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Sensitivity to DMI, SDHI and phenylpyrrole fungicides of Helminthosporium solani causing silver scurf on potato in the US

Noah Rosenzweig: Michigan State University

<div>Silver scurf (SS) caused by <em>Helminthosporium solani</em> is a persistent problem for potato production and is of particular importance in temperate regions. The disease is common in potato seed tubers and can become an issue in storage and negatively impact marketability of ware potatoes that are stored for longer periods of time. Therefore, effective fungicides for management of the disease are essential. The objectives of this monitoring program were to: 1. collect SS-infected potato tubers from commercial storages across US production regions; and 2. determine the sensitivity of the populations of <em>H. solani</em>. to fungicides in three classes, including: DMI fungicides [difenoconazole (DFZ)]; SDHI fungicides [solatenol, (STL), sedaxane (SDX)]; and phenylpyrrole fungicides [fludioxonil (FDL)]. Tubers with symptoms of SS were sampled from potato storages in Idaho, Michigan, Oregon and Wisconsin. Mono-conidial isolates were obtained to determine sensitivity to each fungicide above. A spiral gradient dilution method was used to estimate the fungicide concentration which caused a 50% inhibition of fungal growth (EC<sub>50</sub>) in vitro for all isolates. A total of 26, 33, 33, and 34 isolates were screened against DFZ, STL, SDX, and FDL respectively and the mean EC<sub>50 </sub>values were 13.9 (min: 0.1, max: 77.6), 9.5 (min: 0.10, max: 105.0), 9.7 (min: 0.10, max: 65.9) and 45.8 (min: 0.1, max: 105.0) for DFZ, STL, SDX, and FDL respectively. The sensitivity response to the two SDHI fungicides SDX and STL, revealed cross sensitivity among isolates.</div>