Soybean Yield Losses Due to Heterodera glycines in Iowa. T. L. Niblack, Department of Plant Pathology, University of Missouri, Columbia 65211. N. K. Baker, and D. C. Norton. Department of Plant Pathology, Iowa State University, Ames 50011. Plant Dis. 76:943-948. Accepted for publication 20 April 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0943.

Natural field infestations of Heterodera glycines race 3 in Boone (central Iowa) and Hancock (north central) counties were used to investigate the effect of the nematode on soybean growth and seed yields in a 2-yr study. Iron deficiency chlorosis, brown stem rot, and Phytophthora rot cause soybean seed yield losses annually in Iowa and may occur in H. glycines-infested fields. Soybean cultivars with known reactions to each disease were chosen, and two plots of each cultivar were planted per replication—one treated with aldicarb for H. glycines control, and the other not treated. Cultivar reaction to other diseases had no effect on H. glycines final populations or soybean seed yields. Seed yields of H. glycines-susceptible cultivars were 5.7–35.8% lower than those of resistant cultivars in all environments and were affected by treatment with aldicarb in 1987 but not in 1986. Artificial infestations of H. glycines were established in microplots in a field in Story County (central Iowa) to determine damage thresholds. One resistant and one susceptible cultivar were planted following infestation with 0, 10, 50, 250, or 1,250 eggs per 100 cm³ of soil. In 1986 and 1987, the H. glycines density explained 92 and 54%, respectively, of the reduction in seed yields of the susceptible cultivar. The damage threshold was between 10 and 50 eggs, and yields were reduced 52 and 19% at the highest egg density in 1986 and 1987, respectively.