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Control of Benzimidazole- and DMI-Resistant Strains of Cercospora beticola with Strobilurin Fungicides

The control efficacy of two new strobilurin fungicides, trifloxystrobin and pyraclostrobin, against Cercospora beticola isolates resistant and sensitive to sterol demethylation-inhibiting (DMI) fungicides and benzimidazole fungicides and the effects on evolution of resistance were tested in the current study. Control efficacy of strobilurin fungicides was measured using three C. beticola isolates, one DMI-resistant (DMIR), one benzimidazole-resistant (BENR), and one of wild-type sensitivity (WCB). Both pyraclostrobin and trifloxystrobin provided satisfactory control of all the three isolates used in the study, when applied at 5 μg ml^-1 and very high levels of control when applied at 10 μg ml^-1. Control was independent of the isolate sensitivity to benomyl and difenoconazole. In contrast, benomyl applied at 10 μg ml^-1 failed to control sufficiently the benzimidazole-resistant isolate, whereas difenoconazole applied at either 5 or 10 μg ml^-1 failed to provide satisfactory control of the DMI-resistant isolate of the pathogen. The effects of strobilurin fungicide applications on the evolution of resistance to benzimidazole and DMI fungicides were tested under field conditions in a 2-year experiment (2003 to 2004). Applications of either trifloxystrobin or pyraclostrobin provided high levels of disease control during both years of the study, whereas applications of either benomyl or difenoconazole provided a moderate control efficacy. Measurements of resistance frequencies to benomyl and to difenoconazole showed that successive applications of benomyl tended to select for high frequencies of benzimidazole-resistant phenotypes, whereas successive applications of difenoconazole tended to select for high frequencies of DMI-resistant phenotypes. In contrast, applications of either trifloxystrobin or pyraclostrobin prevented an increase of benzimidazole- or DMI-resistant phenotypes compared with the plots treated with benomyl or difenoconazole, respectively, and decreased frequency of resistance compared with untreated control plots.