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

Four isolates of Colletotrichum lindemuthianum, pathogenic on previous ly resistant dry bean (Phaseolus vulgaris) cultivars, were collected i n 1993 in Michigan and North Dakota from seeds produced in Michigan. C haracterization of the isolates on two sets of differential dry bean c ultivars demonstrated that three isolates were similar and were classi fied as race 73. These isolates resembled the alpha-Brazil race recent ly reported in Ontario. The fourth isolate was unique and was classifi ed as race 7. This isolate resembled most closely the delta race ident ified in Ontario in 1976. This is the first report of the occurrence o f either race 7 or 73 of C. lindemuthianum in Michigan. Although the o rigin of these races is unknown, race 73 appeared to have been present in Michigan State University bean breeding lines since 1991 but was n ot detected until 1993, when resistant cultivars showed typical anthra cnose symptoms. The presence of these races in Michigan threatens curr ent commercial cultivars, since race 73 overcomes the Are gene and rac e 7 overcomes the A gene, both of which have been extensively used in the breeding program. The occurrence of these new races in North Ameri ca challenges current breeding strategies of using single gene resista nce to control anthracnose. Gene pyramiding using molecular markers as a disease resistance strategy is discussed, since the A/Are gene comb ination affords resistance to both races.