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.

Effects of Inoculum Density and Cultivar Susceptibility on Rhizoctonia Damping-Off and Crown and Root Rot in Sugar Beet.

Damping-off and crown and root rot of sugar beet caused by Rhizoctonia solani anastomosis group 2-2 (AG 2-2) are important soilborne diseases in Minnesota and North Dakota. Management involves an integrated approach, including crop rotation, use of resistant cultivars, and timely fungicide application. Our objectives were to evaluate the role of inoculum density and cultivar susceptibility on the onset and development of Rhizoctonia diseases and on yield and quality in sugar beet. Three cultivars varying in susceptibility were sown in field plots inoculated with 0, 20, 40, or 60 kg/ha of R. solani AG 2-2 IIIB infested barley during 2013 and 2015. In both years, there was a significant linear effect of inoculum density with decreasing area under the stand establishment curve (AUSEC), root yield, and sucrose quality as inoculum density increased. Cultivar susceptibility significantly affected AUSEC as well as sucrose quality in both years and root yield in 2013. In both years, there was an inoculum density by cultivar interaction on disease ratings, with the partially resistant cultivar resulting in lower ratings than the moderate and susceptible cultivars, especially as inoculum density increased. These results have implications for cultivar selection and for use and timing of postemergence fungicide application based on field history of inoculum pressure.