PATHOGENIC VARIATION IN, SOURCES OF, AND BREEDING FOR RESISTANCE TO PHAEOISARIOPSIS-GRISEOLA CAUSING ANGULAR LEAF-SPOT IN COMMON BEAN

If we are to breed common bean (Phaseolus vulgaris L.) for durable res istance to diseases, we must understand pathogenic variation and find sources of resistance. Our first objective was to determine the patter ns of pathogenic variation found among isolates of Phaeoisariopsis gri seola (PG), the fungus that causes angular leaf spot (ALS) in common b ean. We characterized 433 PG isolates from 11 Latin American and 10 Af rican countries, using differential cultivars, isozymes, and/or random amplified polymorphic DNA (RAPD) markers. We also systematically scre ened, for ALS resistance, common bean accessions from the world collec tion held at CIAT, and assessed the progress so far made in breeding f or resistance to ALS. Despite their great diversity within and between countries on both continents, the PG isolates were classified into tw o major groups: Andean, and Middle American. Although each group had i nternal differences for virulence, and biochemical and molecular chara cteristics, the 'Andean' PG isolates were more virulent on common bean s of Andean origin, than on those of Middle American origin, thus, sug gesting a host-pathogen co-evolution. The 'Middle American' PG isolate s, although more virulent on common beans from Middle America, also at tacked Andean beans, thus, exhibiting a much broader virulence spectru m. To find sources of resistance, we tested 22,832 common bean accessi ons against naturally occurring PG isolates in the field at CIAT's Exp eriment Station, Quilichao, Colombia, between 1985 and 1992. The resul ting 123 intermediate (scores of 4 to 6) and resistant (scores of 1 to 3) accessions were then tested in the greenhouse against selected 14 PG isolates of diverse origins. Nineteen accessions were intermediate or resistant to at least 13 of 14 PG isolates. Similarly, of 13,219 br ed lines tested in the field between 1978 and 1996, 89 were intermedia te or resistant. Of these, 33 bred lines proved intermediate or resist ant to at least eight of nine PG isolates to which they were challenge d in the greenhouse. We suggest that, to breed for durable resistance to ALS, common bean populations should be developed from crosses betwe en Andean and Middle American gene pools. The populations should then be systematically evaluated and selected against the broadest range of the most virulent PG isolates of diverse evolutionary origins.