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.