Part 8

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.

Symptoms

The characteristic symptoms of the bunch disease are mainly the production of brooms or sucker shoot growth on the tree trunk and main branches and the tufting of terminals, profusion of small branches from axillary buds, the dwarfing and narrowing of the leaflets, and the dying back of the trees resulting sometimes in the death of the trees. The principal symptom is the production during summer of bushy, wiry growth caused by the breaking into growth of lateral buds that normally would remain dormant over the winter. These buds produce shoots that again branch from lateral buds and the process may be repeated for three or four times, resulting in a tightly packed mass or bunch of small, wiry twigs and undersized leaves. Another characteristic symptom is that this growth proliferation continues unabated until the first frost, and, since the wood of these shoots is thus not properly matured, killing back of the diseased portions of the tree usually occurs with the first hard freezes of winter.

As the disease progresses, the wood in the main branches becomes very brittle and is easily broken by wind or ice. This condition is followed by the dying back of branches and finally the death of the tree. Trees even moderately affected soon become worthless for nut production, as few nuts are set and those that mature are usually poorly filled.

Susceptibility of Species

Extended observations show that of the walnut species now grown in eastern United States, the Japanese walnuts, i.e., the Siebold and the heartnut, are by far the most subject to attack by this disease. These walnuts are so susceptible that in localities where this disease is present the planting of young trees is inadvisable, as they are almost certain to be short lived. Once infected, will endanger other walnut trees in the area.

Observations at Beltsville show that the butternut is almost as susceptible to attack as is the Japanese walnut. Some workers are inclined to believe that the rather serious decrease in numbers of butternut trees in some areas is due to the bunch disease. The Persian (English) walnut is also quite susceptible, although probably not so much so as the butternut or the Japanese walnut. The eastern black walnut seems to be the most resistant of all, although some evidence indicates that at least certain trees of this species may have the disease but not show symptoms of it. Gravatt and Stout[17] report that walnut trees may be affected for a considerable length of time without showing recognizable symptoms. Out of a lot of 300 healthy-appearing trees, 37 per cent showed bunch disease symptoms following pruning. Only four percent of the unpruned check trees developed similar symptoms during the same period of time.

Distribution

At the present time bunch disease is quite widespread in eastern United States, occurring in Maryland, District of Columbia, Delaware, New Jersey, New York, Michigan, Ohio, Indiana, Illinois, Pennsylvania, West Virginia, Virginia, North and South Carolina, Georgia, Tennessee, Alabama, and probably other States. No special surveys have been made for bunch disease, and all distribution information has been obtained from observations of U. S. Department of Agriculture or State workers or from specimens submitted.

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.

_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.

4. A total of 156 trees of approximately 36 horticultural varieties has been grown at Beltsville, and only one tree of the variety Graham has shown well developed symptoms of the bunch disease. Two other Graham trees have shown slight or questionable symptoms of the disease.

It should be pointed out that a considerable number of heartnut and butternut trees were planted at random in the same orchards with the black walnut trees used in these experiments and at the same time (1932). In many cases black walnut trees grew within 50 or 100 feet of the heartnut trees. The bunch disease first appeared on heartnut trees, the most susceptible walnut species, and spread quickly to butternut, which is also very susceptible. By 1940 most of the diseased heartnuts had been removed from the orchards, but it was not until after the top-working experiments described above were completed that the orchards were cleared of all diseased trees. It is therefore possible that insect vectors or other agencies may have spread the disease to the scions of the topworked seedlings from the infected heartnut and butternut trees.

Number 795 is the only _J. nigra_ tree on the station farm that has consistently shown symptoms of the disease during the past eight years, and in 1950 only a few limbs are affected. On the basis of the admittedly meager information reported here, it can be stated that the black walnut varieties used in these experiments are more resistant to the bunch disease than are varieties and seedlings of heartnut and butternut. That this is generally true is also borne out by the fact that in the vicinity of Beltsville, Maryland, and the District of Columbia, practically all dooryard trees of the Japanese walnut are infected with bunch disease, many of them having already been killed, whereas relatively few black walnut trees in the area show symptoms of the disease.

The suggestion has been made that most varieties and seedlings of black walnut are symptomless carriers of the disease, and only under certain adverse conditions of environment would symptoms appear. This would explain why trees that are cut back severely, as was the case with tree Number 838 described above, show symptoms on the excessively vigorous shoots of the next year's growth.

Little can be said at the present time about the relative resistance of black walnut varieties to the bunch disease because nothing is known about how it is spread from one individual tree to another. The case histories of trees described in the present paper are considered to be worth recording because they show that black walnut trees may support diseased scions and later regenerate apparently healthy tops. In these cases the trees showed a type of resistance to the disease. However, there are many cases known, the majority of which are seedlings, in which black walnut trees became so badly infected with the disease that nut production ceased and the trees later died. Whether the type of resistance described in this paper is widely prevalent in the black walnut as a species will be impossible to determine until more is known about how the disease is spread.

