WALNUT DISEASES 



2009 



isolated the organism, which has since 

 been isolated many times and its cultural 

 characteristics described. Pure cultures 

 of the organism, when atomized on 

 healthy walnut tissue, will cause typical 

 walnut blight. The organism produces a 

 yellow growth on media, has a single 

 polar flagellum. Its winter habitat is in 

 the diseased branches of the previous 

 year's growth. It probably does not live 

 over in the fallen nuts and leaves. 



Loss • 

 The loss from the disease varies from 

 year to year. In one of the first bad blight 

 years, that of 1903, the crop in California 

 was only about one-half that of the pre- 

 vious year. It is probably conservative to 

 estimate the average loss of nuts during 

 the past ten years from blight in the seed- 



ling groves of Southern California as 

 from 30 to 50 per cent. During the last 

 decade the walnut acreage has multiplied 

 many times, yet the total crop has in- 

 creased but little during this period. 



Favorable Conditions 



The amount of walnut blight infection 

 corresponds closely to the amount of mois- 

 ture, such as fogs, dews and other humid 

 conditions. Rains rarely occur in Califor- 

 nia after the appearance of the small nuts, 

 but foggy weather is more or less fre- 

 quent. Low fogs and especially foggy 

 nights are very favorable for blight dis- 

 semination and infection. At first only a 

 few infections on young leaves or nuts 

 are evident. From these there are other 

 infections that may involve a large num- 

 ber of nuts. 



Fig. 2. 



Walnut Blight. 

 Walnut Bacteriosis on Small and Half-Grown Nuts. 



