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.