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QoI sensitivity in Alternaria solani, causal agent of potato early blight, is dependent upon the quantity of wildtype cytochrome b

Shunping Ding: University of Wisconsin-Madison

<div>Potato early blight caused by <em>Alternaria solani</em> is a perennial and destructive fungal disease. Management currently relies on fungicides due to lack of cultivar resistance. A commonly used class of fungicides is quinone outside inhibitors (QoIs). Mutations at amino acid position 129 of cytochrome<em> b</em> in <em>A. solani</em> isolates lead to partial resistance to QoIs. In this study, three <em>A. solani</em> isolates with different quantities of the mutated single nucleotide polymorphisms (SNPs) were subcultured onto clarified V8 juice agar (CV8) and CV8 amended with azoxystrobin (AZ) at 1mg/L every ten days for a total of 5 subcultures. Mycelial growth rate, EC50 of conidial germination rate, and the quantity of the mutated SNPs were monitored. Results showed no significant difference in mycelial growth rate, EC50, or quantity of mutated SNPs across subcultures when grown on CV8. However, when grown on CV8 amended with AZ, the mycelial growth rate increased by 60%, the quantity of mutated SNPs increased by 28%, and the EC50 increased by 25%. EC50 was positively related to the increasing quantity of mutated SNPs (r<sup>2</sup>=0.68); isolates became resistant when mutated SNPs accumulated. While the increase in mutated SNPs occurs relatively quickly, AZ may still be effective if a population has a low level of mutations prior to selection. Fungicide resistance management and resistance monitoring are critical in maintaining effectiveness of AZ and other single site mode of action fungicides.</div>