rooming Disease of Walnut+

A systemic brooming disease, observed on planted walnuts as early as
1917, has been the subject of considerable discussion during recent
years, because it has now spread widely into the native black walnut
growth. In 1932 Waite published that he had been observing the disease
for some 15 years but that "it was unknown on the black walnut in the
wild in this country or on planted trees away from the Japanese walnut."
The disease has continued to increase in prevalence in recent years and
is now widely distributed in native black walnut growth in Tennessee,
Virginia, District of Columbia, Maryland, Delaware, and New Jersey. This
extensive spread into the native growth during the last 15 or 20 years
and the fact that reports indicate that all of the early cases of the
disease were found near nursery-grown trees offer some evidence that the
disease is an importation from another area or continent into the
eastern black walnut zone. From the literature and oral reports, it
seems that the disease is now present also in North Carolina, West
Virginia, Pennsylvania, New York, Ohio, and Michigan. Surveys probably
would uncover the disease among native wild and planted walnuts in other
States.

[Footnote 11: Crandall, B. S., G. F. Gravatt, and M. M. Ryan. Root
diseases of Castanea species and some coniferous and broadleaf nursery
stocks, caused by Phytophthora cinnamomi. Phytopathology 35: 162-180.
Illus. 1945.]

+Economic Importance and Hosts+

The black walnut is a valuable native forest tree, widely but not
abundantly distributed in the eastern United States. It is extensively
planted as a forest tree. The numerous plantings and natural stands
around farm homes, along fences, and in pastures are also very valuable.
More and more grafted ornamentals, and orchards of black walnut are
being planted. For these the per-tree investment is high.

[Illustration: Figure 2.--The brooming disease of walnut. Severe
brooming on Japanese walnut.]

The ultimate effect of the brooming disease on the black walnut is not
known. Dr. Waite stated, "Trees even moderately attacked soon become
worthless for nut production." Some affected black walnut trees,
however, continue to produce small crops of nuts. Visible symptoms have
been known to disappear. In addition, some seedlings, and probably large
trees also, are infected without showing symptoms. Such observations
indicate the complex nature of the disease. Det