4. A total of 156 trees of approximately 36 horticultural varieties has been grown at Beltsville, and only one tree of the variety Graham has shown well developed symptoms of the bunch disease. Two other Graham trees have shown slight or questionable symptoms of the disease.
It should be pointed out that a considerable number of heartnut and butternut trees were planted at random in the same orchards with the black walnut trees used in these experiments and at the same time (1932). In many cases black walnut trees grew within 50 or 100 feet of the heartnut trees. The bunch disease first appeared on heartnut trees, the most susceptible walnut species, and spread quickly to butternut, which is also very susceptible. By 1940 most of the diseased heartnuts had been removed from the orchards, but it was not until after the top-working experiments described above were completed that the orchards were cleared of all diseased trees. It is therefore possible that insect vectors or other agencies may have spread the disease to the scions of the topworked seedlings from the infected heartnut and butternut trees.
Number 795 is the only J. nigra tree on the station farm that has consistently shown symptoms of the disease during the past eight years, and in 1950 only a few limbs are affected. On the basis of the admittedly meager information reported here, it can be stated that the black walnut varieties used in these experiments are more resistant to the bunch disease than are varieties and seedlings of heartnut and butternut. That this is generally true is also borne out by the fact that in the vicinity of Beltsville, Maryland, and the District of Columbia, practically all dooryard trees of the Japanese walnut are infected with bunch disease, many of them having already been killed, whereas relatively few black walnut trees in the area show symptoms of the disease.
The suggestion has been made that most varieties and seedlings of black walnut are symptomless carriers of the disease, and only under certain adverse conditions of environment would symptoms appear. This would explain why trees that are cut back severely, as was the case with tree Number 838 described above, show symptoms on the excessively vigorous shoots of the next year's growth.
Little can be said at the present time about the relative resistance of black walnut varieties to the bunch disease because nothing is known about how it is spread from one individual tree to another. The case histories of trees described in the present paper are considered to be worth recording because they show that black walnut trees may support diseased scions and later regenerate apparently healthy tops. In these cases the trees showed a type of resistance to the disease. However, there are many cases known, the majority of which are seedlings, in which black walnut trees became so badly infected with the disease that nut production ceased and the trees later died. Whether the type of resistance described in this paper is widely prevalent in the black walnut as a species will be impossible to determine until more is known about how the disease is spread.
FOOTNOTES:
[13] Several common names have been applied to this disease, among which "bunch" and "brooming" have most frequently been used. The authors strongly feel that the accepted common name should be "bunch" for the following reasons: (1). The term is very descriptive of the symptoms of the disorder. (2). It is the accepted name of a disease of pecan and hickory species that is very similar if not identical to the one occurring on walnut species. (3). The names "brooming" and "witches'-broom" have already been applied to diseases caused by fungi.
[14] Waite, M. B. Notes on Some Nut Diseases with Special Reference to the Black Walnut. Ann. Rept. Northern Nut Growers Assoc. 23:60-67, 1932.
[15] Becker, Gilbert, My Observations on Witches Broom Disease of Black Walnut Trees. Annual Report Northern Nut Growers Assoc. 31:106-109, 1940.
[16] Hutchins, Lee M., and Wester, Horace V. Graft—transmissible Brooming Disease of Walnut (Abstract.) Phytopathology 37: 11, Jan. 1947.