Characterizing Reaction of Soybean to Phialophora gregata Using Pathogen Population Density and DNA Quantity in Stems

Evaluation of soybean germplasm for resistance to brown stem rot (BSR) is typically based on symptom severity. However, this approach may not reflect the level of colonization of soybean by the casual agent, Phialophora gregata. A potentially more accurate method to characterize resistance to BSR is to estimate pathogen quantity. The primary goal of this study was to evaluate soybean accessions for resistance to BSR based on the quantity of pathogen in stems. Plants were collected from experiments in field and controlled environments, and CFU and pathogen DNA quantity were determined using dilution plating techniques and real-time quantitative PCR (qPCR), respectively. In the field, the BSR-susceptible cultivars Corsoy 79 and Century 84 expressed greater than 73% foliar and stem symptom severity and had the highest pathogen population density, with a range from log₁₀ 4.3 to 4.7 CFU per gram of stem tissue. The resistant cultivar Bell expressed less than 10% foliar symptom severity, but had a pathogen population density that was not statistically different from the susceptible accessions. CFU measured in Dwight and L84-5873 were consistently lower than CFU in susceptible accessions and several resistant accessions. The amount of pathogen DNA differed among accessions in controlled environments. For example, Corsoy 79 and Century 84 had the highest pathogen DNA quantity, ranging from log₁₀ 6.19 to 6.65 copies, whereas the resistant cultivars Bell, Dwight, and L84-5873 had significantly lower DNA quantities, ranging from log₁₀ 2.04 to 2.91 copies. PI 437833 and IA2008R expressed low symptom severity but contained high DNA quantities. Pella 86, a highly symptomatic cultivar, had fewer CFU and lower DNA quantity in comparison to two other highly symptomatic cultivars and some cultivars with low symptom severity. These results suggest that some accessions express resistance to both pathogen colonization and symptom development, while others are resistant to symptom development but not to pathogen colonization. Results also indicate that resistant and susceptible accessions can be distinguished based on DNA quantity in controlled environments. In the field, differences between the pathogen population in resistant and susceptible cultivars were less distinct, possibly due to when plants were assayed.