Reaction of cultivars of populus spp. to radiation induced virulent mutants of melampsora medusae

Summary Infection Type (IT) and Uredinial Number per Leaf Disk (ULD), induced on eleven cultivars of poplar in vitro, were employed to compare five radiation induced, mono-uredinial mutant lines of Melampsora medusae Thum. with the wild type race 5A from which they were derived. IT produced by the mutants was higher than (eight cultivars), similar to (two cultivars) and pronouncedly less than (IT from 4 to 1 in P. deltoides cv. 7–2) that of race 5A. Although the five mutants produced a uniform IT within a particular cultivar. ULD varied significantly between mutants on individual cultivars and there was a significant differential interaction of the mutant lines with cultivars. The ranking of mutant lines for aggressiveness on cultivars was not consistent while the differences among the mutants in mean ULD over all cultivars were not associated with the dosage level of irradiation from which they were isolated. The implications of the results in the interaction of leaf rust with poplar cultivars are discussed.