A Revised Classification Scheme for Genetically Diverse Populations of Heterodera glycines.

Heterodera glycines, the soybean cyst nematode, is a major yield-limiting pathogen in most soybean production areas worldwide. Field populations of H. glycines exhibit diversity in their ability to develop on resistant soybean cultivars. Since 1970, this diversity has been characterized by a bioassay used to assign a race classification to a population. The value of the race scheme is reflected in the number and quality of resistant soybean cultivars that have been developed and released by soybean breeders and nematologists working in concert. However, the race scheme also has been misapplied as a means of studying H. glycines genotypes, in part due to the use of the term "race." For fungal and bacterial pathogen species, "race" can theoretically be applied to individuals of a population, thus allowing inference of individual genotypes. Application of a race designation to an individual egg or second-stage juvenile (J2) of H. glycines is not possible because a single J2 cannot be tested on multiple hosts. For other nematode species, "race" is defined by host ranges involving different plant species, whereas the H. glycines race test involves a set of lines of the same plant species. Nonetheless, because H. glycines populations vary in genetic diversity, and this variation has implications for management strategies, a mechanism is needed for documenting and discussing population differences. The HG Type scheme described herein avoids the implication of genetic uniformity or predictability in contrast to the way the race scheme has been used.