UW Soybean Plant Health
The discovery of soybean rust in 2004 resulted in an unprecedented interest in the use of foliar-applied fungicides for disease control in soybean. Although soybean rust has not occurred in Wisconsin, there is considerable interest in learning whether foliar-applied fungicides could have a role in soybean production in Wisconsin even in the absence of soybean rust.
Staff at the University of Wisconsin-Madison have worked with county extension agents, private consultants, and soybean growers to conduct on-farm fungicide trials in 2005 and 2006. Nine on-farm trials were conducted in 2005 and six were conducted in 2006. In addition to on-farm trials, University of Wisconsin-Madison staff conducted smaller-scale fungicide trials at University research stations. Key questions regarding the use of fungicides are:
- Will use of registered (section 3) fungicide products result in an economic return?
- Which soybean diseases are controlled by foliar applied fungicides?
- At which growth stage should fungicides be applied?
A total of 15 fungicide trials were conducted in 2005 and 2006. When averaged over the 2 years, soybeans treated with a fungicide have responded with an overall 1.7 bushels per acre improvement in soybean yield compared to untreated soybeans in all 15 trials (Figure 1). We have calculated 3.8 bushels per acre as a baseline needed for an economic response to the application of a fungicide. Thus, the application of a fungicide was not cost effective in most trials. However, yield improvements of 6.1 and 6.8 bushels per acre were achieved in two of 15 trials during the 2-year study.
Figure 1. Yield of soybeans treated and untreated with a fungicide for 15 trials conducted in 2005 and 2006. Yield differences ranged from 0 to 6.5 bushels per acre in 2005 and 1.0 to 6.1 bushels per acre in 2006.
 |
The most prevalent fungal leaf diseases observed during the two years of trials were brown spot, downy mildew, powdery mildew, and frogeye leaf spot. However, it is not clear which, if any, of these soybean diseases were controlled by the fungicides and if control resulted in increased yields. Although less apparent in 2005 than in 2006, soybeans treated with a fungicide were slower to reach physiological maturity (GS R8) when compared to untreated soybeans (Figure 2).
 |
Figure 2. The effect of foliar fungicides on soybean health and yield were studied at multiple locations in 2005 and 2006. Soybeans in bottom photo were treated with a fungicide and soybeans in top photo were not treated in a trial at the Marshfield Agricultural Research Station in 2006. The fungicide-treated soybean average 56 bushels per acre compared to a yield of 50 bushels per acre for the untreated soybeans. Photos are courtesy of Michael Bertram, Marshfield Agricultural Research Station. |
We have not identified a single key factor that would predict whether a fungicide application would result in an economic return. One of several variables under consideration is soybean variety. In 2005, 22 of 123 soybean varieties in the UW-Agronomy Department soybean yield trial produced from 3.8 to 11.3 bushels per acre more when treated with a fungicide. The importance of soybean variety is not known, but will be investigated in future trials. Varieties with varying degrees of genetic resistance to common leaf diseases may be included in future comparisons.
Results in Wisconsin and other states suggest that fungicides are more effective if applied at growth stage R3 (first pod stage) rather than at R1 or R2 (flowering). Research will continue to determine which growth stage is most suitable for the application of a fungicide. Funding for soybean fungicide research has been provided by the Wisconsin Soybean Marketing Board and participating companies.
Although foliar-applied fungicides have resulted in increased yields, the probability of an economic return has been low. Staff at the University of Wisconsin-Madison will continue to research the feasibility of foliar-applied fungicides because some success has been achieved and soybean rust made advances in the Southern states in 2006. Further research is needed to meet this challenge should it occur.
Information on fungicides registered for use on soybean can be found on the page Fungicides for soybean rust available in Wisconsin, or in the UW-Extension bulletin Pest Management in Wisconsin Field Crops (online in pdf format or obtain from UWEX Publications - ask for UWEX No. A3646).
Information from this site can be copied and distributed for educational use. Please credit the source with our name and URL: University of Wisconsin-Madison, Departments of Agronomy, Entomology, and Plant Pathology at www.plantpath.wisc.edu/soyhealth.
3/07