Effects of irrigation and benomyl treatment on chocolate spot (Botrytis fabae) and yield of winter-sown field beans (Vicia faba)

In 1976-7, 1977-8 and 1978-9 plots of winter-sown field beans, either irrigated in June and July or unirrigated, were treated with one or more sprays of benomyl or were untreated. The amounts of chocolate spot on field bean leaves and flowers were increased by irrigation and decreased by benomyl in all seasons. These effects were greater in 1977 and 1978, when there was respectively 76-5 and 82-4 mm rainfall in June, than in 1979, when there was 30 mm rainfall in June and less chocolate spot developed. Leaves half-way up stems of plants in irrigated, unsprayed plots had 11 % of their area affected in July 1977, 17 % affected in July 1978, but only 4 % affected in August 1979. By contrast, middle leaves of plants in unirrigated, sprayed plots had 3, 4 and 0-3% of their leaves affected in 1977, 1978 and 1979 respectively. Irrigation increased the number of pods per stem in 1977 and 1979 and decreased it in 1978; it decreased grain yield in 1977 and 1978, but increased yield in 1979. Benomyl treatments affected the number of pods little in any year; they increased yield in 1977 and and 1978, but not in 1979.