RESPONSE OF COLLETOTRICHUM-COCCODES TO FUNGICIDES IN-VITRO

Black dot is an important disease of potato that affects all plant parts. The causal agent, Colletotrichum coccodes, is commonly found on potato tubers used for seed. Our objective was to determine the in vitro sensitivity of C. coccodes to the fungicides maneb, thiabendazole, imazalil and CGA 173506, a phenylpyrrole. These fungicides are currently used, or are being tested for use, as potato seed treatments in the United States. All four fungicides reduced radial growth, sclerotial germination and spore germination of C. coccodes at 10 mug/ml and higher. The fungicide concentrations that inhibited radial growth by 50% were 2 mug/ml, 5 mug/ml, 35 mug/ml, and 40 mug/ml for imazalil, CGA 173506, maneb and thiabendazole, respectively. TBZ was less inhibitory than the other three fungicides and stimulated sclerotial production in vitro. Isolates of C coccodes differed in their sensitivity to maneb, thiabendazole and imazalil in radial growth, but not spore and sclerotial germination, assays. On CGA 173506, radial growth of four C coccodes isolates was inhibited, but these isolates produced sectors which grew normally. Germination of most sclerotia from these isolates was inhibited, but some germinated and produced normal colonies in the presence of CGA 173506. One C coccodes isolate was resistant to this fungicide in all assays.