Some asexual isolates of the bean rust fungus, Uromyces appendiculatus, exhibit unusual isozyme and host-specific virulence traits. As a hybridization method not involving donor spermatia, urediniospores were applied to receptor pycnia. Aecia developed in a minority (< 10%) of the trials. Inoculations with first sibling generation urediniospores of the putative hybrid progeny on five differential bean lines often produced infection types that matched those of the maternal parent; therefore virulence did not clearly indicate successful crossing. However, urediniospore isozyme patterns containing bands from both parents for six enzymes confirmed hybridization in 7 of 14 progeny. Bands for the remaining progeny matched the maternal parent, indicating origin by selling. Thus, the electrophoretic results indicate that, in the bean rust fungus, urediniospores sometimes can act as spermatia (nuclear donors) for fertilization of pycnia. Pycnium × uredinium crossing should prove to be a useful technique for genetic studies of asexual rust populations.
Isolates of Uromyces appendiculatus with Specific Virulence to Landraces of Phaseolus vulgaris of Andean Origin
