Submitted to: Journal of Nematology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/14/2002
Publication Date: 12/1/2002
Citation: Niblack, T.L., Arelli, P.R., Noel, G.R., Opperman, C.H., Orf, J.H. 2002. A revised classification scheme for genetically diverse populations of heterodera glycines. Journal of Nematology. 34(4):279-288. Interpretive Summary: The soybean cyst nematode (SCN) is a major yield-limiting pathogen of soybean production areas worldwide. Field populations of SCN exhibit diversity in their ability to develop on resistant soybean cultivars. Currently, this diversity has been characterized by a greenhouse evaluation used to assign a race classification, however, the race classification is inadequate to describe the genetic variation in the field populations of the nematode. The race scheme is being evolved to include HG type test to provide many new features valuable to the soybean community, including cultivar recommendations to growers. The HG type will indicate the sources of resistance that will be effective or ineffective against the tested field populations. The system is simple to interpret HG type designation and eliminates redundant indicator hosts. It is open-ended to include new sources of resistance deployed for developing soybean germplasm. Importantly, the new system avoids use of potentially unwieldy numerical designations to classify nematode races. This information is important for extension agents to make recommendations to growers on what resistant cultivars to plant in their fields to control this pest.
Technical Abstract: 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. However, the race scheme also has been misapplied as a means of studying H. glycines genotypes. 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. 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 is described which avoids the implication of genetic uniformity or predictability in contrast to the way the race scheme has been used.