Identifying New Sources of Resistance to Brown Stem Rot in Soybean.

Breeding for resistance to diseases and pests is an important objective to improve and protect soybean yields. In 2010, 14.4% of total soybean yield valued at $5.59 billion was suppressed by pathogenic diseases caused by fungus, microbes, and nematodes. Brown stem rot (BSR), caused by the fungus Phialophora gregata, may cause yield loss of up to 38%. To date, three dominant BSR resistant genes have been identified: Rbs1, Rbs2, and Rbs3. In addition, there may be other loci in the soybean genome contributing to BSR resistance. This research was conducted to determine the inheritance of BSR resistance in soybean accessions PI 594858B, PI 594637, PI 594638B, and PI 594650A when crossed to the three known sources of resistance. BSR severity was assessed in growth chambers five weeks after inoculation based on three measurements: plant vigor, incidence of discoloration, and recovery of P. gregata.  Allelism tests from F_2:3 plants from PI 594638B, PI 594858B and PI 594650A fit a 15:1 segregation ratio, indicating non-allelism to Rbs1, Rbs2, and Rbs3. There might be more genes in these accessions conferring resistance to the fungus. Progeny from PI 594637 segregate in a 3:1 ratio, indicating that PI 594637 could be susceptible to BSR.  Molecular characterization of the segregating populations obtained with these accessions is in progress, and will provide additional information about resistance to BSR.