was very severely attacked. The writer saw this variety severely attacked by this rust and defoliated in the late nineties and. vas informed by older fruit growers that it had been so attacked for some 20 years or more. Pryor Red, however, was not an important commercial variety. Except on Pryor Red and a few localized attacks on other apples the disease was not abundant enough to be of economic importance at the beginning of this century.
In 1905 and 1906, however, this fungus attacked the Wealthy apple in Iowa and Nebraska so as to attract attention as an economic disease. In the summer of 1908 localized outbreaks began to occur in the vicinity of Winchester, Va., and the writer made a personal examination of three small colonies involving a few hundred trees of the York Imperial. That same year reports came of its increase in the Appalachian fruit belt. The York Imperial is the leading commercial variety in this entire district. Since 1908 the disease has increased east of the Alleghanies from New York to North Carolina, and attacked the cultivated apples so severely as to become a major economic disease. It has also increased in many other sections of the eastern United States. Attacks on the York Imperial were followed by its spread to other varieties—Jonathan and Rome Beauty; later Ben Davis, Yellow Newtown, and other sorts were severely attacked. The Grimes Golden and the Winesap group appeared immune at first but the Grimes Golden has been severely attacked and localized attacks on the Winesap and Mammoth Black Twig have begun.
In 1912 the disease destroyed the fruit crop in many blocks of trees in Virginia and adjacent States. That season a few enterprising orchardists in the neighborhood of Winchester, Va., had cut down the red cedars around their apple orchards, and the result was a striking demonstration of the efficiency of this method of control.
In 1914 the disease had become so severe that in the Winchester, Va., district alone it was estimated that it destroyed 100,000 barrels of apples, reducing production estimated at 500,000 to 400,000. As a result, recommendations of the Department of Agriculture and of several of the State experiment stations for eradicating the red cedars began to be carried out. Virginia passed a special cedar-rust law in 1914, West Virginia having passed a similar law the year before, and other States have followed. Whatever theory or explanation may be advanced, the facts are that this fungus gradually attacked one variety after another with increasing severity. Since localized infections have already begun on the Winesap group and other varieties previously resistant, it is doubtful whether any variety of apple can be counted on as resistant to this disease.
Beginning with the little brown galls on the cedar trees in early spring, these begin to exude their orange-colored gelatinous spore masses and form their secondary spores or sporidia on the first rain after the apples reach the pink-bud stage. They continue to throw off sporidia each rainy spell for about six weeks. These sporidia can not attack the red cedar but can grow only on the apple and its relatives. They can attack only young, newly formed leaves within a few days after they are expanded. They are very light and minute and are easily blown in all directions by the wind, but in general the quantity reaching any given number of apple leaves varies inversely as the square of the distance. The nearest infected cedar trees are, of course, the most dangerous. Those twice as far away are about one-fourth as dangerous but increase in the number of trees and the amount of infection may compensate for greater distance. After a fungous thread-forming spore succeeds in entering an apple leaf it produces a visible orange-colored thickened spot. This spot grows to about one-eighth or one-fourth of an inch in diameter and about the middle of July forms fringed cluster cups on the under surface, each crop filled with the summer spores of this fungus.
Leaves carrying a few spots may live through the season and function, but when heavily infested they turn yellow and fall to the ground in mid-summer. The fruit on heavily infested trees stops growing, is poorly colored and often less than half normal size. The fruit itself is sometimes attacked, especially near the calyx end, a similar but larger orange-colored spot being produced. Young tender twigs of some varieties are also occasionally attacked. The cluster-cup spores from the apple leaves mature in July and early August. They can not attack the apple, but can only germinate and enter the young, tender leaves and twigs of the red cedar trees. These summer infections on the red cedars remain dormant and invisible through the summer, fall, and following winter.
When the infected cedars start to grow the next spring the fungus grows through the tissues and the gall forms and grows with the tree. These galls, made up of a mixture of the host and the fungus, each practically full size by fall, and pass through the second winter in this condition. They reach full maturity and exude their spore masses the following spring and then die, thus completing the two-year cycle. The fungus, therefore, spends 2 to 4 months of late spring and summer on the leaves or perhaps the fruit of the apple, and about 21 to 22 months on the red cedar, the first winter as an invisible dormant infection, the second winter as a full-grown gall.
