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Resistance mechanisms of SDHI fungicides in Sclerotinia homoeocarpa field isolates

Geunhwa Jung: University of Massachusetts

<div>Dollar spot, caused by <em>S. homoeocarpa</em>, is the most economically important disease on turfgrass. Succinate dehydrogenase inhibitor (SDHI) fungicides have been widely applied to control dollar spot. Updated resistance mechanisms in golf course isolates from Japan and Rhode Island are presented. The Japanese isolates harbor a H-to-Y mutation at position 267 or silent mutation at 181 in the SdhB gene and a G-to-R mutation at 150 in the SdhC gene. The Rhode Island isolates harbor a mutation at position G91R or G159W in the SdhC gene. The SdhB H267Y mutation was functionally validated and exhibited high resistance to boscalid, moderate resistance to penthiopyrad and fluxapyroxad, weak resistance to isofetamid, and a hypersensitive response to fluopyram. The SdhC G91R mutation was functionally validated and exhibited high resistance to boscalid, fluxapyroxad and penthiopyrad, moderate resistance to isofetamid, and weak resistance to fluopyram. In addition, a single Japanese isolate with the SdhC G150R mutation had a similar response to the SdhC G91R isolate. In contrast, the Rhode Island isolate with both G159W and silent mutations had a similar SDHI resistance profile to the H267Y isolate group. A silent mutation was also found at position 181 in SdhB and no other mutations that cause an amino acid substitution were detected in the SdhB, SdhC and SdhD subunits. Updated results indicate that the SDHI cross- or negative-resistance profile is dependent on the target gene mutation (i.e. among subunits and mutation position within subunit) and considerable variability exists.</div>

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