Relationship Between Increased Virulence and the Aggressiveness Traits ofMelampsora medusae

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Abstract
Five radiation-induced, mono-uredium derived mutants of Melampsora medusae were compared with the wild-type race 5A (from which they were derived) for certain traits of aggressiveness (period to flecking, formation of first uredium, production of 50% of the uredia; number of uredia and urediospores produced per unit leaf area; and number of uredia produced per day) on leaf disks of both a resistant (on which the mutants are virulent) and susceptible cultivar of poplar [Populus .times. euramericana, P. deltoides] (on which the mutants can arise). The cultivar, the mutant line, and their interaction were significant contributors to variation in the traits of aggressiveness. On the susceptible cultivar, the wild type was more aggressive (incubation period, numbers of uredia, and spore production) than the mutants; hence, the traits of aggressiveness, other than the latent period, were negatively correlated with the increase in virulence. Although there were quantitative differences among the mutants for traits of aggressiveness on the susceptible cultivar, when ranked for these traits the mutants formed a cluster distinctly removed for the wild type. Hence, with these isolates, the range in aggressiveness appears to be related to the virulence makeup of the genotype. The delayed initiation and slower progress of disease indicated the "slow-rusting" nature of the resistant cultivar. It is concluded that virulent races are relatively less fit to survive on the susceptible cultivar, while their fitness on resistant cultivars depends on the background genotypes of the host.