Sensitivity of Rhizoctonia solani Anastomosis Group 2-2 Isolates from Soybean and Sugar Beet to Selected SDHI and QoI Fungicides

Rhizoctonia solani causes root and stem diseases on soybean and sugar beet, and fungicides are commonly used to manage these diseases. Quinone outside inhibitor (QoI) fungicides (pyraclostrobin and azoxystrobin) have been used for in-furrow and postemergence application since 2000. Succinate dehydrogenase inhibitor (SDHI) fungicides (sedaxane, penthiopyrad, and fluxapyroxad) became popular seed treatments after their registration in Minnesota and North Dakota between 2012 and 2016. Periodic monitoring of sensitivity to these fungicides in R. solani anastomosis group (AG) 2-2 is important to detect potential shifts in sensitivity over time. R. solani AG 2-2 isolates (n = 35) collected from soybean and sugar beet in Minnesota and North Dakota were evaluated in vitro for sensitivity. Isolates were considered as baseline or nonbaseline for the above-mentioned fungicides based on previous potential exposure. The effective concentration (EC₅₀) required to suppress radial fungal growth by 50% was determined. The mean EC₅₀ values for sedaxane, penthiopyrad, fluxapyroxad, and pyraclostrobin were 0.1, 0.15, 0.16, and 0.25 (µg ml^-1), respectively. The mean EC₅₀ value for azoxystrobin for 22 isolates was 0.76 to 1.56 µg ml^-1; and EC₅₀ could not be determined for 13 isolates because of <50% inhibition at the highest concentrations used. The EC₅₀ values for the QoI fungicides did not differ significantly between baseline and nonbaseline isolates. EC₅₀ values for SDHI fungicides were significantly higher for isolates collected from soybean than from sugar beet, and isolates collected from both crops had similar EC₅₀ values for pyraclostrobin. All SDHI fungicides and pyraclostrobin effectively suppressed R. solani isolates from soybean and sugar beet at low concentrations in vitro.