The effect of carbendazim‐generating fungicides with and without a mineral oil additive on tomato stem lesions caused by carbendazim‐sensitive and tolerant strains of Botrytis cinerea

Isolates of Botrytis cinerea were obtained from tomatoes in several localities in the West Scotland. Some isolates grew on agar containing 100 mg/1 benomyl (carbendazim‐tolerant), while others did not (carbendazim‐sensitive). Pot‐grown tomato plants treated with benomyl and other carbendazim‐generating fungicides, applied either as sprays or soil drenches, were inoculated on the leaf scars with some of these isolates. On treated plants the carbendazim‐tolerant isolates formed lesions which were about as large as those on untreated plants. Sensitive isolates formed much smaller lesions on treated plants. There was evidence that the increase in lesion size during the period 7–14 days after inoculation with a carbendazim‐sensitive isolate was less on plants sprayed with benomyl or carbendazim with added mineral oil (2% Actipron) than on plants to which the fungicides alone had been applied. No such effect was recorded with thio‐phanate‐methyl. There was also an indication that the addition of Actipron to a benomyl spray improved the effect of the fungicide against two tolerant isolates, though there was no effect on the relative increase in lesion size during the second week after inoculation. In two tests the addition of 2% and 4% Actipron to benomyl soil drenches did not improve the level of leaf scar lesion control.