Cultivar Preference and Genotype Distribution of the Brown Stem Rot Pathogen Phialophora gregata in the Midwestern United States

Abstract

Brown stem rot (BSR), caused by Phialophora gregata f. sp. sojae, is an important yield-limiting disease of soybean (Glycine max) in the midwestern United States. Midwestern populations of P. gregata are separated into genotypes A and B based on intergenic spacer sequences of nuclear ribosomal DNA. Genotype A causes both leaf and stem symptoms, and genotype B typically causes internal stem symptoms only. Data are limited on the geographic distribution of genotypes A and B. It is not well understood whether cultivars may be infected preferentially by a genotype. Field plots were established at five sites in Illinois, three sites in Wisconsin, and two sites in Minnesota in two different years. Soybean cvs. Bell, BSR101, Dwight, Sturdy, Williams 82, LN92-12033, and LN92-12054 were sown with two to four replications at each field site. From each plot, 5 to 10 stems were harvested arbitrarily at the R8 growth stage and assayed by polymerase chain reaction to detect the A and B genotypes. Both pathogen genotypes were detected at all locations except Urbana, where only genotype A was detected, and St. Paul, where only B was detected. Genotype A was the predominant genotype detected in susceptible cvs. Williams 82 and LN92-12054, with 70 and 78% of infected stems, respectively, positive for A. The other susceptible cultivar, Sturdy, yielded predominantly genotype A at four of the seven Illinois and Wisconsin locations where both pathogen genotypes were present, but yielded predominantly B at the Minnesota location where both genotypes were detected. Genotype B was the predominant type detected in partially resistant cvs. Dwight, LN92-12033, and Bell, with 56, 85, and 99% of the infected stems, respectively, testing positive for B.

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

“…Bell (Table 5). The results confirmed previous reports of field experiments (Chen et al ., 2000; Malvick et al ., 2003). Some inconsistencies observed among field experimental sites (Malvick et al ., 2003) were probably caused by different relative proportions of the two populations in the natural inoculum present in the fields.…”

“…It is slow‐growing in culture and is thought to be a poor saprophytic competitor (Adee & Grau, 1997). It causes stem and foliar symptoms that may not always occur together, depending on pathogen isolate, host genotype, and environmental conditions (Gray, 1971; Mengistu et al ., 1986; Willmot et al ., 1989; Hughes et al ., 2002; Malvick et al ., 2003). Stem symptoms include brown discoloration of the pith and vascular tissue; foliar symptoms include interveinal necrosis and defoliation (Gray & Grau, 1999).…”

“…sojae is divided into A and B populations using a codominant genetic marker located in the intergenic spacer region of rDNA (rDNA marker) (Chen et al ., 2000). This rDNA marker can be used not only to place isolates in populations, but also to detect strains of either population in infected plants without obtaining pure cultures (Chen et al ., 2000; Malvick et al ., 2003). The two populations also differ at three microsatellite markers defined by three pairs of specific primers (Chen et al ., 2002).…”

“…In addition, because of their close proximity, the two populations may exchange genetic materials through anastomoses (Leslie, 1993), blurring genetic distinctiveness and undermining the usefulness of the rDNA marker. The rDNA marker is playing increasingly important roles in studies of P. gregata and soybean– P. gregata interactions (Malvick et al ., 2003; C.R.G., unpublished data). In order to further ascertain that the rDNA marker identifies genetically distinct populations, additional DNA markers, generated using techniques that randomly sample variation of the whole genome, are needed.…”

“…Field isolation data showed that most isolates obtained from susceptible cultivars belong to population A, whereas most isolates obtained from resistant cultivars belong to population B (Chen et al ., 2000). Malvick et al . (2003) used seven soybean germplasm lines in replicated plots at 10 locations across three states to show that several germplasm lines are preferentially infected by either one of the two populations.…”