Preliminary Evaluation of a Molecular Method for Pathotyping Phytophthora sojae

Abstract

Phytophthora sojae, which causes Phytophthora stem and root rot (PSRR) of soybean, is classified into pathotypes based on the interaction of its avirulence (Avr) genes with resistance to P. sojae (Rps) genes present in soybean. Effective management of P. sojae to minimize PSRR in fields under soybean production includes accurate pathotype identification to select soybean cultivars with appropriate Rps resistance. Currently, P. sojae pathotype is evaluated by inoculating the hypocotyls of soybean differentials, which each contain a single Rps gene, with the pathogen to test for virulence. There are various sets of differentials that differ in their genetic background, for example, a Harosoy or Williams background. The hypocotyl inoculation assay requires a significant amount of time and labor before the pathotype of P. sojae can be determined. Recently, a molecular assay that predicts pathotype based on discriminant mutations within seven Avr genes was published. The goal of this study was to evaluate the molecular assay to predict the virulence and thus pathotypes of a subset of isolates of P. sojae in the Robertson Lab collection. A total of 25 isolates from Iowa, Nebraska and Ohio that varied in pathotype were used. Four isolates that were used in the development of the molecular assay were used as checks. The objectives of the study were to: 1. Determine the pathotype of the isolates on Harosoy and Williams sets of differentials using the hypocotyl inoculation assay. 2. Compare the virulence of each isolate on differential lines in a Harosoy background compared to a Williams background. 3. Use the molecular assay to predict the pathotype of each isolate. 4. Compare the predicted pathotype with the pathotype of each isolate determined using the hypocotyl inoculation assay.

For some isolates, virulence varied among the differential lines of Harosoy and Williams carrying the same Rps gene. In fact, the pathotype determined by hypocotyl inoculation assay of the Harosoy and Williams differential lines was identical for only 5 isolates. These data suggest that differentials of a specific Rps gene in distinct genetic backgrounds may actually reflect the activity of distinct Rps alleles or even a different Rps gene that may be tightly linked. While virulence was predicted correctly for some Avr genes of some isolates using the molecular assay, it was not correctly predicted for many others. Absence or presence of the amplicon in the molecular assay was not associated with the actual virulence or avirulence observed on the differentials. These results may reflect that Avr mutations present in the P. sojae isolates tested here differ from those occurring in the isolates used in the development of the molecular assay. Additional evaluation of the molecular assay is recommended before its widespread use for determining the pathotypes of P. sojae present in a population.