Phomopsis Seed Decay and Nutrient Accumulation in Soybean under Two Soil Moisture Levels

Infection of soybean [Glycine max (L.) Merr.] by Phomopsis longicolla Hobbs is associated with physiological changes that occur during senescence. The purpose of this research was to determine the relationship of carbohydrate and mineral nutrient composition of pod tissue to seed infection by P. longicolla during senescence and maturity. Soybean pod nutrient levels were altered by N deficiency and water stress by growing nodulating and non-nodulating isolines of soybean on a Typic Fragiudalf soil at high and low soil moisture (approximately 100 and 1500 kPa tension, respectively) and with high and low N (75 kg ha⁻¹ and 0 kg ha⁻¹, respectively). Nitrogen fertilizer-isoline treatments affected plant growth and yield but not seed infection by P. longicolla or seed quality. Physiological maturity occurred 1 to 2 weeks earlier in plants grown under low compared to high soil moisture. Seed produced under low soil moisture exhibited less infection by P. longicolla (18 vs. 58% maximum) and greater germinability (66 vs. 47% minimum) than seed from high moisture soil. Total nonstructural carbohydrates (TNC), N, and P were greater in pod walls from plants grown on high compared to low moisture soil. A lower incidence of P. longicolla in upper nodes (3–32%) compared to lower nodes (13–58%) of plants was associated with higher germination levels, lower K, and greater Ca and TNC levels in pod walls. Decreases in seed germination of 14%, 20 weeks after harvest maturity in 1981, and 7%, 8 weeks after harvest maturity in 1982, were only associated with increased seed infection by P. longicolla (10–15%) in 1982. This difference in P. longicolla between years due to harvest delay was associated with higher levels of TNC in pod walls from high soil moisture treatments in 1982 (86–145 vs. 55–58 g kg⁻¹ dry weight). Reducing seed infection by P. longicolla might be achieved by preventing pod wall accumulation of TNC, N, and P, increasing pod wall accumulation of Ca, and shortening the time from physiological maturity to harvest maturity.