Soilborne pathogens / Agents pathogènes telluriques Association between genotypes of the brown stem rot pathogen Phialophora gregata and resistant and susceptible soybean cultivars in the north- central United States and Ontario

587 Abstract: The severity of brown stem rot (BSR) on soybean (Glycine max) is influenced by environment, genotype of the pathogen, and genetic resistance to BSR. BSR is caused by two genotypes of Phialophora gregata: genotype A causes pith browning and leaf necrosis, and genotype B typically causes only pith browning. The distribution of the genotypes and their interactions with resistant and susceptible soybean cultivars was investigated in multiple field environments. Plots were established with susceptible and resistant cultivars in the states of Illinois, Indiana, Iowa, Kansas, Minnesota, Michigan, Missouri, North Dakota, Ohio, and Wisconsin and in Ontario, Canada. Plants were sampled arbitrarily without regard to BSR symptoms at the R7 growth stage from the plots and from production fields in Illinois, Kansas, North Dakota, Nebraska, and South Dakota. DNA was extracted from stems and tested for the presence of P. gregata using a genotype-specific PCR assay. Phialophora gregata was detected in 26% of all samples, except in Missouri, Nebraska, and North Dakota where it was not detected. Genotypes A, B, and A and B were detected in 19%, 75%, and 6% of the samples confirmed to be infected, respectively, although their relative prevalence varied among locations. Genotype A was detected most frequently in a susceptible cultivar (69% A vs. 52% B), whereas genotype B was predominant in resistant cultivars (82% B vs. 35% A) (P < 0.05). A trend was observed for differential infection of different sources of BSR resistance by the two genotypes of P. gregata. Both genotypes are widespread, and the ability of genotype B to infect without producing leaf symptoms suggests that BSR may affect soybean health more often than is recognized and that optimal management may require resistance to both genotypes.