568 FUNDAMENTALS OF FRUIT PRODUCTION 



Disease Resistance. — Cole'*'' recommends the "Kentish sucker as a 

 cherry stock for fruit growers in Victoria because many varieties are 

 less likely to gum when worked upon this stock than on Mazzard seed- 

 lings." Presumably the gumming to which Cole refers is the physiologi- 

 cal type. Barss,^* in Oregon, recommends the genuine Mazzard stock 

 as freer from bacterial gumming than miscellaneous seedlings from the 

 ordinary sweet varieties. This, however, is another case of substitution 

 in part of the tree rather than of change in the part grafted in, since to 

 secure the greater freedom from the disease it is necessary to grow the 

 tree two or three seasons in the nursery or the orchard and then graft 

 it over in the limbs. 



Sometimes increased resistance to fungous diseases is claimed from 

 top working, as in the gooseberry on Ribes aureum, but no evidence is 

 available of any direct influence. In the case just cited any increased 

 resistance is due probably to the changed habit of the plant, the increased 

 height securing better aeration. 



In California the black walnut is used as a stock for the English 

 walnut {Juglans regia), in large part because of its resistance to a soil 

 fungus, the mushroom root rot {Armillaria mellea), to which the English 

 walnut roots are very susceptible. This is, again, a case of substitution 

 and not an influence of stock on cion. 



The claim is sometimes made that certain stocks make the top more 

 or less resistant to insect or fungous attack. Since vigorously growing 

 trees are more subject to aphis or to fire blight and perhaps less subject 

 to certain cankers, it is quite conceivable that a stock affecting growth 

 may indirectly have such an influence. The same effect, however, can 

 be secured by cultvu^al practice and no available evidence indicates 

 any modification of a specific nature in the cion by the stock making it 

 more or less liable to insect or fungus attack. 



Physiological Diseases. — Diseases of a mosaic nature are, of course, 

 transmitted in either direction by grafting. DanieP^ states that some 

 cases of court noue in the grape can be traced to grafting and expresses 

 the belief that it is due to "a kind of physiological trouble induced by 

 osmotic changes caused by the union of plants of different chemical 

 functional capacities." Daniel's statement that the characteristic 

 shortened internode appears also on shoots from the stock suggests a 

 condition similar to the transmission of pathological variegation rather 

 than a specific change due to grafting. Daniel states that grafted beans 

 grown in nutrient solution were free from chlorosis longer than check 

 plants which had absorbed more of the solution. ''Since the chlorosis 

 could not be attributed," he states, "to anything but the presence of 

 an excess of a salt (carbonate of lime, or another), it is necessary to 

 admit that this salt has passed in less quantity because of the different 

 osmosis and because of its utilization at the graft-union to neutralize the 



