86 PHYSIOLOGICAL RELATIONS 



the attention of growers and pathologists from early times. It is, 

 in fact, seldom that all the individuals of even a well-established 

 variety are equally susceptible to disease, and the differences be- 

 tween closely related varieties are often surprisingly great. The 

 Iron cowpea has been shown to be far more resistant than other 

 varieties to the wilt disease, and a new strain of cotton, the Dillon, 

 possesses similar qualities with respect to the same fungus. Every 

 carnation grower became familiar a few years ago with the fact that 

 the Scott carnation was peculiarly susceptible to carnation rust, and 

 that under ordinary conditions the Enchantress was peculiarly re- 

 sistant. The Kieffer pear is far less attacked by blight and leaf 

 spot fungi than other varieties commonly grown. Nearly all fruits, 

 vegetables, field crops, and floricultural plants will, upon careful 

 investigation, give evidence of more or less striking qualities of re- 

 sistance. This resistance may be inherited, or it may be a charac- 

 teristic which changes markedly as the climatic or soil conditions 

 vary under which the host plant may be growing. The relations 

 to disease may, therefore, be complex, and it is not the purpose of 

 this summary account of disease control to describe at length the 

 diverse relations of host and parasite. 



Seed selection. Seed selection is, in many cases, the easiest 

 and most natural method of disease control. The anthracnose of 

 beans is carried over from crop to crop very largely by means of 

 diseased seed, and it has been shown that diseased pods mean as 

 a rule diseased seed, that treatment of such diseased seed is not 

 effective, and that, therefore, the most rational method of combat- 

 ing the organism is to plant seed from selected pods. It is very 

 probable that the anthracnose of cotton is similarly transferred 

 from year to year. Certainly the appearance of the anthracnose 

 abundantly upon the seedlings, especially upon the cotton leaves, 

 suggests the presence of the organism in the seed. The late blight 

 of potato seems to be commonly, if not entirely, carried over from 

 season to season by means of diseased tubers, the latter being in- 

 fected with a form of the disease known as the potato rot. The 

 selection of seed from a field in which no blight has been present 

 to a very large extent insures a crop free from blight. Seed selec- 

 tion is already practiced to a considerable extent, but there is no 

 line of disease control requiring more attention at the present time. 



