Walker—Nature of Disease Resistance* in Plants. 
227 
often exist biologic strains of the parasite possessing distinct 
infective properties. This is especially true with the rusts where 
the existence of distinct biologic strains of Puccinia graminis tritici 
Erikss. & Henn. (74, 76), Puccinia graminis avenae Erikss. & 
Henn. (75), Puccinia graminis secalis Erikss. & Henn. (47), and 
Puccinia triticina Erikss. (51) has already been demonstrated. 
Kanred, a wheat resistant to Puccinia graminis tritici in Kansas, 
is not highly resistant in South Dakota nor in Minnesota. The 
dilference in behavior is not attributed to any material difference 
in the host plant under the three environments, but to a difference 
in infective properties of the biologic strains of the parasite in the 
respective regions (54). In the case of the bean anthraenose 
organism (Colletotrichum lindemuthianum (Sacc. & Magn.) B. 
& C.) two biologic strains have been differentiated which behave 
quite differently in their infective properties on individual varie¬ 
ties of bean (5). Much more study is necessary on the effect of 
environing conditions upon parasitism and the expression of re¬ 
sistance over a wide range of cases. 
Genetic Behavior of Resistance 
The hereditary nature of resistance has been shown to be a 
common phenomenon; its genetic behavior, however, from the 
limited results at hand, does not appear to follow any single 
genetic law. Chief limiting factors in the advancement of our 
knowledge of this phase of the subject have been the necessity of 
assuming an arbitrary division between resistance and susceptibil¬ 
ity in analyzing the behavior of any group of plants, the present 
imperfect understanding of the effect of environmental factors 
upon the expression of resistant characters, and the lack of recog¬ 
nition, until recently, of the possible existence in many cases 
of several distinct biologic strains of the parasite. Biffen (7) 
found resistance in the case of yellow rust of wheat (Puccinia 
glumarum) to be recessive and determined by a single factor. 
Nilsson-Ehle (57), on the other hand, would explain the genetic 
behavior of resistance to yellow rust in his crosses of wheat on 
the basis of multiple factors. It is entirely possible that the 
variance in results in these two cases is due to the existence of 
different biologic strains in the two localities or to differences in 
environment. Recent work with black stem rust of wheat (Puc¬ 
cinia graminis tritici) (1, 55) and of oat (Puccinia graminis 
