BULLETIN 231] WALNUT CULTURE IN CALIFORNIA. 383 



pollination of late varieties is hindered by extremely dry weather which 

 is likely to occur at the time when they are in bloom, and it is also 

 true that during the season of 1911 late spring frosts occurred in 

 many localities in the northern part of the State, which may have 

 interfered with the proper setting of the crop. Aphis has likewise 

 been very abundant during the last two or three years, and this con- 

 dition has been coincident with short rainfall and lack of soil moisture, 

 especially in the subsoil. While the disease cannot at present be 

 attacked specifically, we believe that its control, along with that of 

 perforation, yellow leaf, and, in fact, most of the troubles which affect 

 the walnut will be greatly contributed to by the control of the aphis 

 and attention to proper irrigation of the subsoil. The latter can usually 

 be readily accomplished where irrigation water is available and the 

 grower should watch carefully the moisture condition of his subsoil in 

 dry years by actual examination down to a depth of at least six feet. 

 Where no irrigation is possible dry farming methods should be sys- 

 tematically pursued, consisting in deep plowing so as to catch as much 

 as possible of the winter rainfall and deep and thorough summer 

 cultivation to retain such moisture in the ground. The control of 

 aphis we discuss under the next heading. 



APHIS. 



The walnut is comparatively free from insect enemies, and we shall 

 not attempt to present a complete description of every insect which 

 may at times be found attacking the tree. One enemy of this sort, 

 however, is of particular importance and should be mentioned here. 

 This is the green aphis or plant louse. This insect has become so 

 abundant during the past two years on walnut trees of almost every 

 species all over the State that it may be considered fully as important 

 as the much dreaded walnut blight. 



The walnut aphis is a small, soft, green insect which occurs on the 

 under side of the leaves, often in considerable abundance. The insect 

 itself is less conspicuous than the secondary effects which it produces. 

 These are, first, an abundant secretion of honeydew, a sticky, liquid 

 substance with which the foliage and the ground or vegetation beneath 

 the trees becomes covered. The second conspicuous effect consists in 

 the development of a black, smutty mold which grows upon this 

 honeydew and covers the leaves and fruit of the walnut and every- 

 thing else upon which the honeydew has dropped. This insect and 

 its attendant effects have always been common in occasional seasons, 

 but during the past two years there has been an unusual prevalence 

 of aphis in most parts of the State, both on English and black walnut 