FOOTNOTES:

[Footnote 13: Several common names have been applied to this disease, among which "bunch" and "brooming" have most frequently been used. The authors strongly feel that the accepted common name should be "bunch" for the following reasons: (1). The term is very descriptive of the symptoms of the disorder. (2). It is the accepted name of a disease of pecan and hickory species that is very similar if not identical to the one occurring on walnut species. (3). The names "brooming" and "witches'-broom" have already been applied to diseases caused by fungi.]

[Footnote 14: Waite, M. B. Notes on Some Nut Diseases with Special Reference to the Black Walnut. Ann. Rept. Northern Nut Growers Assoc. 23:60-67, 1932.]

[Footnote 15: Becker, Gilbert, My Observations on Witches Broom Disease of Black Walnut Trees. Annual Report Northern Nut Growers Assoc. 31:106-109, 1940.]

[Footnote 16: Hutchins, Lee M., and Wester, Horace V. Graft--transmissible Brooming Disease of Walnut (Abstract.) Phytopathology 37: 11, Jan. 1947.]

[Footnote 17: Gravatt, G. F., and Stout, Donald C. Diseases Affecting the Success of Tree Crop Plantings. Ann. Rept. Northern Nut Growers Assoc. 39: 60-68. 1948]

WEDNESDAY MORNING SESSION

A Forester Looks at the Timber Value of Nut Trees

CHARLES S. WALTERS, _Forestry Department, University of Illinois_

What I am going to say will apply mostly to black walnut since it is one of our most valuable timber trees, but it also will apply to other species like hickory, pecan, persimmon. I've never seen papaw or hazel nut large enough for timber, but the Persian walnut has some value and the Chinese chestnut is a fair timber tree. All of these species should be commercially useful if there is sufficient quality and volume involved to warrant a sale.

What I have to say may not apply five years from now. Persimmon used to be the main source of material for golf club heads and shuttles for the textile industry. It no longer is.

Today golf club heads are being made of "Compreg," a wood which has been impregnated with phenolic resins and cured with heat. The resin is similar to Bakelite. Thin sheets of wood are glued together to build up the head, rather than using a single solid piece, and it makes a considerably better golf club head. The developments in wood use are progressing just as in many other fields. What the wood specialists are trying to do is to take low quality material and change it over to a form which is suitable for many uses for which high-quality expensive material is now used. The timber buyer now wants a tree of long, clean, bole with few knots, of large size,--at least 16 inches in diameter at breast height. In short, he wants high quality material.

What I am saying may not apply to nut growing. Foresters grow trees for the wood crop, with nuts as a by-product. The first 16 feet of trunk or the butt log is his main interest. It should be completely free of limbs, knots, and other defects for at least 16 feet. You can use the logs above the butt-cut but they usually produce lower grade material.

You have two courses to follow. You can grow wood either in natural stands or in plantations, and the end product is very little different. It is probably easier to grow a high quality tree in a plantation than in the wild. What can be easier than growing a timber tree in the woodlands? It eventually reaches merchantable size and is harvested. Well, nature can do better if you give her help. Your chances of growing a high quality tree to merchantable size are better in the plantation.

About ten years ago Dr. R. W. Lorenz of our Department made a study of 150 plantations growing on prairie soil in Illinois. Thirty-six were walnut which ranged in age from 22 to 75 years. The one thing we had the most trouble with was determining their ages. One day we stopped at a farm and talked to a farmer, and we asked him when the trees were planted. This man said he could tell us the exact day. "I was a young lad and a neighbor drove by and said, 'Yesterday Abe Lincoln was shot.'" So we had the historical records to determine the age of that particular plantation.

These plantations ranged in number of trees per acre from 46 to 330. The number of trees per acre has a direct influence on the size or diameter growth of the timber tree. An eight by eight spacing, or 680 trees per acre, eventually will be thinned to 200 trees per acre. That gives each tree proper spacing for best height and diameter growth.

The trees ranged in height from about 31 feet to 85, averaging about a foot and a quarter in height each year. The average diameters ranged from about 12 inches to 15 inches. Individual trees, however, ranged up to 24 inches at breast height (4-1/2' above ground level). Each plantation had had very little or no care. If some of them had been cared for, or "managed", their owners would have had a better wood crop--higher quality and higher quantity too.

Now, as to the growth in the managed plantations. We believe it is possible to grow 300 board feet per acre per year. Compared with upland oak, walnut exceeded it in almost all growth factors up to 70 years of age and then they were about the same.