RESPONSE OF COLLETOTRICHUM-COCCODES TO FUNGICIDES IN-VITRO

Black dot is an important disease of potato that affects all plant par ts. The causal agent, Colletotrichum coccodes, is commonly found on po tato tubers used for seed. Our objective was to determine the in vitro sensitivity of C. coccodes to the fungicides maneb, thiabendazole, im azalil and CGA 173506, a phenylpyrrole. These fungicides are currently used, or are being tested for use, as potato seed treatments in the U nited States. All four fungicides reduced radial growth, sclerotial ge rmination and spore germination of C. coccodes at 10 mug/ml and higher . The fungicide concentrations that inhibited radial growth by 50% wer e 2 mug/ml, 5 mug/ml, 35 mug/ml, and 40 mug/ml for imazalil, CGA 17350 6, maneb and thiabendazole, respectively. TBZ was less inhibitory than the other three fungicides and stimulated sclerotial production in vi tro. Isolates of C coccodes differed in their sensitivity to maneb, th iabendazole and imazalil in radial growth, but not spore and sclerotia l germination, assays. On CGA 173506, radial growth of four C coccodes isolates was inhibited, but these isolates produced sectors which gre w normally. Germination of most sclerotia from these isolates was inhi bited, but some germinated and produced normal colonies in the presenc e of CGA 173506. One C coccodes isolate was resistant to this fungicid e in all assays.