Biological interactions among soybean cyst nematode, herbicide injury, and insect defoliation in soybean

Field experiments were conducted during 1988-1991 for quantifying interactions among soybean, Glycine max, stresses from soybean cyst nematode (SCN), Heterodera glycines, acifluorfen plus bentazon herbicides, and green cloverworm (GCW), Plathypena scabra (Lepidoptera: Noctuidae). SCN and soybean responses were evaluated. Treatments were combinations of SCN density, acifluorfen plus bentazon rate, and simulated GCW defoliation level. A method developed for infesting soil with an SCN-root-soil inoculum was used in 1990 and 1991. Herbicide applications and defoliation were imposed at V6 and R2 to R4 stages of soybean development, respectively. Herbicides always stressed soybean, as indicated by visual evaluations of foliar injury. Likewise, defoliation reduced soybean leaf area in 1989, 1990, and 1991. No determinations of leaf area were made in 1988. Herbicides consistently decreased SCN soil densities, although applications never increased soybean seed yield. SCN soil densities generally were unaffected by defoliation. Because of extreme confounding from iron deficiency chlorosis and drought in 1988, data for that year were not used for evaluations of soybean response. Herbicides and defoliation limited soybean growth and yield in 1990 and 1991. Excessive injury from SCN likely precluded similar responses in 1989. Preharvest growth parameters reduced by herbicides included leaf area, plant height, pod number, and leaf, pod, and support (stem and petiole) dry weights; while defoliation reduced plant height and leaf dry weight. Moreover, herbicides and defoliation interacted to reduce seed yield over 1990 and 1991. Yield reductions primarily were attributed to decreases in pod number (defoliation) and weight per seed (herbicides). When desired stress from SCN was achieved (1990), interaction with herbicides decreased leaf stomatal conductance, increased visual crop injury, and reduced preharvest growth (leaf area, leaf number, pod number, and pod weight) and seed yield. Additionally, SCN and defoliation interacted in 1990, reducing seed yield. Canopy interception of photosynthetically active radiation was a key mechanism underlying yield reductions from SCN, herbicides, and defoliation. Management recommendations, including adjusted economic decision levels for GCW, were made to deal with SCN x herbicides, SCN x defoliation, and herbicides x defoliation effects