Classification methods and identification of reniform nematode resistance in known soybean cyst nematode resistant soybean genotypes.

Plant parasitic nematodes are a major yield-limiting factor of soybean in the United States and Canada. It has been indicated that soybean cyst nematode (SCN, Heterodera glycines Ichinohe) and reniform nematode (RN, Rotylenchulus reniformis Linford and Oliveira) resistance could be genetically related. For many years fragmentary data has shown this relationship. This report evaluates RN reproduction on 418 plant introductions (PIs) selected from the USDA Soybean Germplasm Collection with reported SCN resistance. The germplasm was divided into two tests of 214 PIs reported as resistant, and 204 PIs moderately resistant to SCN. The defining and reporting of RN resistance changed several times in the last 30 years, causing inconsistencies in RN resistance classification among multiple experiments. Comparison of four RN resistance classification methods was performed: (1) ≤10% as compared to the susceptible check, (2) using normalized reproduction index (RI) values, and transformed data (3) log10 (x) and (4) log10 (x+1), in an optimal univariate k-means clustering analysis. The method of transformed data log10 (x) was selected as the most accurate for classification of RN resistance. Among 418 PIs with reported SCN resistance, the log10 (x) method grouped 59 PIs (15%) as resistant, and 130 PIs (31%) as moderately resistant to RN. Genotyping of a subset of the most resistant PIs to both nematode species revealed their strong correlation with rhg1-a allele. This research identified genotypes with resistance to two nematode species and potential new sources of RN resistance that could be valuable to breeders in developing resistant cultivars.