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 fo r disease resistance has been difficult because there is substantial p athogenic variation among fungal isolates. Random amplified polymorphi c DNA (RAPD) markers were used to characterize 62 P. griseola isolates from three countries (Malawi, the United States, and Brazil). The gen e pool of the bean plants from which the isolates were obtained was de termined by isozyme and phaseolin analysis. Eleven primers generated r eproducible and distinct RAPD patterns that divided the P. griseola is olates into two major groups. Group 1 (Andean) isolates were generally recovered from Andean gene pool materials, whereas group 2 (Mesoameri can) isolates were recovered from Mesoamerican materials. Phaeoisariop sis griseola isolates representing groups 1 and 2 were inoculated onto selected Andean and Mesoamerican bean genotypes. Group 1 isolates wer e 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 P. griseola fungus and its c ommon bean host has resulted in increased levels of disease in this ho st-pathogen interaction. The results have implications in development of ALS breeding strategies.