CROSS-RESISTANCE OF BOTRYTIS-CINEREA TO CAPTAN, THIRAM, CHLOROTHALONIL, AND RELATED FUNGICIDES

Six isolates of B. cinerea were studied for their cross-resistance to 7 fungicides. Fungicide inhibition of mycelial growth followed the order: iprodione, chlorothalonil (EC500.2-8.5 .mu.M), captafol, thiram (1-18 .mu.M), captan, etem and folpet (4-130 .mu.M). Inhibition of spore germination usually required lower amounts of the chemicals but relatively similar concentrations were needed for all fungicides (EC500.06-1 .mu.M), except for iprodione, which was ineffective at this stage of fungal development (EC5040-370 .mu.M). Significant cross resistance among all isolates was demonstrated both for mycelial growth and spore germination with captan, folpet, captafol, etem, thiram and chlorothalonil. No cross resistance was observed with iprodione. Cross resistance may be due to excessive production of thiols which detoxify the fungicides. Thus, if resistance develops against any one of these multisite fungicides, the others will also likely prove ineffective.