Damage Caused
Trees with bunch disease may live for several years in a stag-horned or tufted condition. Affected trees generally set few nuts and the nuts that mature are usually poorly filled and hence low in oil content. It is likely that a part of the unsatisfactory growth and fruiting performance of certain eastern black walnut trees may be due to the disease, even though they do not show the symptoms as they are now known. Severely affected trees are subject to cold injury, and in addition the wood becomes very brittle and is easily broken by storms. Although this disease has been known for several years, it is believed that its seriousness has not been fully appreciated, as it does not cause death as soon as symptoms appear. Several years must elapse before the tree succumbs. In the nut tree plantings made at the Plant Industry Station at Beltsville, Maryland, large numbers of butternut, Japanese walnut, and Persian walnut trees were planted. During the following years, although no records have been kept, several hundred of these trees have become affected and have been removed. Consequently at the present time we do not have any butternut or Japanese walnut trees, and only a few Persian (English) walnut trees left in the plantings. So far, not a single eastern black walnut tree has been removed from the orchards because of the bunch disease. Some trees have shown characteristic symptoms of the disease, but following the removal of the entire diseased limbs the symptoms have not reappeared.
Possible Effects of Bunch Disease on the Walnut Industry
This disease is known to spread to nearby healthy walnut trees, but the means by which it is spread or how infection occurs is not known. No survey has been made to determine whether the disease is present in the various regions in which walnut trees are grown, and hence it is not known how widely it is distributed at present. Its spread is probably associated with an insect vector, and the presence of the vector would determine whether or not local spread would occur. Much more must be learned about this disease before its importance and destructive nature can be fully determined. It seems certain that in localities where the disease is already present there is little use in planting young trees of the most susceptible species unless trees in the vicinity that are already diseased are destroyed. Nurserymen growing trees of the Japanese walnut, butternut, and Persian walnut should be sure that no diseased trees which might infect the nursery trees are close to their nurseries. It is not known how far the inoculum may be carried, but at this time it would seem that in order to be reasonably safe no diseased tree should be allowed to grow within a mile radius of a nursery. Infected nursery trees (or scions) probably constitute the most important means of long-distance spread for a disease of this type.
Control
The only known method of control of the bunch disease is to prevent healthy trees from becoming infected. This can be done only by destroying completely all diseased trees. In the early stage of the disease, sometimes only one branch on a tree may show symptoms; and complete removal of this branch may result in the tree's not showing additional symptoms for a year or more. Except in the case of black walnut, the disease breaks out again; hence cutting out diseased limbs cannot be considered a satisfactory control measure, except possibly on the eastern black walnut.
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.