Characterization of variability in the fungus Phaeoisariopsis griseola suggests coevolution with the common bean (Phaseolus vulgaris)

Angular leaf spot (ALS) disease of common bean (Phaseolus vulgaris) is caused by the imperfect fungus Phaeoisariopsis griseola and severely reduces bean yields in tropical and subtropical countries. Breeding for disease resistance has been difficult because there is substantial pathogenic variation among fungal isolates. Random amplified polymorphic DNA (RAPD) markers were used to characterize 62 P. griseola isolates from three countries (Malawi, the United States, and Brazil). The gene pool of the bean plants from which the isolates were obtained was determined by isozyme and phaseolin analysis. Eleven primers generated reproducible and distinct RAPD patterns that divided the P. griseola isolates into two major groups. Group 1 (Andean) isolates were generally recovered from Andean gene pool materials, whereas group 2 (Mesoamerican) isolates were recovered from Mesoamerican materials. Phaeoisariopsis griseola isolates representing groups 1 and 2 were inoculated onto selected Andean and Mesoamerican bean genotypes. Group 1 isolates were more pathogenic on Andean beans, whereas group 2 isolates were more pathogenic on Mesoamerican beans. RAPD and pathogenicity data suggest that groups 1 and 2 may have originated in the Andes and Mesoamerica, respectively, and that coevolution of the R. griseola fungus and its common bean host has resulted in increased levels of disease in this host-pathogen interaction. The results have implications in development of ALS breeding strategies