GENETIC-ANALYSIS OF THE INHERITANCE OF BROWN STEM ROT RESISTANCE IN THE SOYBEAN CULTIVAR ASGROW-A3733

Disease resistance is an important attribute for soybean [Glycine max (L.) Merr.] cultivars. Diversification of resistance employed to control disease is necessary to provide yield stability against pathogen variability. Cultivars currently registered as resistant to brown stem rot (BSR), caused by Phialophora gregata (Allington and Chamberlain) W. Gams, derive their resistance from Pl 84.946-2. The BSR resistance found in Asgrow A3733 is not derived from known sources of resistance. The progeny of three crosses involving A3733 and susceptible parents (Pella 86 for cross 1, CenturY 84 for cross 2, and Pl 437.654 for cross 3) were evaluated in the greenhouse for BSR reaction. The wide range of disease reactions for leaf symptoms suggests that more than one gene is involved in BSR resistance in A3733. A two-gene model, based on F3 family data from crosses 1 and 2, was proposed, with the A allele being the major gene conferring BSR resistance and the B allele acting as a modifier or minor gene. Data from cross 3 indicates the susceptible parent, Pl 437.654, may be lacking a third additional allele found in A3733 and possibly in Pella 86 and Century 84 that affects disease development. Progress in selection of resistant genotypes from A3733 x BSR-susceptible crosses will be greater based on F3 family means than with F2 plant selection. The cultivar A3733 may provide a useful and unique source of resistance to BSR.