Cultivar preference exhibited by two sympatric and genetically distinct populations of the soybean fungal pathogen Phialophora gregata f.sp. sojae

Abstract

Phialophora gregata f.sp. sojae, a soilborne vascular pathogen causing brown stem rot of soybean, has been divided into A and B populations based on variation in the intergenic spacer region of nuclear rDNA (rDNA marker). The A and B populations correlate with defoliating and nondefoliating pathotypes, respectively. In this study, eight additional polymorphic anonymous marker loci (five inter simple sequence repeat loci and three long‐primer random amplified polymorphic DNA loci) were identified and applied to a total of 189 isolates. Alleles of these eight loci were invariant within, but different between the A and B populations, providing further evidence that the rDNA marker identifies genetically distinct populations. The two populations were sympatric, residing not only in the same field, but also in the same plants under field conditions. Representative strains of the two populations, when used individually in inoculations, infected both resistant cv. Bell and susceptible cv. Sturdy. However, when the same representatives of the two populations were mixed in a 1 : 1 ratio and used as a mixed inoculum in a competitive bioassay, differential cultivar preference was revealed using PCR detection of populations in infected plants. Population A was detected significantly more often (18 out of 24 plants) in the susceptible cv. Sturdy, whereas population B was detected significantly more often (17 out of 24 plants) in the resistant cv. Bell, corroborating earlier field studies. This is the first controlled experiment to demonstrate a differential host preference of P. gregata f.sp. sojae toward different cultivars of the same host species. Unification of terminologies used in P. gregata f.sp. sojae is discussed.