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Evaluating Helminthosporium solani, causal agent of potato silver scurf blemish disease, for sensitivity to the fungicide azoxystrobin

Sofía Macchiavelli Girón: University of Wisconsin-Madison

<div>Silver scurf of potato, caused by the ascomycetous fungus <em>Helminthosporium solani, </em>is a tuber blemish disease causing negative impacts of increasing concern in potato production. Growers have utilized variably-effective cultural controls and fungicides for disease management. After <em>H. solani</em> developed resistance to the fungicide thiabendazole, QoI fungicides (such as azoxystrobin) were adopted with some control success. However, recently there has been an increase in disease severely affecting quality and storability. Genetic mutations associated with QoI fungicide resistance have been identified in other ascomycetes, such as <em>Alternaria solani</em>. For this reason, we hypothesized that <em>H. solani</em> might currently be poorly controlled due to QoI resistance after over 2 decades of use. In our preliminary screening we selected five <em>H. solani</em> isolates representing different production regions and management regimes. Five of these isolates were exposed to azoxystrobin at 8 concentrations to determine fungicide sensitivity. Isolates were grown on clarified V8 agar amended with azoxystrobin (0-100 μg/mL). Relative growth was measured after 20 and 40 days of incubation at 23ºC under darkness. There were significant differences in growth between isolates, as well as between treatments. One of the isolates grew on all concentrations, including the highest rate, suggesting some level of insensitivity. Further studies will focus on evaluating QoI sensitivity in a larger population and we will investigate the presence of mutations associated with QoI sensitivity in other ascomycetes pathogens in our <em>H. solani</em> isolates. Our results could greatly influence continued use of azoxystrobin in potato production for the control silver scurf.</div>