Botrytis allii, grey mould, is the major cause of bulb and seed yield losses of growing onion seeds in Israel. Nursery treatments with iprodione or vinclozolin significantly decreased the number of diseased onion bulbs which were supplied to the farmers from 24% in the control to 8% in the treated plots and resulted in higher yield of seed with increases of 45% in seed weight per plant. Onion bulbs under non-irrigated conditions in the mountains were found to be more susceptible to the disease, as well as to secondary infection, than irrigated onions that were grown in the plains. Chemical treatment of the bulbs before planting significantly reduced the rate of secondary infection in the field and decreased seed losses. The survival of B. allii propagules and their importance as a primary source of inoculum in growing onions for seed were studied in the central mountainous region of Israel. From 197 to 1532 B. allii propagules per g soil were found in the field plots, this as the result of only one season growing onion seed. Ninety-one per cent of the total planted healthy bulbs were infected in the highly contaminated plots after 150 growing days. The surviving B. allii propagules reduced the seed yield, from 14·4 in the less contaminated plots to 10·4 g/plant in the highly contaminated.