Studies on the biology of Hemileia vastatrix Berk. & Br

Abstract

Germination of the uredospores of Hemileia vastatrix can occur only in the presence of liquid water, either in the dark or in light of low intensity, the latter stimulating the onset of germination at low, but not at optimal temperatures. On agar the optimum is 22° C, the minimum 15·5° and the maximum 28·5°. On leaf surfaces, however, germination is markedly depressed at ca. 24°, resulting in a bimodal relation with temperature. After subjection to moderately low temperatures a marked stimulation of germination follows on transferrence to the optimum, the amount of the stimulus being proportional to the duration of the exposure to the lower temperature. Spores of H. vastatrix cannot infect except at comparatively high concentrations, hence the threshold period for successful infection is longer than the time necessary for the formation of appres-soria by individual spores. Lesions can occasionally develop after 3½ hr. exposure to conditions which permit germination, but the number increases with the length of exposure up to at least 12 hr. Germination is more frequent on young leaves than on old, and at the margins than near the mid-rib. When stored dry the spores are relatively short-lived, losing 50 % of their viability in less than 2 days. When wetted for very short periods and then dried for equally short ones, their germinative capacity is very greatly reduced. The relation of these findings to conditions in the field is discussed.