Case Histories at Beltsville
As a part of walnut breeding work carried on during the past 14 years, approximately 20 large nigra trees of named horticultural varieties have been topworked to seedlings of natural first-generation hybrids between J. regia and J. nigra for the purpose of forcing the seedling scions into early fruiting. Of these 20 trees, 3 have shown such unusual behavior as to merit a description of each in the form of a case history.
Tree Number 838. This tree was cut back severely in the spring of 1942, and on August 26, 1943 vigorous new shoots were budded to 47.11-P17, a second-generation seedling of the O'Conner natural hybrid. The buds grew vigorously in 1944 and early in the season developed symptoms of the bunch disease. By the end of the growing season of 1944 the scion limbs were heavy with the typical proliferated shoots characteristic of the disease. Also, a few vigorous sucker limbs of the stock tree that grew out from below the point of union of the scions showed typical symptoms of the disease, although these limbs were later outgrown by normal shoots and are not now to be seen. In the early spring of 1945 the diseased limbs were all removed from the tree to prevent the further spread of the disease in the area.
At the same time that the above seedling was budded in the top of this tree, a large lateral limb of the stock tree was budded to seedling number 40.70-P1. This seedling originated from a nut of the Ohio variety of black walnut that was only about 1/4 the size of nuts typical of the variety. At the time it was thought that this nut resulted from a cross of Ohio with pollen of the Persian walnut, as it was produced under bag and following hand-pollination. Later growth of the seedling indicated, however, that the pistillate flower was probably pollinated by J. nigra before the bagging occurred, since only J. nigra characteristics have shown up in the seedling. In 1950, one bud of the nigra seedling 40.70-P1 has almost completely regenerated the top of the tree and no symptom of the disease is evident. By contrast in 1944, almost all of the top of the tree was occupied by diseased limbs, five in number, of the O'Conner seedling.
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.