Submitted to: Plant Pathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/18/2004
Publication Date: 1/1/2005
Citation: Meng, X.Q., Grau, C., Chen, W. 2005. Cultivar preference exhibited by two sympatric and genetically distinct populations of the soybean fungal pathogen phialophora gregata f.sp.sojae. Plant Pathology. 54:180-188. Interpretive Summary: Brown stem rot of soybean is an important disease causing 15 to 40% yield loss in affected areas. The pathogen Phialophora gregata is separated into two genotypes A and B. This research was carried out to determine the genetic differentiation between the two genotypes and to determine their differential cultivar preference. Results showed that the two genotypes are completely genetically isolated and there were no genetic recombination between them. Furthermore, data of controlled experoments demonstrated that the two genotypes have differential cultivar preference. Genotype A showed preference to susceptible cultivar Sturdy, whereas genotype B showed preference to resistant cultivar Bell. The implications of the cultivar preference in managing brown stem rot were discussed.
Technical Abstract: Phialophora gregata f. sp. sojae, a soilborne vascular pathogen causing brown stem rot of soybean, has been subdivided into A and B genotypes based on variation in the intergenic spacer region of nuclear rDNA. The A and B genotypes correlate with defoliating and non-defoliating pathotypes, respectively. In this study, we identified eight additional polymorphic anonymous marker loci (five inter-simple sequence repeat loci and three long-primer random amplified polymorphic DNA loci) and applied them to a total of 189 isolates. Alleles of these eight loci were invariant within, but different between the rDNA genotypes, suggesting a lack of gene flow between the two populations. Representative strains of the two genotypes, when used individually in inoculations, infected both resistant Bell and susceptible Sturdy cultivars. However, when the same representatives of the two genotypes were mixed at a 1:1 ratio and used as a mixed inoculum in a competitive bioassay, differential cultivar preference was revealed using qualitative PCR detection of genotypes in infected plants. Genotype A was detected more often in the susceptible cv. Sturdy, whereas genotype B was detected more often in the resistant cv. Bell . This is the first controlled experiment to demonstrate a differential cultivar preference of P. gregata f. sp. sojae and corroborates results from field studies.