Phialophora gregata f.sp. sojae IGS-genotype B and the development of brown stem rot of soybean

The causal agent of soybean brown stem rot (BSR) is the soil-borne fungal pathogen Phialophora gregata fsp. sojae (Pgs). Two genotypes of Pgs, A and B, are now recognized. The goal of this research was to determine the influence of genotype B on BSR development and soybean production. A multiplex, quantitative real-time PCR (qPCR) assay was developed that allowed for the quantification and differentiation of A and B in soil and host tissue. The influence of genotype B on soybean yield was assessed in field microplots that were non-infested or infested with A, B, or both A and B. In the presence of B, soybean yields were reduced as much as or more than soybean yield in the presence of A alone. The degree of yield loss was dependent on the genetics of the soybean accession. The influence of soybean acession on genotype of Pgs was assessed by monoculturing BSR-resistant and susceptible accessions in an area never previously planted to soybean. Pgs populations were evaluated by isolation of Pgs from stems and genotyping through standard PCR and by qPCR of DNA extracted from soil and stem samples. Pgs genotype B was isolated at a higher frequency (∼4:1) than A from the monoculture of a BSR-resistant accession while isolates of A and B were isolated at a similar frequency (1:1) from the monoculture of a BSR-susceptible accession. Similar results were obtained with qPCR except susceptible accession had a higher population of A than B. Pgs soil populations were slightly larger following a susceptible soybean monoculture than a resistant soybean monoculture. Overall, Pgs soil populations had a higher population of B than A. Isolates of Pgs genotype B from these monocultures were screened for aggressiveness against a resistant and susceptible soybean accession. Isolates obtained from susceptible or resistant monocultures were more aggressive than isolates from a mixed soybean monoculture. The resistant accession expressed greater symptoms severity than the susceptible accession when inoculated with B. Results of this research suggest Pgs genotype B influences soybean production and should be included in future research and breeding programs.