BULLETIN 231] WALNUT CULTURE IN CALIFORNIA. 329 



a source of infection. The test did not seem to show that the blight 

 organism was present in any great abundance in this liquid. Pierce 

 states* that the blight organism winters in the pith cavity, but we have 

 not found the organism any more abundant in this place than in the 

 partly live wood and bark of the diseased lesion. The diseased twigs of 

 the previous year are without doubt the chief source of the initial infec- 

 tion each spring. From our study we believe the old lesions to be the 

 only place where the organism can live over the dormant period of 

 winter and start the first infection the following spring, although Pierce 

 states that the old nuts and soil are a source of infection. 



Bacteriosis is not Die-Back. Walnut blight causes very characteristic, 

 comparatively small, sunken, black areas on the small shoots of the tree. 

 It does not attack branches of any size and does not injure them to such 

 an extent that they die back for several feet, as in the case of die-back. 

 See Die-Back, page 372. 



On Nursery Trees. Bacteriosis is especially severe in rapid-growing 

 trees in the nursery. Walnuts are grown for two years in the nursery, 

 being grafted the second season. The scions, after they start, usually 

 grow rapidly and are in a very favorable condition for this disease to 

 attack them. This infection in many cases comes from using diseased 

 scions. Rapid sucker growth in the orchard is very apt to become 

 infected and show the disease in a manner similar to that which occurs 

 in nursery trees. 



Leaves. The leaves are sometimes diseased, especially the petioles and 

 veins, which become a blackish or brownish color. The disease also 

 attacks the parenchyma or soft tissue of the leaf, causing spots of a 

 brownish color and of various shapes. These spots are rather common 

 on rapidly-growing leaves, and especially on the very large leaflets of 

 young nursery trees. The spots are circular or angular in outline and 

 by confluence often cause quite large diseased areas. Here the small 

 lateral veins in the diseased areas are a browner color than healthy ones. 

 Fig. 80. The disease does not cause serious defoliation of the tree and 

 should not be confused with the falling of the leaves that sometimes takes 

 place after a period of hot weather during the summer months. 



Catkins. The catkins are probably not diseased by walnut bacteriosis. 

 They often turn black, but this is probably only due to the natural 

 process of dying and drying up after their work is done. Various 

 attempts have been made to obtain the blight organism from these dark- 

 ened catkins, but without success. 



Nuts. Bacteriosis on the young nuts is especially virulent and 

 destructive under favorable conditions. The disease on the twigs and 



^Pacific Rural Press, 1899; Vol. 57, No. 25, p. 387. 



