History of the Disease
The bunch disease of walnut has been known for years. Waite[14] in 1932 said, "It turned up in Delaware several years ago, where quite a variety of walnuts, including the Persian, the Japanese Group, and the American Black Walnut, were found to be affected. At Arlington Farm, Virginia, during the past 15 years it has boldly riddled the collection of nut trees assembled in the grounds for study and ornamental purposes." Photographs made in 1914 of Japanese walnut trees growing in Georgia and thought to be affected by rosette (now known to be caused by zinc deficiency) have been found in the files of the U. S. Department of Agriculture. Now that the symptoms of the two different disorders are known, it seems clear that the bunch disease was present in those two states at that early date.
Becker,[15] of Climax, Michigan in 1940 reported on his observation of this disease in that area. He reports that he observed several cases of it on Persian walnut, Japanese walnut, and butternut, in addition to many diseased eastern black walnuts. He says, "My conclusions are that in witches'-broom (bunch disease) we have a very bad disease that threatens the black walnut trees everywhere".
In 1939, the late Howard E. Parsons, pathologist of the U. S. Department of Agriculture, made an inspection trip to Climax and other areas in Michigan where he studied and photographed diseased trees. Parsons at that time was working on a similar disease of pecan and
water hickory and was of the opinion that the disease found on the various species of walnuts in Michigan was similar to the one he was studying.
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.