Growth of Soybean Stressed by Nematodes, Herbicides, and Simulated Insect Defoliation

Soybean [Glycine max (L.) Merr.] responses to combinations of stress factors generally are unknown. Research was conducted in 1989, 1990, and 1991 to quantify growth responses to stresses from soybean cyst nematode (SCN; Heterodera glycines Ichinohe), acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid} plus bentazon [3-(l-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] herbicides, and simulated green cloverworm [GCW; Plathypena scabra (F.)] defoliation. Treatments were combinations of at-planting SCN soil density, acifluorfen plus bentazon rate, and simulated GCW defoliation level. Herbicides were applied at V6 soybean development, and GCW feeding was simulated from R2 to R4 soybean development. Plant growth was quantified at V4, R2, and R4 developmental stages. Herbicide stress was assessed by leaf stomatal conductance and visible foliar injury. Herbicides reduced conductance and caused visible injury each year, and limited growth (plant height, leaf area, pod number, and dry weight of leaf, pod, and stem plus petiole) in 1990 and 1991. Likewise, defoliation reduced leaf area each year, but reduced growth (plant height and leaf dry weight) in 1990 and 1991 only. Uncontrolled environmental factors probably confounded growth responses in 1989. Although no herbicide × defoliation effects on growth were found, defoliation caused greater reductions in canopy quantity and quality for herbicide-injured plants in 1990 and 1991. Targeted stress from SCN was achieved in 1990, and interaction with herbicides decreased conductance, increased visible herbicide injury, and reduced leaf area, plant height, pod number, and pod dry weight. These data support the potential for stresses from SCN and acifluorfen plus bentazon to cause more-than-additive reductions in soybean growth. Journal Paper no. J-15442 of the Iowa Agric. Exp. Stn., Projects 2285, 2580, 2871, and 2903.