Pathogenic Characterization of Genotypes A and B of Phialophora gregata f. sp. sojae

Genetic studies of Phialophora gregata f. sp. sojae, the causal agent of brown stem rot (BSR) of soybean, have led to the development of species-specific primers capable of separating isolates into two distinct genotypes, A and B. To determine whether genotypic characterization could be related to differences in BSR symptom expression, five soybean cultivars, Pioneer 9234, Corsoy 79 (both BSR susceptible), Williams, BSR 101, and Jack and plant introduction (PI) 437970 (all BSR resistant), were inoculated with a total of 27 isolates of each genotype in four greenhouse experiments conducted from February to November 2000. BSR severity was calculated as the percentage of symptomatic foliar, internal stem, and internal root tissue. Genotype A isolates caused significantly more severe (P < 0.0001) BSR foliar symptoms than genotype B isolates on Pioneer 9234, Corsoy 79, Williams, and BSR 101, while Jack and PI 437970 expressed minimal foliar symptoms regardless of isolate genotype. Overall, internal stem symptoms caused by genotype A isolates were more severe than those caused by genotype B isolates on Pioneer 9234, Corsoy 79, Williams, and BSR 101. Conversely, Jack and PI 437970 did not differ significantly in severity of stem symptoms when inoculated with isolates of genotype A or B. Internal root symptoms for genotype A isolates were generally more severe than for genotype B isolates on all soybean genotypes tested. Our data strongly suggest that A and B genotypes of P. gregata f. sp. sojae differ in the severity of symptoms they cause, and that these genotypes correspond to the Type I (defoliating) and Type II (nondefoliating), respectively, pathotypes previously proposed for this vascular pathogen of soybean.