Interactions of Weeds, Herbicides, and Aerial Blight (Rhizoctonia Solani) in Soybeans.

Abstract

Field studies evaluated soybean response to Rhizoctonia foliar blight (RFB) (Rhizoctonia solani AG-1 IA and IB) and interference from common cocklebur, hemp sesbania, or johnsongrass. Soybean maturity was delayed both years by hemp sesbania whether or not RFB was present. Soybean yield was reduced 20% in plots inoculated with the RFB pathogen in 1993, but not in 1994. Soybean yield was reduced only when inoculated plots were infested with common cocklebur and johnsongrass in 1993 and with hemp sesbania and johnsongrass in 1994. Acifluorfen, glufosinate, glyphosate, paraquat, and pendimethalin were evaluated for effects on mycelial growth and sclerotia/microsclerotia production by R. solani AG-1 IA and IB in culture and on severity of RFB of soybean in the field. In laboratory studies, all herbicides reduced colony radius of both isolates. Growth reductions for IB were greater than those for IA in the presence of pendimethalin, alachlor, or acifluorfen. However, glufosinate reduced growth of IA more than IB. Sclerotia production was completely inhibited by paraquat and greatly reduced by glufosinate. In field studies, single degree-of-freedom contrasts indicated a reduction of RFB severity in soybean when paraquat was applied. Greenhouse experiments evaluated barnyardgrass, broadleaf signalgrass, common cocklebur, entireleaf morningglory, hemp sesbania, itchgrass, johnsongrass, large crabgrass, northern jointvetch, prickly sida, purple nutsedge, redweed, sicklepod, and smooth pigweed as hosts for R. solani AG-1 IA and IB. In the first study, sclerotia of IA were recovered from all weed species except pigweed, and mycelia of IA were recovered from tissue of all weeds except pigweed and redweed. Microsclerotia or mycelia of IB were not recovered from sicklepod, barnyardgrass, or large crabgrass, and only microsclerotia were recovered from itchgrass and purple nutsedge in the first study. Sclerotia/microsclerotia and mycelia of IA and IB were recovered from all weed species in the second study. R. solani spread from at least 6 of 7 infected weed species to a noninfected soybean plant growing in close proximity. Results emphasize the importance of weed control, not only for reducing plant competition, but also for the potential impact on RFB development.