Resistance of Botrytis cinerea to Dichlofluanid in Greenhouse Vegetables

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Abstract
During a survey in Crete in 1986, samples of various vegetables infected by Botrytis cinerea were taken from 76 greenhouses, and the sensitivity of the fungis to dichlofluanid was tested by a spore germination test. Also, 52 single-spore isolates obtained from 10 greenhouses were tested for their sensitivity to dichlofluanid with respect to mycelial growth, spore germination, and disease control by foliar sprays. Thirteen single-spore isolates of B. cinerea, obtained from vineyards in which no control against gray mold had ever been applied, were compared with the vegetable isolates. On the basis of the inhibitory concentration (IC) of dichlofluanid for mycelial growth, the greenhouse single-spore isolates were divided into four classes with IC values of 3, 9, 27, and .gtoreq. 81 .mu.g/ml respectively. The mean ED50 values of dichlofluanid for the inhibition of spore germination for the isolates of each class and the group of vineyard isolates were: class 1, 0.063 .+-. 0.016; class 2, 0.058 .+-. 0.016; class 3, 0.043 .+-. 0.016; class 4, 0.267 .+-. 0.147 and vineyard isolates, 0.056 .+-. 0.025 .mu.g/ml. The mean ED50 values for the protection of young bean plants (Vicia faba L.) by foliar sprays for the same isolates were: class 1,65.0 .+-. 10.4; class 2, 7.7 .+-. 39.4; class 3, 110.3 .+-. 75.0; class 4, 369.5 .+-. 177.8 and vineyard isolates, 65 .+-. 22.1 .mu.g/ml. The isolates of classes 1, 2, and 3 were as sensitive to dichlofluanid as the vineyard isolates, while those of class 4 were significantly less sensitive. In a greenhouse experiment, dichlofluanid, captan, and chlorothealonil applied at label rates were not effective against gray mold in cucumber caused by an isolate of class 4 with a dichlofluanid ED50 value of 0.150 .mu.g/ml for spore germination. Fifty-five of the 76 samples obtained during the survey were classified in the least sensitive class 4. This accounts for the reduced effectiveness of this fungicide against gray mold that had been noticed by the growers.