The effect of seed- and soilborne inoculum of Colletotrichum coccodes on the incidence of black dot on potatoes

Soil in a potato field naturally infested with black dot (Colletotrichum coccodes) was fumigated with methyl bromide at 126 g m(-2) or left unfumigated. Potato seed tubers (cv. BP1) uninfected, lightly infected (1-25% surface affected), severely infected (26-100% surface affected) and severely infected with C. coccodes but dusted with prochloraz manganese chloride as Octave 2.5% DP at 750 g per 100 kg seed were planted in fumigated and unfumigated soil. When harvested, the incidence of black dot on the progeny of infected seed planted in unfumigated soil was twice that of progeny in fumigated soil, with progeny of uninfected seed having a 68.5 times higher disease incidence in unfumigated soil. Black dot on progeny tubers was reduced by pre-treatment of seed with prochloraz in fumigated soil only. With black dot infested fields, planting disease-free seed or treating seed with fungicides would not decrease disease on progeny tubers.