Genetic Gain x Management Interactions in Soybean: IV. Disease Effects.

United States soybean [Glycine max (L.) Merr.] yields have increased at an annual rate of 23.4 kg ha-1 yr-1.  These gains have come from a variety of sources including genetic, agronomic, and environmental changes.  While it is assumed that genetic gains through breeding efforts have contributed the most to this increase, the relative contribution of each source of gain is difficult to estimate. The objective of the disease effect component a larger genetic gain by management study was to compare yield across year of release attributed to greater seedling and foliar disease tolerance.  One hundred and sixteen cultivars in maturity groups (MG) II and III were intensively managed for disease with seed treatments and foliar fungicides, and compared to untreated controls.  Fifty-nine MG II cultivars (released 1928-2008) and fifty-seven MG III cultivars (released 1923-2007) were selected to represent a distribution of historically significant cultivars. The annual rate of yield gain was 1.1% and was unaffected by the fungicide treatment.  Treated cultivars produced seed with a 6 g kg-1higher protein and 5 g kg-1 lower oil concentration. Seed number increased 1.0% over time, confirming previous reports that increasing seed number, and not seed mass, has contributed to the observed yield gain over time. Chemical disease management did not influence yield or interact with the rate of yield gain over time.