Sensitivity of <i>Helminthosporium solani</i> Isolates from Northwestern Washington to Azoxystrobin, Difenoconazole, and Fludioxonil

Silver scurf, caused by Helminthosporium solani, is a surface blemish disease of potato ( Solanum tuberosum L.) that reduces tuber quality and market value. We obtained 75 H. solani monoconidial isolates from northwestern Washington-grown potato tubers and confirmed they were free of Sarocladium strictum using restriction enzyme digestion. These isolates were then evaluated using mycelial growth inhibition assays for their sensitivity to azoxystrobin (FRAC 11), difenoconazole (FRAC 3), and fludioxonil (FRAC 12), which are used commonly to manage silver scurf of potato. The mean absolute EC 50 values for azoxystrobin and difenoconazole were 0.72 and 0.26 µg/ml, respectively, with one isolate exhibiting an EC 50 value &gt;10 µg/ml to azoxystrobin. In contrast, 89% of H. solani isolates tested were able to grow on an agar medium amended with 100 μg/ml of fludioxonil. The in vitro insensitivity of H. solani isolates to fludioxonil emphasizes the need for monitoring the field efficacy of fungicides with fludioxonil for management of silver scurf of potato.