NEW RACES OF COLLETOTRICHUM-LINDEMUTHIANUM IN MICHIGAN AND IMPLICATIONS IN DRY BEAN RESISTANCE BREEDING

Four isolates of Colletotrichum lindemuthianum, pathogenic on previously resistant dry bean (Phaseolus vulgaris) cultivars, were collected in 1993 in Michigan and North Dakota from seeds produced in Michigan. Characterization of the isolates on two sets of differential dry bean cultivars demonstrated that three isolates were similar and were classified as race 73. These isolates resembled the alpha-Brazil race recently reported in Ontario. The fourth isolate was unique and was classified as race 7. This isolate resembled most closely the delta race identified in Ontario in 1976. This is the first report of the occurrence of either race 7 or 73 of C. lindemuthianum in Michigan. Although the origin of these races is unknown, race 73 appeared to have been present in Michigan State University bean breeding lines since 1991 but was not detected until 1993, when resistant cultivars showed typical anthracnose symptoms. The presence of these races in Michigan threatens current commercial cultivars, since race 73 overcomes the Are gene and race 7 overcomes the A gene, both of which have been extensively used in the breeding program. The occurrence of these new races in North America challenges current breeding strategies of using single gene resistance to control anthracnose. Gene pyramiding using molecular markers as a disease resistance strategy is discussed, since the A/Are gene combination affords resistance to both races.