254 THE MONTHLY BULLETIN. 



WALNUT BLIGHT OR BACTERIOSIS.* 



By Clayton O. Smith^ Assistant Plant Pathologist, University of California. 



Walnut blight has been known in California for about 25 years. 

 Its severity has varied from year to year according to the existing 

 climatic conditions. Some years the loss has been very heavy and 

 in other seasons it has been almost entirely absent from the groves. 

 The ravages from the disease have entirely revolutionized walnut cul- 

 ture. All new plantings are of selected varieties grafted on the Cali- 

 fornia black walnut. It is probable that these selected varieties will 

 in most cases be more free from the disease than is the ordinary seed- 

 ling grove. 



The trouble is found on the branches, leaves and nuts. Little or 

 no injury is done to the leaves and branches, the economic importance 

 being the diseased nuts which are often attacked at their different 

 stages of development. Many of these fall while small; others may 

 remain until the harvest time, only to be placed among the culls. 

 Nuts infected late in the season are not likely to be so seriously injured 

 that the grade is much lowered. 



The disease is called by either one of two popular terms, Wahiut 

 Blight being in more general use, while AValnut Bacteriosis is the 

 more descriptive term. 



APPEARANCE OF THE DISEASE. 



The disease is characterized by the appearance at different points 

 of blackish colored spots of a round or irregular shape; these when 

 examined with a microscope show numerous bacteria present in the 

 tissue. The infections are superficial, especially if they start late in 

 the summer, or may extend more deeply in the tissue, often reaching 

 the kernel, which is blackened and destroyed. The size of the diseased 

 areas varies but may cover a comparatively large surface of the nut, 

 due to the coalescence of several smaller ones. 



Walnut bacteriosis is found on the English walnut and its hybrids 

 with the blacks. It has never been reported as occurring naturally 

 on the California or Eastern blacks, but can be produced on tliem 

 artificially. Leaf spots occasionally are seen on the leaves of the Cali- 

 fornia blacks which, upon superficial examination, might be regarded 

 as walnut blight. These are. however, caused by a leaf-spot fungus. 



DISTRIBUTION. 



The disease is distributed more or less in California and Oregon ; 

 also is found on the west coast of Mexico on trees imported from Cali- 

 fornia. It has been reported from New Zealand, and was seen in 

 France by Professor R. E. Smith during his visit of last year, but 

 does not cause so widespread injury as with us. The blight was intro- 

 duced in California about 1890, in nursery stock imported from 

 France. The first published account of its presence was in 1893 in 

 the Report of Secretary of Agriculture, Division of Vegetable Pathol- 

 ogy. The trouble became establi.shed first in Orange and Los Angeles 

 counties, and from there has spread to almost all walnut-growing sec- 

 tions of the State. The severity of the disease differs according to 



♦Address before State Fruit Growers' Convention, Davis, Cal., June 1-6, 1914. 



