SCREENING SOYBEAN CULTIVARS FOR RESISTANCE TO AERIAL BLIGHT CAUSED BY RHIZOCTONIA SOLANI AG1-IA

Aerial blight, caused by the fungus Rhizoctonia solani AG1-IA, is an economically important soybean disease in the mid-South. The same pathogen also infects other crops commonly rotated with soybeans like rice, corn, and grain sorghum. Rhizoctonia solani AG1-IA is difficult to manage because of its soilborne nature and the production of long-term survival structures, sclerotia. Management has relied on fungicide applications during the season. Still, there is an increasing prevalence of resistance to commonly used strobilurin fungicides and an urgent need to identify soybean cultivars resistant to aerial blight. Since the patchy distribution of the pathogen complicates field cultivar screening, the present study aimed to develop a greenhouse screening protocol to identify soybean cultivars resistant to aerial blight. As a first step towards this, 88 isolates of the pathogen were collected from symptomatic soybean plants in commercial fields and research farms across five Louisiana parishes. DNA was extracted, and 77 % of the isolates were confirmed to be R. solani AG1-IA. The remaining isolates did not amplify with any primers tested, likely due to poor DNA quality. Three polymorphic co-dominant microsatellite markers were used to explore the genetic diversity of 44 isolates that amplified with the markers. Results showed high genetic diversity with 35 haplotypes in total, with only two haplotypes present in two other locations. Six genetically diverse isolates were used in ongoing cultivar screening trials. We simultaneously tested several artificial inoculation protocols and identified a millet inoculation protocol to cause reproducible disease. Two grams of millet colonized by one R. solani AG1-IA isolate, when placed at the soil line of soybean plants (V5–V7 growth stage) and maintained under high humidity, caused symptoms in six to eight days. Of seven cultivars tested with this method, the cultivars LS5909CFS and S17-2193C were classified as moderately susceptible, and the rest of the cultivars were susceptible or highly susceptible. Of the 18 cultivars screened from the 2022 core block, the cultivars Delta Grow 48X45 and Armor 48-D25 were identified as moderately resistant to resistant to aerial blight. The cultivars identified as resistant may provide short-term solutions for growers to choose less susceptible cultivars for planting. At the same time, the newly developed protocols lay the foundation to characterize host resistance against this important soybean pathogen