BULLETIN 231] WALNUT CULTURE IN CALIFORNIA. 323 



at present only troubled with this disease to a limited extent. In 

 northern California and Oregon there are few commercial bearing 

 groves at the present time. This disease shows the greatest variation in 

 the amount of infection from year to year, and while it may seem to be 

 scattered throughout a grove, yet there are trees here and there that 

 are almost free from attack. Some years there is almost no blight or 

 only a small amount of early infection. There is less infection in a 

 bright spring that is free from fogs and cloudy days. 



The disease is now distributed all over California, being often found 

 even upon isolated walnut trees situated a long distance from any others 

 of the same kind. It has no doubt been generally disseminated through 

 nursery stock, which is often infected, and may also be carried through 

 the air. Localities with much foggy or moist weather during the spring 

 show the most severe attacks of blight, but in places where the crop is 

 largely grown, as in the southern part of the State, the disease becomes 

 more conspicuous, although it is really more prevalent and virulent in 

 some other parts of the State where the walnut crop is less important. 

 The occurrence of the disease varies everywhere in direct proportion to 

 the amount of atmospheric moisture during the spring and early sum- 

 mer months, from the time the trees start into growth until the nuts 

 are well advanced in development. The amount of blight likely to occur 

 in a given locality or given season can be closely estimated on this basis, 

 as the connection between the occurrence of the disease and the amount 

 of atmospheric moisture is well established. Localities with much spring 

 and summer fog may expect a considerable development of walnut 

 blight, while those where such weather does not prevail may safely count 

 upon being reasonably free from this trouble, even though it be fre- 

 quently introduced. The same rule applies as to the occurrence of the 

 disease in different seasons. 



Losses from Blight. The amount of loss actually caused by this dis- 

 ease has been every year very large for the past ten years or more, but 

 has varied considerably from year to year. It is also true that much 

 has been attributed to blight which in reality had nothing to do with 

 this disease, such as the effects of drouth, frost and other unfavorable 

 climatic or soil conditions. At the same time the losses directly attribu- 

 table to blight have been extremely large. One significant fact in this 

 connection is that while the walnut acreage in southern California has 

 multiplied many times during the past decade, the total walnut crop of 

 the State has increased very little during this time. In fact, the crop 

 has been actually much less during several years of this period than it 

 was in some of the best years of the preceding decade. This loss or 

 failure of the crop to increase has not been entirely attributable to 

 blight, yet it has certainly been due to the disease much more than to 



