Effects of fungicides on the growth and conidial germination of Colletotrichum coccodes and on the development of black dot disease of potatoes

The effects of 10 fungicides on the growth of Colletotrichum coccodes in agar culture and on the germination of conidia was investigated. In field experiments in 1990 and 1991 the extent to which treating black dot-affected potato seed tubers with fungicides affected the development of the disease on stem bases, roots and tubers was assessed. Black dot was also assessed on plants from field trials in 1990 which were designed to investigate the efficacy of the soil sterilant 1,3-dichloropropene (Telone) and two nematicides, aldicarb (Temik) and ethoprophos (Mocap). Prochloraz and fenpiclonil were the most effective fungicides in decreasing the size of C. coccodes colonies on agar. Imazalil, propiconazole and dichlorophen were also effective, but at higher concentrations, whereas tolclofos-methyl, thiabendazole and benomyl were only moderately effective. Resistant sectors developed from inhibited colonies on agar containing fenpiclonil and tolclofos-methyl. Conidial germination was prevented at 1 mg/litre fenpiclonil and 5 mg/ litre dichlorophen; imazalil, benomyl and thiabendazole were also moderately effective. Fenpiclonil and propiconazole seed tuber treatments consistently decreased black dot infection on roots, stem bases and daughter tubers early in the season, but only fenpiclonil decreased disease on tubers at harvest in October. Propiconazole also delayed emergence and decreased stem numbers. Soil treatment with 1,3-dichloropropene, aldicarb or ethoprophos had no effect on black dot but Rhizoctonia solani tuber infection and black scurf were increased.