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 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 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-1 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, and 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 moderate and susceptible cultivars, especially as inoculum density increased. Results have implications for cultivar selection and use and timing of postemergence fungicide application based on field history of inoculum pressure.