Genetic Gain X Management Interactions In Soybean: IV Disease Effects.

U.S. soybean [Glycine max (L.) Merr.] yields have increased at a yearly rate of 23.4 kg ha^-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 not well understood.  A comprehensive four-state study was conducted in Illinois, Indiana, Wisconsin, and Minnesota to test the relative contribution of four management practices; seeding rate, nitrogen use, planting date, and fungicide use.  The objective of this component of the larger study was to compare yield across year of release attributed to greater seedling and foliar disease tolerance.  One hundred and sixteen varieties in maturity groups II and III were intensively managed for disease with a seed treatment and foliar fungicides.  Preliminary results show the rate of yield gain has not changed as a result of disease management practices even though the treatment showed a significant reduction in disease severity.  Treated varieties had a 13% higher plant density among older varieties and a 3% higher plant density among new varieties.  Newer varieties had 11% lower disease incidence than older varieties.  This likely indicates the effectiveness of disease resistance genes in newer varieties.  These results suggest the rate of yield gain has not changed due to chemical disease management.  Rather, durable disease tolerance remains the goal for reducing yield lost due to disease.