FIGURE 17.—The extent of the principal area known to be invaded by the Dutch elm disease, and the total number of diseased elms confirmed in each county, as of October 24, 1934.
DUTCH Elm Disease Must be Eradicated to Save American Elm
Wide-spread destruction faces the American elm through the spread of the Dutch elm disease, caused by a deadly fungous parasite introduced from Europe. The presence of this disease in the vicinity of New York Harbor was discovered in June 1933, but subsequent observations indicate that it may have become established there as early as 1929. It is now known to have invaded an area of approximately 2,500 square miles in New Jersey, New York, and Connecticut, within a 40- to 50-mile radius of New York City. By October 1934 more than 7,500 diseased trees had been located in this center of infection. Presumably many more are diseased but had not at that time developed characteristic external symptoms.
In practically all the States east of the Rocky Mountains the American and other species of elm constitute an irreplaceable public asset. In the Northeastern States particularly the American elm is the characteristic shade tree along streets and about dwelling houses. As such, this species has an economic value that runs into many millions of dollars. The enhanced value of real estate due to the presence of elm shade trees in many parts of the United States may hinge on the success of the campaign against this disease in the restricted area at present infected.1
Observations of the effect of the Dutch elm disease in Europe, as well as in the infected area around New York City, indicate that this disease is capable of wiping out all our native species of elms. There is no known cure for the Dutch elm disease. The only present hope of preserving our elm plantings rests on the eradication of the disease from this country, which present information on the means of its spread indicates may be possible. The accomplishment of this task necessitates immediate action to check the spread of the disease while it is confined to a comparatively small area, and the cost of destroying infected trees is not prohibitive. Another year’s delay will dissipate the only chance of saving the elms, or at least will multiply the cost of an adequate eradication program in the future.
The Dutch elm disease is caused by the parasitic fungus Ceratostomella ulmi (Schwarz) Buisman, which lives and develops in the sapwood of elms. The presence of this parasite in a tree results in the growth of obstructions in water-conducting vessels, first of the branch originally attacked and eventually of the entire tree.
The first external symptom of the disease is the wilting or dying of the foliage of the infected twig or branch, and this may occur as early as 10 days after the part is attacked. Apparently, however, these symptoms may not be in evidence for some time. Field observations in 1934 indicate that the disease does not usually enter a large proportion of the water-conducting vessels of the tree until the spring following infection. Early in the spring the American elm develops a new ring of such vessels. The fungus may cross into this new zone of vessels and may spread with great rapidity to all its parts in the roots as well as in the aerial portions of the tree. The foliage wilts and dies, and finally either the entire tree dies or there may be a temporary recovery as scattered new vessels laid down in the summer wood per- mit partial circulation of water.
Soon after an elm branch or tree begins to die, it may be invaded for breeding purposes by bark beetles and other wood-boring insects. One of these bark beetles, Scolytus multistriatus Marsh., is of European origin, but was reported in the United States as early as 1909. This beetle has been found at various points from northeastern Massachu“setts to southeastern Pennsylvania, and it is well established in most of the infected areas in New York, New Jersey, and Connecticut. This bark beetle has been demonstrated to be an important agent in the spread of the Dutch elm disease in this country. When adults emerge from the bark of a diseased elm, they may carry viable fragments or spores of the fungus in or on their bodies. These adults fly to young twigs of elm, and in feeding on the succulent tissues, especially in the crotches of such twigs, they may inoculate healthy elms with the fungus. As these trees wilt and begin to die, they in turn are entered by bark beetles seeking to establish new broods. Thus the cycle continues, with rapid multiplication of both the beetle population and the number of diseased trees.
