Field experiments in 1988–1990 determined the (1) effects of defoliation by Pseudoplusia includens (Walker) (soybean looper) or basal stem girdling by Spissistilus festinus (Say) (threecornered alfalfa hopper) on soybean stem canker severity and (2) effects of the soybean looper defoliation on red crown rot incidence. Compared to ungirdled plants, stem canker severity was increased while stem length, stem diameter, and yield were reduced for girdled plants. Stem canker severity, number of perithecia, yield, stem length, and stem diameter were reduced on defoliated soybean, compared to undefoliated soybean. Red crown rot incidence, perithecial production, and yields were reduced on defoliated soybean compared to undefoliated soybean. This suggests different types of insect injury may either increase or decrease the severity of soybean diseases.

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Acifluorfen, alachlor, glufosinate, glyphosate, paraquat, and pendimethalin were evaluated for their effects on mycelial growth and sclerotia/microsclerotia production byRhizoctonia solaniAG-1 IA and IB in culture. All of these herbicides except glufosinate and glyphosate were evaluated for effects on severity of Rhizoctonia foliar blight of soybean in the field. In laboratory studies, all herbicides reduced colony radius ofR. solani.Growth reductions for IB were greater than for IA in the presence of pendimethalin, alachlor, and acifluorfen, but glufosinate reduced growth of IA more than IB. Sclerotia production by both isolates was prevented by paraquat, greatly reduced by glufosinate, but markedly less affected by the other herbicides tested. In field studies, all tested herbicides influenced severity of Rhizoctonia foliar blight when disease pressure was low, but only paraquat reduced severity when disease pressure was high.