Dicarboximide resistance in Botrytis cinerea in protected lettuce

In surveys of protected lettuce crops in the Humberside area of the UK during 1984‐5, 47.3% ofBotrytis cinerea isolates were resistant to dicarboximides and 25% to benomyl. Dicarboximide‐resistant isolates obtained direct from the field had a moderate degree of resistance with ED50 values for mycelial growth on agar of 1.0‐2.4 μg/ml vinclozolin for all except two of 30 isolates. Isolates which were resistant to vinclozolin were also resistant to iprodione, myclozolin and procymidone. Thirty dicarboximide‐resistant isolates obtained direct from the field were capable of infecting cucumber cotyledons sprayed with field‐strength Ronilan (250 μg/ml vinclozolin) whereas sensitive strains were not. Strains showing different degrees of resistance to dicarboximides developed after incubation of sensitive and resistant strains on agar supplemented with vinclozolin. Some of these strains had ED50 values for mycelial growth on agar of more than 100 μg/ml vinclozolin. No highly resistant isolates were obtained direct from the field. Resistance to dicarboximides in field isolates was stable.