BULLETIN 231] WALNUT CULTURE IN CALIFORNIA. 377 



The hard, outer shell is deficient and fails to develop properly. This 

 failure, or in other words the cause of perforation, is not thoroughly 

 understood, but is probably connected with one of two factors, or 

 possibly both. These are, the fertilization or pollination of the nut dur- 

 ing the blooming period and its nutrition during development. Per- 

 foration has been especially abundant under two conditions: first, in 

 unusually dry years, especially when the trees were badly affected with 

 the walnut aphis; second, in young trees which are making a particu- 

 larly thrifty growth. The trouble also varies to a considerable extent 

 in individual trees, some being affected every year and others never 

 showing this trouble. In some instances there has been some indication 

 that the trouble was worse in years when pollination was interfered 

 with, either by extremely dry weather or frost during the blooming 

 period. This, however, is not well established. It seems most probable 

 that the disease is due to lack of nutrition of the nuts due to dryness of 

 the soil, exhaustion of the trees by aphis attacks, or, in the case of young 

 trees, a vigorous development of wood at the expense of the nuts. If 

 this be true it may be expected in the case of young trees that they 

 will gradually outgrow the trouble as they become older and divert 

 their energies more toward crop production, while in the case of older 

 trees abundant irrigation throughout the year and the control of aphis 

 (which we discuss elsewhere) offers most promise in the prevention of 

 perforation. 



CROWN GALL. 



This disease which is common in most fruit trees does not often 

 affect the walnut seriously, although now and then a badly affected 

 tree like that shown in Fig. 94 may be found. The trouble manifests 

 itself as a large swelling or gall at the base of the trunk either just 

 above or just below ground or both. It is caused by a bacterial organism 

 of a decidedly parasitic, infectious nature. 



Occasionally a nursery tree is found affected with this disease, 

 although it is very much less common in the walnut than in the peach, 

 apricot, almond, apple and other trees. It has generally been sup- 

 posed that only the English walnut root is susceptible to crown gall, 

 but we have recently seen a case of a northern California black walnut 

 root badly affected with this trouble. This, however, is decidedly 

 uncommon. Any nursery tree which shows this trouble should be 

 discarded. Trees affected in the orchard show a failure of growth, 

 gradually falling behind the other trees in size, and should be dug 

 out and replaced as soon as detected. The galls may be chiseled out 



