Response to Gamma Irradiation and Induced Virulent Mutation in Melampsora medusae of Poplars

When the urediniospores of three races of Melampsora medusae, the causal agent of leaf rust of poplars, were exposed to Co⁶⁰ gamma irradiation (range 100–1000 Gy), there were significant differences in the radiosensitivity of the races as assessed, in vitro, by incubation period to flecking and the uredinial survivability percentage. In race 5A the frequency of mutation to virulence on Populus deltoides cv. T-173, normally a resistant cultivar, was maximum (1.688 %) at an irradiation dose of 400 Gy. Importance of mutations as source of variability in pathosystems and usefullness of artificially induced mutations in the context of host-pathogen interactions are discussed.