WALNUT OBSERVATIONS IN CALIFORNIA[3]

WALNUT OBSERVATIONS IN CALIFORNIA[3]

L. D. Batchelor, University of California, Citrus Experiment Station, Riverside, California.

The walnut industry of California is just entering a transition period from the planting of seedling groves to the established plantings of grafted trees. Just as other seedling fruit trees, such as the orange, apple, peach, almond, etc., have been eliminated, so too, the seedling walnut groves of California seem doomed to be replaced by clonal varieties. In many ways this industry is as much in its infancy as the apple industry of New York was sixty-five years ago, when varieties first began to be propagated in a commercial way by grafting and budding. This readjustment in the walnut industry is well started, and, although it is likely to be gradual in its evolution, and wisely so, the change seems nevertheless certain. There are but a very few seedling trees for sale at the present time by the progressive nurseries, and, in fact, only a very few such trees have been set out in groves during the past four or five years. The demand for grafted trees has been brought about largely by the wide range of variation in walnut seedlings as regards their productivity, commercial value of the nuts, season of harvest and ability of the trees to resist the walnut blight.

In view of the very recent propagation of the walnut by grafting, which has extended over only about ten to twelve years, it is reasonable to expect that the majority of the varieties thus propagated so early in the development of the industry are only partially suited to the needs of the walnut grower. The nuts from many of these grafted varieties fall considerably short of the commercial standard for high-grade walnuts. Some of the heaviest-bearing sorts, such as the Chase, Prolific and El Monte, produce nuts that cannot be sold in the very best grade of the commercial product. On the other hand, the Placentia, which produces one of the most nearly ideal commercial nuts, is not a heavy-producing variety, especially in the northern walnut sections, and is quite as susceptible to walnut blight as the average seedling tree. Again, the Eureka variety, which seems to successfully avoid the walnut blight during many seasons by its lateness in coming into bloom, is a very moderately yielding variety in the southern sections. The above examples are only a few of many that might be cited to show the short-comings of most of the varieties of walnuts now being propagated.

The wide range of climatic and soil conditions makes the eventual propagation of quite a large number of varieties inevitable. While the coast regions are bathed in fog nearly every morning during the growing season, the inland valleys experience an extremely dry climate with high maximum temperatures. Walnuts are being grown at the present time on soil types varying from the extremes of sand to heavy clay loams. Many of the future varieties must be especially adapted to some one of these particular environments if they are to stand the test of time.

Many of the present seedling groves are of uncertain origin and represent greatly varying values. No doubt some of these groves are the progeny of especially selected trees known to have considerable merit. On the other hand, it is very apparent that many of them are the result of a great demand for seedling trees when the industry was in its infancy twenty or thirty years ago. At that time without doubt, great quantities of walnuts were planted without due regard for their parentage. Again, there is a wide range of variability among the individual trees of any grove, as variations in type of tree, blooming season, character of foliage, resistance to disease, productivity and character of the nuts.

Type of Tree

The tree types vary from the upright, sturdy individual to the more or less spreading, weeping types which droop nearly to the ground under the burden of the crop. The upright, vigorous growing type is well exemplified in the Eureka. On the other hand, such varieties as the Prolific have a spreading, bushy habit and an almost semi-dwarfness characterizes their growth.

Blooming Season

It is not unusual to find the blooming season in an ordinary seedling grove extending over a period of from a month to six weeks. A few individual trees leaf out and blossom with the first signs of spring. Then the great majority of the trees in the grove come out in full leaf. But there are frequently trees still leafless after the nuts on the early individuals are of the size of a marble. This variation in the blooming season has considerable economic importance in relation to the harvesting and marketing of the nuts as well as the avoidance of diseases and frost which may be more prevalent during certain periods in the spring.

Foliage Characteristics

The character of the foliage varies from the broad-leaved types, whose foliage somewhat resembles that of the horse-chestnut, to the narrow-leaved varieties whose leaves have a tendency to curl up like the foliage of the Winesap apple. The broad-leaved types are much more densely foliated and this factor has considerable bearing on the problems of sun-scald on the twigs and trunks of the tree and the exposure of the nuts to this injury. For this reason, the densely foliated varieties may prove best adapted to the inland valleys, where the difficulties of sun-scald are most prevalent. The more sparsely foliated types often appear to have less blight on the nuts and leaves because of their exposure to the sunshine.

Disease Resistance

Probably one of the most important limiting factors in walnut production in California, and especially in the older walnut sections, is the bacterial disease commonly known as walnut blight. The inroads of this disease have caused a very heavy dropping of the nuts during many seasons of the past, and although a great deal of time and scientific effort has been devoted to the control of the trouble, there is no satisfactory known means for the prevention of walnut blight at the present time.

It is a well-known fact that in the vegetable kingdom closely related species suffer in different degrees from the attacks of the same parasite. This difference in resistance is often as marked among different varieties of the same species as between the species themselves. The absence of blight is not necessarily an indication of immunity. There is a great deal of difference in the amount of blight prevalent at the present season in the different walnut growing sections. Again, the immunity from blight of a particular tree for one season may be followed by more or less prevalency of blight on the same tree the next season. The degree of resistance must be tested out through a number of years before any variety can be pronounced resistant to this disease. The observations must also be carried out in different localities as certain varieties seem to behave differently on different soils and when growing under different climatic conditions.

