Soybean (Glycine max) response to weed interference and Rhizoctonia foliar blight (Rhizoctonia solani)

Field studies evaluated response of soybean to Rhizoctonia foliar blight (RFB) disease in combination with varying densities of common cocklebur, hemp sesbania, or johnsongrass. Soybean plants at both V10 and R1 growth stages were not inoculated or inoculated with suspensions containing equal concentrations of Rhizoctonia solani AG-1 IA and IB mycelia. Intensity of RFB was rated weekly beginning at V1 soybean growth stage, and data were used to determine area under disease progress curves. Intensity of RFB was greater in 1993 than in 1994. When averaged across weed species and weed densities, soybean yield in 1993 was reduced 18% in plots inoculated with R. solani compared with those not inoculated. Intensity of RFB, however, did not differ between inoculated and noninoculated plots in 1994. Interactions between R. solani and weed density for RFB intensity and yield were not significant either year. Soybean yields in 1994, however, were reduced by hemp sesbania and johnsongrass in inoculated plots. Soybean maturity was delayed both years when hemp sesbania was present.