Population Characteristics of Heterodera glycines in Iowa. M. E. Zirakparvar, Post-Doctoral Associate, Department of Plant Pathology, Seed and Weed Sciences, Iowa State University, Ames 50011. Don C. Norton, Professor, Department of Plant Pathology, Seed and Weed Sciences, Iowa State University, Ames 50011. Plant Dis. 65:807-809. Accepted for publication 2 February 1981. Copyright 1981 American Phytopathological Society. DOI: 10.1094/PD-65-807.

Four field populations and three populations of Heterodera glycines from single cysts were tested for ability to reproduce on six soybean differentials. The three populations from single cysts and one field population were classified as race 3. However, three field populations gave new patterns of response. After race identification of field population 2, cysts that developed on resistant cultivars were used to study changes in parasitism through selection on the resistant host from which they were obtained. The index of parasitism increased from 3.6 and 1.3 to 75.6 and 69.1 in two generations for larvae and eggs from cysts produced on resistant Pickett 71 and PI 88788, respectively. Four generations were required for larvae and eggs from cysts produced on resistant Peking and PI 90763 to increases their indexes of parasitism from 0.1 to 38.9 and 18.2, respectively. This study suggested that three groups of genes for parasitism on resistant cultivars are present in one field population and that the race status of the population may change in response to selection forces.