Identifying New Sources of Resistance to Brown Stem Rot in Soybean

Breeding for pathogen resistance is an important objective to improve and protect soybean yields. In 2010, 14.4% of total soybean yield was suppressed by diseases. Brown stem rot (BSR), caused by the fungus Phialophora gregata, reduces yield up to 38%. To date, three dominant BSR resistance genes have been identified: Rbs1, Rbs2, and Rbs3. Other resistance loci might be present within the soybean germplasm collection. This research was conducted to determine the inheritance of BSR resistance in soybean accessions PI 594858B, PI 594637, PI 594638B, and PI 594650A crossed to the three known resistance genes. BSR severity was assessed in growth chambers 5 weeks after inoculation based on three measurements: 1) plant vigor, 2) incidence of discoloration and 3) recovery of P. gregata. Allelism tests from F_2:3 plants of PI 594638B, PI 594858B and PI 594650A fit a 15:1 segregation ratio, indicating we have identified at least one novel source of BSR resistance. Molecular characterization of these populations is in progress. Whole plant microscopy was conducted and P. gregatahyphae were localized in xylem vessels of all three resistant cultivars, confirming that current resistance genes only offer partial resistance to BSR. Xylem vessels of the susceptible cultivar were more degraded and broken than those of the resistant, degradation of cells is a common effect of a toxin. We are conducting RNA-seq of BSR infected and mock infected leaf, stem, and root tissue of both a resistant and a susceptible genotype. This will provide additional information about mechanisms of BSR resistance.