368 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION. 



freer from blight than those which open their buds earlier in the 

 season. The reason for this difference was easily apparent, being con- 

 nected with conditions of atmospheric moisture. Trees coming out late 

 escape the spring rains and fogs, which period is by far the worst for 

 blight infection. Some observations seem to indicate that trees com- 

 ing out unusually early in the spring are also freer from blight than 

 the average, and there appears to be some truth in this fact. If this 

 is correct, it is probable that these early trees escape blight infection 

 owing to the lower temperature which prevails at the time when their 

 shoots and nuts are young and easily susceptible. We have shown 

 elsewhere that the blight organism requires a fairly high temperature 

 for its most vigorous development. These early and late blight-free 

 trees really escape infection therefore, rather than actually resisting 

 it, or rather they escape it at the time when they are most susceptible 

 and then are able to resist any later infection on account of the maturity 

 of their tissues. It is true, however, that certain individual trees have 

 some actual resistance to the disease, even at susceptible periods, and 

 those having such immunity are of course the most desirable type. 



In seeking blight-resistant or immune walnut trees there must at the 

 same time be kept in mind the fact that freedom from blight is not 

 the only quality necessary in the walnut, since we must also have a 

 tree of heavy production, with nuts of desirable size, form, color, full- 

 ness of meat, flavor and other qualities which go to make up an ideal 

 walnut. The strongest emphasis should also be laid on the fact that 

 the performance of a single tree in a given locality, either as to blight, 

 bearing qualities, or character of the nuts does not justify its exploita- 

 tion for all parts of California under the widely different conditions 

 which exist in different localities. Freedom from blight in a certain 

 tree or variety in a given locality may be due simply to the fact that 

 the disease is not present or conditions not favorable to its development 

 in that particular place. When planted in another locality the same 

 variety may prove to be one of the very worst affected by blight. Again, 

 a given variety may produce very heavily in one locality under certain 

 climatic conditions and not in another. Again, a tree may be free from 

 blight and produce a very large crop of nuts one year or even for several 

 years, and then later on develop the disease very badly and produce 

 only a very few good nuts. Again, a tree or variety may be actually 

 immune to blight to a large extent and still be a very small producer 

 or produce nuts of an undesirable type in size, form, color, flavor, or 

 some other characteristic. Still again, a variety may have only slight 

 immunity to blight and yet have so much vigor of growth and tendency 

 toward heavy production that the resulting crop, in spite of the blight, 

 will be much larger than that of a more resistant but less productive 

 tree. Still further, one variety may produce very heavy crops of nuts 