It should be noted that there are three living plants necessary for this cycle—the red cedar, the apple or its relatives, and the cedar-rust fungus. Without the presence of the cedar-rust fungus, red cedars and apples grow harmlessly together, neither endangering the other. However, when the cedar-rust fungus is present the red cedar becomes a pest tree to the apple orchards; and, on the other hand, the diseased apple tree serves to infect the red cedar.
Two other related species of cedar-rust fungi attack the apple in the eastern United States. Both are unimportant as apple pests as they occur only occasionally. One of these, however, also attacks the cultivated quince, producing an orange rust of economic importance.
Efforts to control the disease by spraying have proved futile. Spraying has reduced the attacks of the fungus in some cases but the rapidity of the development of the new leaves and the repeated infections that occur are probably the explanation of the failure of spraying. It should be noted that cedar rust has spread in Virginia and West Virginia orchards that were annually successfully sprayed for apple scab, leaf spot, and other fungous diseases. On the other hand, efforts to control the disease by cutting down the red cedars have been uniformly successful just in proportion to the thoroughness of the cedar eradication and the distance. At first, during 1912 and 1914 good results were obtained in Virginia by cutting the cedars within half a mile of the apple orchard. Later, it was found necessary to cut the cedars within 1 or 2 miles and finally as infection increased even the 2-mile distance has been found unsatisfactory here hillsides and mountain sides were found covered with large bodies of cedars. Ordinarily a distance of 2 to 4 miles may be recorded as satisfactory, with the latter distance as probably the only safe one.
Since it is only infected cedars which transmit this disease, and since the large spored form of fungus apparently is not blown by the wind as readily from the apple onto the cedar as the sporidia from the cedar to the apple, it is probably important where a large quantity of cedars at a considerable distance can not be removed, to keep the apples away from them. In other words, a large group of cedars may receive their rust infection from a few apple trees near them and then deliver the wind-blown spores in vast quantities to distant apple orchards. Without the few near-by apple trees they would remain uninfested or little infested.
In view of the above facts the biological conditions call for the following procedures: (1) The eradication of the red cedar in the vicinity of apple orchards wherever the cedar-rust fungus is present; (2) the abandonment of the planting of red cedars in the vicinity of apple orchards in the eastern United States and perhaps elsewhere in the country; (3) the substitution of other species of conifers of somewhat similar appearance for ornamental, park, and roadside trees; and (4) the segregation of red cedar plantings for forest or other purposes into districts where apples are not grown and preferably where apples and crab apples are eradicated and with a zone of 4 or 5 miles surrounding the forest area where neither host plant is permitted to grow.
Possibly also quarantine action may be necessary in preventing the shipment and planting of red cedars. Certainly red cedars from cedar-rust infested territory should not be shipped into districts west of its natural range, that is, west of the Great Plains. Certain details in connection with the handling of these matters may be mentioned. The presence of the cedar-rust fungus on cedar trees from infested districts is not determinable when there are dormant infections. It is not easy to find moderate infection of the mature galls on account of the density of the tops. There is no likelihood of the disease ever being transmitted from dormant nursery trees of the apple, as it is only the summer form that occurs on this host. In eradicating the red cedar trees those over 3 or 4 inches in diameter may be simply cut down and the stumps will die, but trees of smaller size usually sprout and the young sprouts are more susceptible to the disease than the mature growth. All young trees, therefore, should be either pulled up or grubbed below the ground line. Many cedar trees occur in hardwood thickets. The best time to find them, therefore, is in the winter, when the leaves are off, especially when there is a light covering of snow on the ground. When the cedars are cut in the fall or winter the galls die, but when cut in March or April so that the tops remain green and the galls fresh they may exude their spore masses from the fallen trees. All tops, therefore, should be burned on trees cut after February.
