BULLETIN 231] WALNUT CULTURE IN CALIFORNIA. 335 



White Deposit on Diseased Tissue. On the surface of the diseased 

 tissue of both the branches and the nuts can often be observed a whitish 

 substance that accumulates during the summer but at length disappears. 

 When this is properly stained and examined with a compound micro- 

 scope, it is found to be composed of countless numbers of bacteria and 

 broken-down plant tissue. Cultures made from this substance gave a 

 large number of typical walnut disease colonies. A somewhat careful 

 study was made of this white deposit in small nuts. Small diseased 

 nuts having this white substance were put in 10 per cent chromic acid 

 and the deposit swelled up and became soft and gum-like. It is thought 

 that this white substance containing so many organisms is an important 

 source of natural infection. Larger nuts, diseased at the blossom end, 

 sometimes show gum-like streaks running down over the nut from the 

 diseased portions. This substance also was found to contain bacterial 

 organisms. From laboratory experiments it is known that the organism 

 is quite resistant to desiccation and probably the condition described 

 is an important factor in bringing this about. 



Winter Habitat of Germ. The germ of the organism without question 

 winters in the old lesions of the branches. Much work has been done 

 in making cultures at short intervals of time throughout the year from 

 the different diseased tissues in order to see if the disease organism was 

 alive and where it best could pass through the winter or dormant period. 

 This work of making artificial cultures is given more in detail on page 

 343. The work began in October, 1907, and continued until the follow- 

 ing spring, at which time the disease again appeared on the new growth. 

 In every series of cultures the disease organism was found, showing 

 conclusively that the disease was still alive in the old lesions of the wood 

 and bark. The cultures made from the old nuts, both those on the 

 ground and some still on the tree, gave some walnut blight cultures 

 early in the autumn, but as the season advanced the organism seeined 

 to die out and toward spring it was almost impossible to secure blight 

 cultures from the old nuts found about the orchard. Pierce* states that 

 the organism "winters in fallen nuts beneath the trees and probably 

 upon fallen leaves and upon the soil." We believe that the old nuts 

 are not an important source of infection, since in all well-cared-for 

 groves these are plowed under before the tree starts out in leaf. This 

 is especially true of those groves where a cover crop is sown. We have 

 never tested soil and leaves to see if the germ could be found wintering 

 in them. 



The most prolific source of new infection is the lesions on diseased 

 twigs. Here the germs remain in almost a dormant condition until the 

 warm weather of spring, which arouses them to a renewed activity, when 

 they exude on the surface and are carried to the new growth, leaves, 



* Pacific Rural Press, Vol. 57, No. 25, p. 387. 



