Soybean [Glycine mar (L.) Merr.] production for greatest yield involves intensive management of resource inputs. Proper cultivar selection, irrigation, and P application on P-deficient soils have all been shown to increase yields. Nitrogen applications have resulted in variable yield effects. Under sprinkler irrigation, several diseases caused by species of Diaporthe [D. phaseolorum var. sojae (Lehman Wehm., and D. phaseolorum var. caulivora Athow. G Caldwell.] and Phomopsis longicolla Hobbs, the Diaporthe/Phomopsis complex may also be yield limiting. This disease complex includes pod and stem blight (PSB), Phomopsis seed decay (PSD), and stem canker, In order to determine the combined influence of cultivar, irrigation level, fertility level, and disease control, field experiments were conducted in 1986, 1987, and 1988 at the South Central Research and Extension Center new Clay Center, NE to evaluate these possible soybean yield limiting factors. All combinations of two levels of benomyl [methyl-l-(butylcarlbamoyl)-2-benzimadazole carbamate] fungicide (0 and 1.12 kg ha−1 applied at R3 and R5), two levels of N (0 and 134.4 kg ha−1) and P (0 and 67.2 kg ha−1), and two cultivars (determinate Hoyt and indeterminate Zane) were tested at two nonreplicated levels of irrigation. Nitrogen and P application seldom increased seed yield. A yield increase (517 kg ha−1, 1986; 396 kg ha−1, 1987; 215 kg ba−1, 1988) occurred when benomyl was applied to Hoyt soybean under a high level of irrigation. Less of a yield response to benomyl was obtained with later-maturing Zane, for both cultivars at the lower irrigation level, and during a year of below normal rainfall. Benomyl application decreased the incidence of PSB and increased seed size, but did not consistently affect PSD based on seed appearance. Stem canker was never observed during the course of this study. Yield increases were primarily the result of increased seed size associated with delayed maturity doe to PSB control and increased number of seeds per pod due to benomyl's possible growth regulatory properties. This research indicates that, contrary to previous reports in the northern USA, pod and stem blight can be a yield-limiting factor with or without the development of Phomopsis seed decay under high-yield environments.
Contribution of Dep. of Agronomy, Univ. of Nebraska, Lincoln, NE. Published as Paper no. 9137, Journal Series, Nebraska Agric. Res. Div. Project no. 48-009.