Effects of Cropping Systems on Population Density of Heterodera glycines and Soybean Yield. S. R. Koenning, Extension Specialist, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. D. P. Schmitt, and K. R. Barker. Former Professor, and Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Plant Dis. 77:780-786. Accepted for publication 15 March 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/PD-77-0780.

We evaluated the effects of soybean planting date and maturity group (MG) on final population density of Heterodera glycines and yield in cultivars susceptible to H. glycines grown in monoculture and in rotations with 1 or 2 yr of nonhosts. Population density of eggs and eggs plus second-stage juveniles of H. glycines declined to barely detectable levels after 2 yr of nonhost culture. Population densities of this nematode were consistently greater (P = 0.05) for an MG VII cultivar than an MG V cultivar. Planting date had variable effects on final population density of H. glycines: early planting resulted in the highest nematode numbers in some years, whereas late planting was associated with significantly greater (P= 0.05) population densities in other years. Yields were higher (P = 0.01) when soybean followed 1 or 2 yr of nonhosts than in monoculture. Yields of an MG V cultivar grown in succession with an MG V cultivar were greater than those of an MG VII cultivar that followed an MG VII cultivar, either in monoculture or in a 1-yr rotation with a nonhost. The higher yield of the earlier-maturing MG V cultivar was the result of low equilibrium densities of H. glycines associated with a shorter growing season. Late planting (June) generally resulted in lower soybean yield than early (May) planting. Yield of an MG V cultivar planted late was generally equivalent to that of an MG VII cultivar planted in May when H. glycines was at damaging levels.