BULLETIN 231] WALNUT CULTURE IN CALIFORNIA. 333 



the infected nuts fall when they are small, but some remain on even until 

 harvest time. In the nuts the disease gradually grows within the tissue 

 until the kernel is reached which at length becomes blackened and then 

 destroyed. The disease may start at any place on the nut and gradu- 

 ally extend through other tissues into the kernel, but by far the most 

 of the infections that injure the kernel are from the blossom end. 



Lateral Nut Infection. The disease on the nuts starts at one or more 

 points on the surface as a small, circular, raised, discolored area that 

 at first has a water-soaked appearance and may not be larger than the 

 point or head of a pin when first visible. The diseased area in its 

 earliest stages is slightly raised above the surrounding healthy tissue, 

 but as the disease progresses, the spot becomes more or less sunken below 

 the normal tissue. The spot gradually enlarges in size and becomes 

 black in appearance. Surrounding the blackened area is a paler zone, 

 having the same water-soaked appearance already referred to. This 

 band of tissue lies between the healthy tissue and the blackened area 

 and represents the cells of the tissue that are being acted upon by the 

 ferments secreted by the organism, which break down the tissue and 

 prepare the way for the further advance of the bacteria. In the early 

 infection,, if climatic conditions are favorable, lesions or dark spots are 

 formed which often extend through the hull and shell-forming tissues 

 into the kernel. The nut in such cases is deformed in shape as the 

 diseased part ceases to grow. Such nuts do not bark clean, as the outer 

 covering clings very tightly to the shell, and the kernel at best is only 

 poorly developed. Such nuts are usually never picked up from the 

 ground, and in any case are only fit for culls. 



Late Infections. It often happens that during the summer months 

 weather conditions are favorable for natural infection of nuts. At 

 this time the outer tissue is beginning to harden and is not in condition 

 for the deep growth of the disease that occurs earlier in the season when 

 the tissue is more tender. Then, too, the disease may start in the sur- 

 face rather late in the season and suddenly conditions become unfavor- 

 able for its further development, thus giving a very shallow, superficial 

 development of the disease. These points of infection appear as small, 

 dark-colored areas scattered over the surface of the nut. Each little 

 infection can be distinctly seen, or its confluence with others may make 

 a large spot. In these late infections the development is shallow and 

 does not penetrate much through the epidermis, and the disease seems 

 to dry out and die. Occasionally a more severe case of late infection 

 occurs where the blackening and lesion extend to the hard shell, causing 

 the hull to cling to the shell of the nut. 