Some varieties seem to avoid the blight the majority of the seasons but really have little or no resistant qualities when the seasonal conditions and the growth of the plant happen to coincide with the most favorable time for the spread of the disease. An example of this is seen in the Eureka variety the present season. While this variety has maintained a reputation during a majority of seasons for freedom from blight, during the present year the Eureka is badly diseased in certain sections of Orange County. This may, perhaps, be explained by the prevalence of damp, cloudy weather for about a week or ten days during the first of May when this variety was in full bloom. In one grove under observation the trees were thought to have lost at least 50 per cent of their blossoms soon after blooming. At the present time on these same trees, 32 per cent of the nuts are afflicted with more or less blight. To be sure, some of these will likely mature, but the appearance of blight on nearly one third of the crop shows that this variety has very little resistant power against walnut blight. Its freedom from disease in the past has no doubt been due largely to its dormancy during the most favorable weather conditions for the spread of blight.

The field for the selection of blight resistant varieties must necessarily be in the badly blighted sections. A tree with only 10 per cent blighted nuts in an orchard having an average of 70 per cent to 80 per cent may really be more resistant to blight than a variety which appears to be positively free from the disease when growing among trees which are only 15 per cent to 20 per cent blighted. In making observations and selections, therefore, it is quite as important to know the amount of blight on the surrounding trees and the grove, as a whole, as it is to know the prevalence of blight on the selected individual. The extreme variation of different seedling trees in their susceptibility to this disease is well illustrated in some of the following observations which were made the present year. The percentages which follow the varieties named were determined by counting at least 100 nuts on a tree just before the blighted nuts began to drop. In a seedling grove in the Whittier district about 300 trees were examined and 100 nuts counted on each tree. The individual trees varied from 2 per cent to 85 per cent blighted nuts, while the grove as a whole averaged 25 per cent. There were at least a dozen or fifteen trees in this grove which were blighted less than 10 per cent, although some of the nearby trees were blighted as high as 60 per cent or 70 per cent.

Another seedling grove in Orange County which was counted in the same way, averaged 47 per cent blighted nuts during the second week in June. In making this determination 105 trees were examined. In this same grove, there were, however, at least three trees which averaged less than 6 per cent blighted nuts.

It is interesting to know that the Placentia variety, growing within a stone's throw of the aforementioned seedling grove and under identical cultural conditions, was blighted to the extent of 71.9 per cent on the same date.

Observations of the Prolific (Ware's) in the vicinity of the above mentioned grove, showed less than 1 per cent blighted nuts on the trees and practically none of the nuts have dropped to the ground at the present time, yet in the past this variety has not had a reputation for disease immunity. The original Chase tree was observed during this time and showed a percentage of 37 per cent blighted nuts. These examples are given neither in support of any particular variety nor to discredit others, but are noted merely to call attention to the wide variation, and this variation is a great source of encouragement in our endeavors to produce a disease resistant variety.

Of course blight immunity is not the only factor to be considered in selecting a variety of walnut. A profitable yield of good commercial nuts is the real test of the superiority of any variety. A very heavy yielding tree with a small amount of blight may prove more profitable than a light yielding variety that is totally immune to this disease.

The production of a medium grade nut which would grade only as a seedling No 1, might prove more profitable if the tree is at least partially blight immune than the production of such a high grade nut as the Placentia with its susceptibility to blight. These things must be considered and weighed carefully by the growers who are planting walnuts in the blight sections. The various areas where walnut blight is not a factor might profitably sacrifice heavy production to superior quality.

From our present knowledge it is very apparent that the disease resistance of individual trees varies considerably from year to year and under different soil and climatic conditions. The thorough testing of resistant varieties will require considerable time.

Nut Characteristics

The character of the nuts is as variable as the trees themselves, not only in the exterior appearance, but in the character of the meats as well. The ideal commercial nut should be of medium size, about one and one-eighth to one and one-half inches in diameter, of regular oval form somewhat elongated, with smooth surface, and light brown color, and uniform for these characters. The cracking quality of the nuts is quite as important as their exterior appearance. The nuts should be well sealed so they will not crack open in shipping. The shells should be thin but strong, so the nut may be easily opened and the whole meat taken out intact. The pellicle surrounding the kernel should be light tan colored or silvery brown with a glossy waxed appearance attractive to look upon. The meat should be smooth, and plump, averaging 50 per cent or more of the total weight of the nut, and with a mild, pleasant flavor, free from any astringency.

The shells vary all the way from extremely rough and unattractive specimens to the smooth commercial type, as the Placentia, while the color of the meats varies from dark brown to nearly white, and so on through the other characteristics mentioned.

In the selection of varieties the walnut breeder is exceptionally favored by the occurrence of large areas of seedling trees. According to the 1910 census there were in the neighborhood of one and a quarter million seedling trees growing in California. With this almost unlimited material for selective use, it seems indeed reasonable that many varieties will be selected in the future which are better adapted to the demands of the industry than some of those now being propagated. By means of hybridizing methods it is also hoped that some of the desirable unit characters of the varieties now in cultivation may be recombined into more nearly ideal varieties for future generations. The fact that walnut breeding is necessarily a long-termed, expensive problem has made it rather unattractive to the practical breeders. Such work will depend largely upon public or specially endowed institutions for its support.