Fortunately, the relation of fungus development to beetle infestation is such as to favor eradication of the disease. Bark beetles do not start to breed in a diseased elm until the affected part is so weakened as to show external symptoms. Then 50 to 60 days elapse before the new adults mature, emerge, and spread the disease to other elms. Therefore, by systematically inspecting all elm trees within and near the infected area once a month during the foliage season, when the beetle is active and disease symptoms are readily apparent, and thoroughly destroying all diseased trees as soon as they are found, it appears practicable to prevent the escape of this disease carrier from every infected tree. Once the spread of the disease has been halted, continuation, for a number of years, of systematic inspection of the infected area and prompt destruction of trees in which belated symptoms appear should result in complete elimination of the disease.
Eighteen elms attacked by the Dutch elm disease have been found outside of the main area of infection in the vicinity of New York City. A single diseased tree was discovered in Cincinnati, Ohio, in 1930. In the same year 3 infected trees were found in Cleveland, Ohio, and additional infected trees have since been discovered, 4 in 1931, 1 in 1933, and 2 in 1934. One infected tree was found in Baltimore, Md., in 1933. New isolated infections in 1934 comprise 1 tree in Old Lyme, Conn., 1 tree at Norfolk, Va., and 4 trees in Indianapolis, Ind.
All these isolated infected trees, except the one near Old Lyme, Conn,, are definitely associated with known shipments of burl elm logs from Europe. Such logs are recognized as the means of entry of the Dutch elm disease into the United States. Present information indicates that the infected tree near Old Lyme resulted from the move- ment of domestic diseased material. There is no indication that any of these spot infections have become centers of spread, evidently because of the absence of the European elm bark beetle from these localities and because the diseased trees were immediately destroyed.
Following the finding of a diseased elm in Maplewood, N. J., in June 1933, extensive scouting was carried on in New Jersey, New York, Connecticut, and neighboring States in an attempt to define the limits of the infected area. During the winter and early in the spring of 1934 scouting for diseased trees and their destruction were continued by various recovery agencies. In May and June the rapid development of serious symptoms in elms infected in 1933 or in previous years necessitated a sharp upward revision of the estimated number of trees affected. The total number of diseased trees that had been found by October 24, 1934, in this infection center had reached 7,557, of which 5,032 were in New Jersey, 2,470 in New York, and 55 in Connecticut. All but approximately 1,450 of these trees had been removed by this date.2 One systematic examination of the entire area known to be infected, plus a survey of a safety border arbitrarily established 10 miles beyond the outlying infections found, was completed. A large part of this area was examined a second time, and a relatively small portion was examined three times at intervals of approximately 1 month.
Because at least one winter is required for the majority of infected trees to develop marked external symptoms, at no time does current information based on these symptoms necessarily represent the current status of the disease. Figure 17 represents the principal infected area and the number of diseased elms as known on October 24, 1934, after the completion of the first systematic examination of the known infected area and its environs.
In addition to the known diseased trees still standing, there is in the work area a large accumulation of dead and dying elms, many of which may be harboring the disease. Elimination of these decadent and dead elms is essential to the success of the disease-eradication program. The completion of this clean-up work before the spring of 1935 will permit concentration of location and eradication activities in 1935 on the new crop of dying elms.
The increased knowledge of the Dutch elm disease situation gained during 1934 has furnishe§ a sounder basis for optimism with respect the ultimate eradication of the disease. However, it is recognized that only a thorough, long-term program of adequate proportions can preserve for the future the stately beauty of this unsurpassed shade tree, the American elm.
1. After this article was written the Public Works Administration on the recommendation of the Department allotted $677,000 for combating the Dutch elm disease. Owing to a provision made by Congress, that the regular appropriation will be reduced by an amount equal to any amount that may be allotted for this purpose from Federal emergency appropriations, the amount actually available for combating the Dutch elm disease, including the location and removal of potentially diseased and dying elm trees, is $527,000. Work under this allotment was started early in 1935.
2. Diseased trees found in this infection center up to Apr. 6, 1935, totaled 7,773, of which 5,134 were in New Jersey, 2,383 in New York, and 56 in Connecticut. Only 6 known diseased trees remained standing.