In a commercial planting of onion, root and basal rots reduced establishment of sets to 44%, which with 30% basal rot in harvested bulbs, represented an overall numerical loss of potential yield of 69%.Fusarium oxysporum, already present in sets at planting, contributed to poor establishment. Isolation from seedlings grown for sets in virgin soil at three locations yielded infection rates of 80 to 90%. In two field trials to evaluate fungicide treatments mean establishment was 47%. In the first trial involving dusting of the sets before planting Granosan 200 (benomyl 15% + mancozeb 60%) increased establishment by 28%, reduced basal rot of harvested bulbs by 77% and increased yield by 106%. Benomyl decreased basal rot and increased yields but captan and thiram treatments were ineffective. When bulbs from this trial were stored for six weeks under ambient conditions losses were 94% in controls and 45% in the benomyl + mancozeb treatment, with losses from other treatments intermediate. In the second trial, using pre-planting dips of benomyl the optimum concentration/time of 100 μ/ml for 15 min, reduced basal rot by 65% and increased yield 54%.