BULLETIN 231] 



WALNUT CULTURE IN CALIFORNIA. 



337 



branches and nuts. From our observation, the young leaves seem to be 

 infected very early and probably are one of the chief sources of the 

 secondary infection. 



Secondary Infection. The first or initial infection may occur on only 

 a few nuts and new growth, then quite suddenly the disease seems to 

 spread and infect many small nuts. This sudden increase of the disease 

 is due to an infection from the earlier diseased nuts and new growth, 

 and can thus be termed the secondary infection. 



Effect of Climate. Walnut bacteriosis is a disease that is quite 

 susceptible to variations in climatic conditions. It is a matter of com- 



FIG. 85. Bacteriosis lesions on older wood. Healing over. 



mon observation among growers that the amount of blight varies from 

 year to year in a given locality, also that some sections are freer from 

 the disease than are others, even during the same periods of time. A 

 given grove may be very bad one year and nearly free from the disease 

 the following year. This is well illustrated by a comparison of the years 

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