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POSTERS: Chemical control

Sensitivity of Rhizoctonia solani isolates from soybean and sugar beet to selected SDHI and QoI fungicides
Pratibha Sharma - Department of Plant Pathology, University of Minnesota. Dean Malvick- University of Minnesota, Department of Plant Pathology, Ashok Chanda- Department of Plant Pathology, University of Minnesota

Rhizoctonia solani can cause significant production losses to sugar beet and soybean. Disease management relies heavily on fungicides. The objective of this study was to determine the sensitivity of R. solani to sedaxane, penthiopyrad, and pyraclostrobin. Thirty-five isolates of R. solani collected during 1986 - 2018 from soybean and sugar beet in Minnesota and North Dakota were selected for this study. Isolates were subjected to poison plate assays with fungicide concentrations of 0, 0.01, 0.1, 1 and 10 ug/ml in potato dextrose agar. For assessing sensitivity to pyraclostrobin, salicylhydroxamic acid (SHAM) was added to the medium at 100 ug/ml. The fungicide concentration that suppressed 50% radial growth of mycelium compared to the growth on non-amended plates (EC50) was estimated for each isolate using ‘drc’ package in R. The EC50 values for sedaxane, penthiopyrad and pyraclostrobin ranged between 0.04 to 0.34, 0.04 to 0.32 and 0.04 to 0.85 ug/ml with means of 0.12, 0.16 and 0.22 ug/ml, respectively. The EC50 values of penthiopyrad and pyraclostrobin varied significantly among the isolates, but not for sedaxane. The EC50 values of penthiopyrad were significantly higher for isolates from soybean compared to the isolates from sugar beet. The three fungicides are effective for suppressing the pathogen growth in vitro, however R. solani isolates appear to be more sensitive to SDHI fungicides than QoI fungicides.