Tree Number 854. This tree has shown behavior almost identical with that of Number 838, but three seedlings were topworked instead of one. All three originated from the Coye hybrid and all were budded on July 27, 1944. Less than one month later all buds had produced a foot or more of growth, and one to two scions of each seedling reached sufficient size and vigor to survive the following winter without damage. None of the scions branched in 1944, and all failed to show symptoms of the disease. Early in 1945 profuse branching occurred on the one surviving scion of seedling number 39.03-P2, and by midsummer excessive proliferation of the buds of primary shoots had resulted in the formation of a mistletoe-like growth characteristic of the disease. Scions of the two other seedlings, 39.03-P8 and 39.03-P11, were lost by wind damage in midsummer, but at the time they showed no signs of the disease. Most of the shoots of 39.03-P2 were killed during the following winter, and in April, 1946, the remaining live portions were removed by the Division of Forest Pathology for use in transmission studies.
On August 18, 1944, four patch buds of the O'Conner natural hybrid were placed on one of the main limbs of this tree. One of these buds grew, and in 1950 has come to occupy more than half the top of the tree. The remainder of the top is made up of the original stock tree. There is no evidence of bunching in the tree at present.
Tree Number 411. This tree was budded to six seedlings of the Fox natural hybrid on April 28, 1943. Only one of these lived, 40.45-P4, and one scion of this seedling in 1950 comprises the entire crown. No symptom of the disease has appeared in this scion, and the tree is healthy at present.
On April 8, 1944, small lateral limbs of the tree were splice-grafted to two Coye seedlings, 39.03-P8 and 41.26-P10. One scions of each grew vigorously during the summer, and 41.26-P10 first became chlorotic, then diseased. Seedling 39.03-P8 became chlorotic but at the end of the season had not shown symptoms of the disease. Both were removed from the tree early in 1945 and the living shoots used for scionwood in transmission studies by the Division of Forest Pathology.
An additional case is Tree Number 795. This is a grafted tree of the Graham variety of black walnut that was planted in 1932 within 100 feet of trees of the Bates and Faust varieties of heartnuts. By 1940 the latter trees were heavily infected with bunch disease, but it was not until 1943 or 1944 that symptoms were discovered in the Graham tree. At this time the heartnuts were removed from the orchard. The Graham tree has shown only a few small diseased limbs during the past six or seven years, and in 1950 a fair crop of nuts is in prospect.
Discussion
The following observations should be mentioned briefly before discussing the questions raised by the case histories:
1. Out of more than one hundred seedling scions from 13 hybrids topworked on large nigra trees, three have become diseased the first or second year after the scions began to grow on black walnut stock.
2. The three susceptible seedlings have all been grafted on different nigra stock trees, and the three stock trees have since regenerated only healthy limbs, after removal of the diseased shoots.
3. Seedlings from a total of 13 natural hybrids between J. nigra and J. regia have been used, and only two of these hybrids have yielded susceptible seedlings. However, only a few seedlings were available from certain hybrids.