208 AGRICULTURE OF MAINE. 



ment, each infecting the flower of its respective host and 

 thus greatly differing from other smuts. The spores of the 

 wheat smut dusted on the flowers of wheat will infect the 

 grain which, if planted, will produce smutterl plants, but if the 

 spores of the barley smut are dusted on the wheat flower no 

 infection will result, nor will any if the spores of wheat smut 

 are introduced onto the barley flowers. The various rusts 

 are likewise strictly confined to the host species they nor- 

 mally attack and they may become so specialized as to infect 

 certain varieties within the species, the other varieties being 

 partly or entirely immune. Salm.on (i) found that certain 

 Erysiphze (powdery mildews) had become so specialized that, 

 while the same species of mildew was found on a number of 

 different hosts, the form on one would not attack the others. 

 These forms, physiologically different, though anatomically 

 alike he calls "biologic forms". The writer in his infection 

 work with the bean anthracnose fungus, found that a given 

 strain of the organism will attack certain varieties of beans 

 and will not infect certain other varieties, yet if another strain 

 of the fungus is used the results may be entirely different. 



Some varieties of plants are, year after year, never attacked 

 by certain parasites, simply because they have their period of 

 growth and reach maturity before the parasite makes its 

 appearance. Varieties that appear later are attacked, as these 

 would be if they were later in their growth or if the parasite 

 were earlier in its appearance. Oats sown early usually escape 

 the rust, and some early varieties of potatoes are nearly mature 

 before the advent of the late blight organism but on the other 

 hand they are liable to attack from the early blight. In some 

 regions where the early blight is prevalent it is the custom to 

 plant potatoes as late as possible in the season in order that 

 they may escape this disease. Such varieties Orton (2) calls 

 "disease-escaping plants." Nevertheless it is true, as numerous 

 instances prove, that certain varieties even when they are sub- 

 jected to disease conditions will become affected but slightly 

 or not at all while other varieties of the same species of plant 



1. Salmon, Ernest S. Cultural Experiments with Biologic Forms of the 

 Erysiphaccac. Proc. Roy. Soc. 73 : 116-118, 1904. 



2. Orton, W. A. The Development of Farm Crops Resistant to Dis- 

 ease. Year Book U. S. Dept. of Agr. 1908 : i. c. 458, 1909. 



