588 FUNDAMENTALS OF FRUIT PRODUCTION 



For such soils the most promising varieties are 106^ in the drier and Aramon X 

 Rupestris No. 1 or 202^ in the wetter locations. In dry, shallow soils 420A 

 and 157^^ give promise of being excellent stocks." 



Some stocks show such cathohcity in taste that it is safe to grow trees 

 on them for planting in all locations that are at all suited. The Cah- 

 fornia black walnut, for example, adapts itself to so many soils that it is 

 almost universally used in California as stock for the English (or Persian) 

 walnut, though its resistance to root rot {Armillaria mellea) is also an 

 important factor. 



Sorauer^^* quotes Lieb to the effect that Pyrus mains prunifolia 

 major and P. m. baccata cerasiformis have been found valuable as stocks 

 for apple in very exposed or dry positions. 



Immunity or Resistance to Soil Parasites. — Adaptation to soil must 

 be paralleled at times by adjustment to diseases. The Damson plum 

 seems rather resistant to crown gall and in special cases might be given 

 preference for this reason. Shaw has found that cion-rooted apple trees 

 show crown gall in different forms according to variety. "Thus," he 

 states, "the Jewett apple shows usually if not always the hard form of the 

 gall, the Red Astrachan the simple form of the hairy root and the Olden- 

 burg the woolly knot form with many soft fleshy root growths. Other 

 varieties show the brown root form and still others often the aerial 

 form. . . . 



"Some varieties on their own roots seem to be largely if not entirely 

 immune to this disease. If this proves to be really the case, here may lie 

 the solution of the problem of the prevention of crown gall. . . . Prob- 

 ably the economic advantage would warrant the extra effort necessary to 

 propagate such trees, only under conditions where the crown gall was 

 especially troublesome. 



"There are other root diseases which are injurious, especially through 

 the southern part of the apple belt, that might possibly be avoided in a 

 similar fashion." '^^ 



The pear affords an interesting example. The so-called Japanese 

 pear (Pyrus serotina) is more resistant to blight than the French stock, but 

 seems rather susceptible to mushroom root rot and is sensitive to soil 

 moisture. Choice between the two may at times involve nice discrimi- 

 nation. In some soils the lemon suffers from root rot to such an extent 

 that other stocks are substituted. In Florida the sweet orange roots 

 formerly used as stocks were so badly attacked by root rot that this 

 stock has been superseded. Similar susceptibility is found in California. 

 In regions subject to pear blight the displacement of French seedling 

 pear stock by other stocks, such as Pyrus serotina, P. ussuriensis and 

 P. calleryana, that are resistant or immune can be forecasted, except as 

 other troubles may develop. 



