+Description of Symptoms+
The entire range of symptoms of the brooming disease has not been determined. Symptoms are recognizable during mid-July but they are most pronounced during September and October. Curling and cupping of leaflets, chlorosis, narrowing and basal tapering of leaflets appear to be associated with early stages of the disease. On severely affected trees there are distinct broomlike growths at branch terminals, along primary or secondary branches, or on the main stem to the ground line (Fig. 2). The broomlike growths are formed by the continuing abnormal development of normally located buds into short, succulent branches. Upright, suckerlike branches appear on primary and secondary branches and on the main stem of the affected tree. (Fig. 3).
The broomed parts usually die back during the dormant period following their appearance. The dead brooms on trees that appear to be healthy during the early months of the growing-season indicate that the trees are infected. Usually the diseased trees, even those severely affected, exhibit normal growth during the early summer months.
Evidence that walnut trees may be infected for a considerable time prior to appearance of recognizable symptoms was obtained when 37 per cent of a total of 300 severely pruned trees exhibited brooming disease symptoms. These trees had looked healthy until they were pruned. Unpruned control trees showed a 4 per cent increase in disease during the same period.
[Footnote 12: Hutchins, Lee M., and Horace V. Wester. Graft-transmissible brooming disease of walnut. Phytopathology. 37 (1): 11. (Abstract) 1947.]
+Summary+
There is strong evidence that a virus disease is active among certain species of walnut in central and eastern United States. The disease exhibits distinctive symptoms and appears to damage infected trees, sometimes severely, over several growing seasons. Present data indicate that recognizable symptoms of the disease may not appear for some time after infection, unless the host is subjected to severe shock. Thus, nursery stock may be one means of spreading the disease into new areas. It is recommended, without experimental work to back up the recommendation, that walnut nurserymen remove infected trees in the vicinity of their nursery sites.
Investigation of this disease to the present time has been limited. General observations indicate that severely broomed trees produce poor nut crops. Mortality caused by the disease appears to be quite low among black walnut trees. Butternut and Japanese walnut trees are, in general, more severely affected by the disease than the black walnut and many seem to be killed by it, although the killing process is slow. As a result of experience with other virus diseases, orchardists who have only a few infected trees among their black walnuts are advised to remove them. Whether the disease can be kept under control by repeated roguing is uncertain. If an owner has just a few trees of value as ornamentals as well as nut producers, one hesitates to advise him to remove a lightly infected tree until more information is obtained concerning the disease.
This Division will welcome information from persons having experience with the brooming disease of walnut, as it is in a position to do only a limited amount of work on the disease.
+Persimmon Wilt+