Biological activity and physical modes of action of the Qo inhibitor fungicides trifloxystrobin and pyraclostrobin against Cercospora beticola

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The effect of trifloxystrobin and pyraclostrobin on spore germination and mycelial growth of Cercospora beticola, were determined in vitro. In addition, the protective, curative, translaminar and post-symptom activity against the pathogen was determined on sugar beet plants in growth chambers with difenoconazole and chlorothalonil as standard fungicide treatments. Both pyraclostrobin and trifloxystrobin were highly active causing complete inhibition of spore germination at concentrations of 0.01 and 0.1μgml−1, respectively, although higher concentrations were required for complete inhibition of fungal growth. Both fungicides were protective against C. beticola when applied 96h and 24h before inoculation of plants. Both fungicides, applied at 20μgml−1 , were superior to difenoconazole applied at 10μgml−1 and to chlorothalonil applied at 100μgml−1. Effective control was obtained when strobilurin fungicides were applied 24h after inoculation, but were less effective at 96h after inoculation. Pyraclostrobin at 20μgml−1 applied 96h after inoculation, was more effective than difenoconazole while control with trifloxystrobin at 20μgml−1 applied 96h after inoculation was similar to that obtained with difenoconazole. Chlorothalonil showed little activity against C. beticola in curative treatments. Tests to evaluate translaminar activity showed that disease severity on leaves treated with strobilurin fungicides on the upper leaf surface and inoculated on the opposite surface was similar to that obtained on leaves treated and inoculated on the same surface. Anti-sporulant activity was good with both strobilurin fungicides applied at concentrations of 10 or 20μgml−1 when applied after the appearance of the symptoms. The anti-sporulant activity of difenoconazole was similar to that of pyraclostrobin and trifloxystrobin applied at 5μgml−1, while chlorothalonil did not provide significant antisporulant activity. Such results encourage the evaluation of trifloxystrobin and pyraclostrobin under field conditions to select optimal partner fungicides for use in mixtures.