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

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 0.1 μg ml−1, respectively, although higher concentrations were required for complete inhibition of fungal growth. Both fungicides were protective against C. beticola when applied 96 h and 24 h before inoculation of plants. Both fungicides, applied at 20 μg ml−1 , were superior to difenoconazole applied at 10 μg ml−1 and to chlorothalonil applied at 100 μg ml−1. Effective control was obtained when strobilurin fungicides were applied 24 h after inoculation, but were less effective at 96 h after inoculation. Pyraclostrobin at 20 μg ml−1 applied 96 h after inoculation, was more effective than difenoconazole while control with trifloxystrobin at 20 μg ml−1 applied 96 h 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 20 μg ml−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 5 μg ml−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.