Effects of inoculum source and irrigation on black dot disease of potatoes (Colletotrichum coccodes (Wallr.) Hughes) and its development during storage

Summary In 1985 and 1986 plots were planted with seed tubers affected with black dot and with healthy seed inoculated or not with cultures ofColletotrichum coccodes at planting. Half the plots were irrigated from shoot emergence to the end of July. Seed tubers rotted more often when they were diseased than when they were not and rotting was increased by irrigation; by contrast, up to 18 weeks after planting irrigation decreased infection of stem-bases, roots and tubers. Tuber infection, first recorded in July, increased throughout the season and most rapidly from September, when the irrigation applied earlier increased disease. Soil-applied inoculum gave most disease in both years and tubers from healthy seed gave least in 1986 but as much as from diseased seed in 1985. Incidence and severity of black dot increased more during storage at 15°C than at 5°C and only on unwashed tubers.