PEAR DISEASES 341 



only a few feet may often be sufficient to determine whether 

 the trees will be severely affected or not. (For a fuller dis- 

 cussion of the question of frost-injury, see under Apple, page 



35.) 



CROWN-GALL 



Caused by Bacterium tumefaciens E. F. Smith and Townsend 



The pear, like most other fruits, is affected with galls both 

 at the crown and at the tips of the roots. While orchard trees 

 show the enlargements, there is a greater tendency for nursery 

 stock to be affected. In any case the destruction to pears is less 

 rapid, and therefore less extensive, than to peaches or apples. 

 Hairy-root, another form of the crown-gall disease, is less 

 common on the pear than on other fruit-trees. Both the galls 

 and hairy-roots may be induced by factors other than Bac- 

 terium tumefaciens; among such agencies may be noted (a) im- 

 proper wrapping of grafts, (b) heavy applications of nitroge- 

 nous fertilizers, and (c) the woolly aphis. A discussion of the 

 galls, the causal factor, and remedial measures are treated 

 more fully under Apple, page 108. 



REFERENCES 



Selby, A. D. Some diseases of orchard and garden fruits. 4. Root or 

 crown gall of pear. Ohio Agr. Exp. Sta. Bui. 79 : 127. 1897. 



Swingle, D. B. Fruit diseases in Montana. Crown-gall of pear. 

 Montana Agr. Exp. Sta. Circ. 37 : 317-318. 1914. 



Martin, G. W. The common diseases of the pear. Crown gall. 

 New Jersey Agr. Exp. Sta. Circ. 52 : 10-11. 1915. 



EASTERN RUST 



Caused by Gymnosporangium glob?.~im Farlow 



Pear rusts occur generally throughout the world. In the 

 United States two important rust diseases of pear exist, but 



