Resistance of Sclerotinia minor Isolates to Cyclic Imides in Lettuce Field Soils of Roussillon, France

Lettuce drop caused by Sclerotinia minor Jagger is the most common disease affecting lettuce crops during the fall and winter in Roussillon. Spraying of cyclic imide fungicides (iprodione, vinclozolin, and less commonly procymidone) generally provides satisfactory protection. However, since about 1985, the mean field efficacy of iprodione and vinclozolin has significantly decreased, whereas that of procymidone is currently not in question. The appearance of fungicide-resistant isolates of S.minor could explain the reduced efficacy of these fungicides. We thus looked for iprodione and procymidone-resistant isolates of S.minor in these problem fields. The investigations were earned on for over 3 years. Most of the isolates showed fungicide sensitivity (iprodione IC 50 = 0.17 mg 1⁻¹), about one quarter showed average resistance (iprodione IC 50 = 0.59 mg 1⁻¹) and a few were found to be highly resistant. Resistant isolates were also obtained in vitro when using high fungicide concentrations. Moreover, isolates resistant to iprodione were found to be resistant to procymidone. The most highly resistant isolates produced fewer and larger-sized sclerotia than the sensitive isolates, they were also more quickly inhibited by high osmotic pressures. Field variability was quite low: 73 of 74 isolates obtained from one field were found to be sensitive, with only one showing mild resistance. There did not seem to be any correlation between the observed decrease in the field efficacy of iprodione and the presence of resistant isolates.