Impact of Population Density of Heterodera glycines on Soybean Canopy Growth and Weed Competition. D. G. Alston, Assistant Professor, Department of Biology, Utah State University, Logan 84322-5305. J. R. Bradley, Jr., H. D. Coble, and D. P. Schmitt. Professor, Department of Entomology, Professor, Department of Crop Science, North Carolina State University, Raleigh 27695; and Professor, Department of Plant Pathology, University of Hawaii, Honolulu 96822. Plant Dis. 75:1016-1018. Accepted for publication 9 April 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-1016.

Soybean (Glycine max) canopy size and subsequent growth of annual weeds were influenced by initial population density (Pi) of the soybean cyst nematode (SCN), Heterodera glycines, in eastern North Carolina field studies. Soybean biomass accumulation and canopy size were inversely related to increasing Pi of SCN, resulting in open canopy soybeans at moderate and high Pi levels. More photosynthetically active solar radiation reached the soil surface in high-SCN, open-canopy soybeans than in low-SCN treatments. Weed biomass was 63–92% greater in soybeans with the highest SCN Pi levels and open canopies.