Funding provided in part by the Kentucky Soybean Promotion Board (KSPB) and the Southern Soybean Research Program (SSRP) has led to the development of an Asian soybean rust (SBR) yield-loss prediction tool.

The tool, developed at the University of Kentucky, utilizes a model that weighs potential yield losses against the cost of fungicidal control in relation to the crop stage. The goal is to help soybean producers make more informed SBR-management decisions, ultimately improving economic returns and limiting the use of unnecessary fungicide applications. The SBR yield-loss tool and model were developed in collaboration with a number of scientists in the U.S. and Brazil.

State Extension plant pathologists formulate general SBR management recommendation based upon three areas: current distribution and severity of SBR, range of local stages of soybean development and disease/weather forecasts. Until now, no tools have existed to help soybean producers make field-specific decisions about the need to spray. The tool is intended to be used in conjunction with the more generalized state-wide or multi-county SBR risk assessments made by state Extension specialists.

In a two-year study conducted in Brazil, SBR-induced yield loss was found to be due to accelerated leaf drop, reduced green leaf area (GLA) and reduced photosynthetic capacity of GLA. Yield reductions were greater the earlier in the crop's development SBR became established. To account for this, the three factors were integrated over time into a factor called effective leaf area duration (ELAD). Reduction in ELAD was a measure of the damage to the crop canopy caused by SBR and was determined to be a good predictor of yield loss.

Independent studies were conducted to validate the accuracy of model predictions over a range of environments, cultivar maturity groups and row widths in the U.S. Trials were planted in Quincy, FL, in 2007, Tifton, GA, in 2008 and southern Brazil in 2006 and 2009. The Brazil trials had severe SBR epidemics and large reductions in ELAD and yield. However, over the past five years, research has shown that lesser SBR epidemics are much more common in the U.S. Consequently, reduced rates of disease progress were estimated and incorporated into the model. The expanded model assumes that severe, moderate and light SBR epidemics have rapid, moderate and slow disease progress. The estimated yield losses were calculated for SBR starting at different growth stages and for different epidemics scenarios.

Estimated yield losses have been combined with economic calculations on an interactive website to assist growers making fungicide spray decisions. The user will be asked to enter per acre costs including fungicide chemical, fungicide application and soybean hauling. Additional information needed includes predicted potential yield and expected sale price.

Once the growth stage when SBR is first reported or expected is selected, the following calculations are preformed:

  • Return per acre with fungicide
  • Return per acre without fungicide
  • Net benefit of spraying
  • Percent difference in return

In addition to the table of results, there is a graphical output of the net benefit of spraying for the three epidemic scenarios at different soybean prices. Fungicide application would be economically justified with a positive net benefit of spraying for the growth stage and anticipated SBR epidemic severity.

The biggest unknown of the tool is how far north the tool applies. Certainly Kentucky and southern portions of Illinois, Indiana and Missouri can find benefit in the tool.

The tool was developed by Saratha Kumudini, Don Hershman and Joseph Omielan through checkoff funding. For more information about the research visit www.kysoy.org under the News section.