THE RELATIVE RESISTANCE OF SOME WHEAT VARIETIES TO TILLETIA CARIES (DC.) TUL. (— T. TBITIGI (BJERK.) WINT.)

SUMMARY Some factors influencing the relative susceptibility of a wheat variety to Tilletia caries are recorded, and it is shown that infection varies with the spore load and with the date of sowing. The relative resistances of certain wheat varieties to bunt when contaminated separately with Little Joss bunt, their own bunt and bunt spores from other varieties are shown. Rye is susceptible to Tilletia caries, but no infection is recorded on Hordeum nudum and Avena nuda. Hybridization experiments between resistant and susceptible wheat varieties are described. The general trend of the experiments indicates that several of the so‐called immune or highly resistant varieties are susceptible when they are contaminated with bunt spores that are produced on those varieties. They indicate that the passage of the pathogen through the host increases the virulency of the disease. These phenomena are accounted for because Tilletia caries is not a single system of one identity but is composed of physiologic forms and the passage of the parasite through the host increases the virulency because this process leads to the selection of a particular form or forms well adapted to the environmental conditions. It is thought that resistance depends not on one single factor but on at least three, namely, the physiologic form of the parasite and its environment, and the strain of the host and its environment.