Uromyces appendiculatus in Honduras: Pathogen Diversity and Host Resistance Screening.

Bean rust, caused by the fungus Uromyces appendiculatus, is a major constraint for common bean production worldwide. Virulence of U. appendiculatus collected from wild and cultivated Phaseolus spp. was examined in 28 locations across Honduras. Host accessions representing wild and domesticated Phaseolus spp. collected at the same sampling locations were evaluated for resistance against U. appendiculatus. In total, 91 pathotypes were identified from 385 U. appendiculatus isolates according to their virulence on each of the 12 host differentials. No significant difference in pathogen total virulence, measured as the mean disease score, was found between locations. However, significant differences were found in pathotype virulence among isolates collected from different Phaseolus spp. within a location. Moreover, when locations were compared on the basis of pathotype occurrence and frequency, differences among locations were evident. No two locations had the same pathotype composition. The most common pathotype was virulent on 9 of the 12 differential lines. A high number of resistant accessions were identified in Phaseolus coccineus and P. lunatus. Although most wild P. vulgaris accessions were highly susceptible, rust resistance was observed in P. vulgaris landraces collected from farmer's fields. Thirty-two (52%) of the accessions screened showed intermediate to high levels of resistance and, of those, 16% were P. coccineus accessions. Our findings support the hypothesis that interaction of U. appendiculatus in host populations composed of diverse Phaseolus spp. and genotypes has favored highly diverse and virulent pathotypes, creating a center for virulence diversity of the pathogen in Honduras. The high percentage of intermediate and highly resistant accessions identified in the present study supports the strategy of collecting plants from the center of diversity of a pathogen or in locations with high incidence of disease and pathogen diversity to maximize the probability of identifying new sources of resistance.