For the past 20 years the bunch disease of walnuts has been under observation by the writers and it seems clear that its incidence has increased greatly during that time. In 1935 scions and buds were taken from diseased eastern black walnut and butternut trees growing at Arlington Farm and grafted or budded on eastern black walnut stock growing in the original nut tree nursery at the Plant Industry Station at Beltsville, Maryland. This was done in an attempt to determine whether the disease was caused by a mineral deficiency or by a virus. All buds and scions died, but the following year two of the seedling rootstocks showed characteristic symptoms of the bunch disease. Since this disease was already present on the station farm it was not definitely known that it was transmitted to the stocks by budding or grafting the diseased material on them.
In December of 1946 Hutchins and Wester[16] presented a paper before the American Phytopathology Society giving the results of their studies on the bunch disease. In this paper they reported that the disease was transmitted by patch bark grafts performed in 1944 and 1945 and that the incubation period varied from several months to two years. It was concluded that since the disease was transmitted by grafting, and in the absence of a visible pathogen, a virus causal agent was indicated.
Symptoms
The characteristic symptoms of the bunch disease are mainly the production of brooms or sucker shoot growth on the tree trunk and main branches and the tufting of terminals, profusion of small branches from axillary buds, the dwarfing and narrowing of the leaflets, and the dying back of the trees resulting sometimes in the death of the trees. The principal symptom is the production during summer of bushy, wiry growth caused by the breaking into growth of lateral buds that normally would remain dormant over the winter. These buds produce shoots that again branch from lateral buds and the process may be repeated for three or four times, resulting in a tightly packed mass or bunch of small, wiry twigs and undersized leaves. Another characteristic symptom is that this growth proliferation continues unabated until the first frost, and, since the wood of these shoots is thus not properly matured, killing back of the diseased portions of the tree usually occurs with the first hard freezes of winter.
As the disease progresses, the wood in the main branches becomes very brittle and is easily broken by wind or ice. This condition is followed by the dying back of branches and finally the death of the tree. Trees even moderately affected soon become worthless for nut production, as few nuts are set and those that mature are usually poorly filled.
Susceptibility of Species
Extended observations show that of the walnut species now grown in eastern United States, the Japanese walnuts, i.e., the Siebold and the heartnut, are by far the most subject to attack by this disease. These walnuts are so susceptible that in localities where this disease is present the planting of young trees is inadvisable, as they are almost certain to be short lived. Once infected, will endanger other walnut trees in the area.
Observations at Beltsville show that the butternut is almost as susceptible to attack as is the Japanese walnut. Some workers are inclined to believe that the rather serious decrease in numbers of butternut trees in some areas is due to the bunch disease. The Persian (English) walnut is also quite susceptible, although probably not so much so as the butternut or the Japanese walnut. The eastern black walnut seems to be the most resistant of all, although some evidence indicates that at least certain trees of this species may have the disease but not show symptoms of it. Gravatt and Stout[17] report that walnut trees may be affected for a considerable length of time without showing recognizable symptoms. Out of a lot of 300 healthy-appearing trees, 37 per cent showed bunch disease symptoms following pruning. Only four percent of the unpruned check trees developed similar symptoms during the same period of time.
Distribution
At the present time bunch disease is quite widespread in eastern United States, occurring in Maryland, District of Columbia, Delaware, New Jersey, New York, Michigan, Ohio, Indiana, Illinois, Pennsylvania, West Virginia, Virginia, North and South Carolina, Georgia, Tennessee, Alabama, and probably other States. No special surveys have been made for bunch disease, and all distribution information has been obtained from observations of U. S. Department of Agriculture or State workers or from specimens submitted.