320 UNIVERSITY OF- CALIFORNIA EXPERIMENT STATION. 



DISEASES AFFECTING THE WALNUT. 



WALNUT BLIGHT OR BACTERIOSIS. 



This is by far the most important trouble affecting the walnut in 

 California. So serious has been this disease that the loss of a large 

 portion of the crop has in some years been charged to this source; 

 legislatures have made special appropriations for its investigation, and 

 the growers have offered a large reward for a practical remedy. The 

 ravages attributed to blight were the occasion of the investigations de- 

 scribed in this bulletin and the prevalence of the disease has resulted 

 in changing the whole aspect of walnut culture in California within 

 a period of comparatively few years. The following account has been 

 prepared mainly by Mr. C. 0. Smith, who has carried out most of this 

 portion of the work. 



The bacterial diseases of plants are more and more engaging the at- 

 tention of plant pathologists. Most of these have only within recent 

 years been well understood. At present, however, there are a number 

 of very definite plant troubles due directly to these micro-organisms 

 living in plant tissues. One of the most important of this class of 

 plant troubles is the bacterial blight of the Persian or English walnut. 



Bacterial diseases of plants are, as a rule, much more difficult to con- 

 trol than those caused by fungi, and do not readily yield to treatment 

 by spraying. Our experiments in spraying will be described in full at 

 another point in this article, but the unsatisfactory results we secured 

 in checking the development and spread of the disease show quite con- 

 clusively that the solution of this problem lies along other lines. 

 The solution of the trouble is quite as much a horticultural as a phy to- 

 pathological problem, involving as it does the growing of varieties that 

 show a certain amount of resistance or in which the nuts escape the 

 blight because of their late period of blooming in the spring after the 

 period of blight infection is largely past. The walnut industry is 

 just now in a state of transition from the seedling orchard to one con- 

 taining only the best and most productive, non-blighting varieties. A 

 number of these have been quite thoroughly studied by the Laboratory 

 of Plant Pathology in southern California and some of our observa- 

 tions are given in detail in another part of this bulletin. 



Host. The natural occurrence of bacteriosis or walnut blight is 

 probably confined to the English walnut, or its hybrids with the blacks. 

 Blighted nuts have been collected from a Paradox walnut tree (a hybrid 

 of English and California black) and the organism has also been 

 isolated from the leaves of Paradox seedlings growing in the Experi- 



