Comparative Virulence of Monokaryotic and Dikaryotic Stages of Five Isolates ofUromyces appendiculatus

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Abstract
The virulence of monokaryotic basidiospores and dikaryotic aeciospores and urediniospores of 5 isolates of U. appendiculatus was compared on 11 lines and cultivars of beans (Phaseolus vulgaris). Virulence of monokaryotic basidiospores of each isolate was determined by placing germinating teliospores over plants of each line or cultivar. Segregating isolates produced both pycnia and flecks; homozygous isolates produced only flecks or pycnia. The fungal isolates were mass-selfed and the bean lines inoculated with bulked S1 aeciospores to detect heterozygosity and dominance in the dikaryotic stage. Segregating dikaryotic S1 aecial collections gave rise to both flecks and uredinia; homozygous isolates produced only flecks or uredina. With only 2 exceptions, in which avirulent basidiospores produced no reaction, isolates that segregated for pycnial infection types also segregated as dikaryotic S1 aeciospores, and isolates that did not segregate as monokaryons did not segregate as S1 aeciospores.