GROWTH OF SOYBEAN STRESSED BY NEMATODES, HERBICIDES, AND SIMULATED INSECT DEFOLIATION

Soybean [Glycine mar (L.) Merr.] responses to combinations of stress f actors generally are unknown. Research was conducted in 1989, 1990 and 1991 to quantify growth responses to stresses from soybean cyst nemat ode (SCN; Heterodera glycines Ichinohe), acifluorfen 2-chloro-4-(trifl uoromethyl)phenoxy]-tritrobenzoic acid} plus bentazon methyl-ethyl)-(1 H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] herbicides, and simula ted green cloverworm [GCW; Plathypena scabra (F.)] defoliation. Treatm ents were combinations of at-planting SCN soil density; acifluorfen pl us 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 sto matal conductance and visible foliar injury. Herbicides reduced conduc tance and caused visible injury each year, and Limited growth (plant h eight, leaf area, Fled number, and drg weight of leaf, pod, and stem p lus petiole) in 1990 and 1991. Likewise, defoliation reduced leaf area each S-ear, but reduced growth (plant height and leaf dry weight) in 1990 and 1991 only. Uncontrolled environmental factors probably confou nded growth responses; in 1989. Although no herbicide x defoliation ef fects on growth were found, defoliation caused greater reductions in c anopy quantity and quality for herbicide-injured plants in 1990 and 19 91. 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. T hese data support the potential for stresses from SCN and acifluorfen plus bentazon to cause more-than-additive reductions in soybean growth .