Soybean and Phialophora gregata interactions: Implications for evaluating resistance to brown stem rot of soybean
Resistance of soybean to brown stem rot (BSR), caused by Phialophora gregata (Allington and Chamberlain) W. Gams, has become an important aspect of soybean breeding programs in the north central United States. The discovery of variability of P. gregata isolates from different states has increased the interest in BSR research. The objectives of this study were to further investigate interactions between P. gregata and soybean by studying: (1) the inheritance of resistance to BSR in the soybean cultivar Asgrow A3733, (2) the effects of culturing the fungus on different sources of soybean media, and (3) the effects of field fertility levels on BSR development on soybean. The inheritance study showed a different genetic control for resistance to BSR than has been previously reported. The relatively large number of intermediate disease reaction types in progeny from crosses involving A3733 and susceptible parents indicates the presence of a minor or modifier gene. A genetic model was proposed based on one gene with a major effect (probably dominant) and a second gene with a minor effect for BSR resistance. This appears to be a unique source of resistance to BSR that can be incorporated into breeding programs along with other known sources of resistance. Culturing P. gregata on soybean stem agar and seed broth prepared from BSR-resistant or -susceptible soybean produced varying results. The use of certain resistant soybean as media source caused the pathogen to be much less aggressive. When susceptible soybean were used to prepare media for inoculum production, typical disease reactions were observed for BSR-resistant and susceptible soybean. Soybean plants grown on a low field fertility level had greater development of BSR stem symptoms on BSR-susceptible and resistant soybean as compared to being grown on a field with a high fertility level. Leaf symptoms are not observed every year and may appear too late in the growing season to be rated on early maturing soybean plants. Differences between resistant and susceptible soybean genotypes were greater in low fertility. Selection for BSR resistant plants in the field may be enhanced with conditions of low fertility.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign