Susceptibility of pure and hybrid willows to isolates ofMelampsora epitea rust

Pure species and F₁ hybrid families ofSalix viminalis andS. dasyclados were tested for resistance to four single uredinium isolates ofMelampsora rust in laboratory experiments using excised leaves. Rust isolates were derived from:S. viminalis, S. dasyclados, aS. viminalis x triandra hybrid, andS. daphnoides. Incidence of infection, number of uredinia per leaf, and numbers of spores per uredinium were measured. As expected, the isolate fromS. daphnoides did not infect any of the willow species or hybrids tested. For the other three rust isolates that were tested, the parent from which the isolate was derived was susceptible, the other parent was resistant, and hybrids were intermediate in resistance for incidence and uredinia per leaf. These patterns indicate additive inheritance of these resistance traits in hybrids. Numbers of spores per uredinium were similar on the hybrids and the susceptible parent for one rust isolate, suggesting dominant inheritance of this trait in the hybrids.