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