Across Iowa many growers are starting to ask if or when to apply fungicide on soybeans in 2011. There are essentially two options being discussed. The first is to apply fungicides at R1 (flowering) growth stage of the soybean plant. This application could be tank-mixed with the last application of glyphosate herbicide. The second option is to apply fungicide at R3 growth stage, which is during pod formation. This application could be tank-mixed with an insecticide or applied alone. Which application timing is best? Which products best protect yield?
The following answer and observations are from Alison Robertson, Iowa State University Extension plant pathologist, and her colleagues at ISU, graduate student Nathan Bestor and plant disease specialist Daren Mueller.
To answer these questions ISU researchers conducted soybean fungicide trials at various locations in Iowa in 2008, 2009 and 2010. "We evaluated the yield response of soybeans to applications of a strobilurin fungicide," explains Mueller. "We tested three different strobilurin products."
The three strobilurin fungicides tested were: Headline, a triazole; flusilazole which is a product named Domark; and a premix of strobilurin + triazole which is a product known as Stratego YLD. "We also assessed foliar disease severity at the R5 stage of soybean growth," he adds. "Finally we calculated the probabilities that an aerial application of a fungicide product would 'break even' at a specific site year when prices of beans were estimated at $10 per bushel, $13 per bushel and $16 per bushel. The cost of fungicides was estimated using information gathered from area cooperatives and ISU Extension field agronomists."
The results of these ISU trials are summarized in Table 1:
• In all years, disease severity was low.
• Brown spot was the most common disease and was found virtually only in the lower canopy.
• The yield response of soybeans was greater with an R3 application rather than an R1 application. So, an R3 application of a fungicide was more likely to break even than an R1 application
• The chance that an R3 application of a fungicide would break even ranged from 7% to 99%.
• Products that contained a strobilurin as one of the active ingredients were more likely to break even compared to a product containing only a triazole.
When should you spray? When disease could impact bean yield
Although these data suggest that nearly 60% to 70% of R3 applications of a fungicide containing a strobilurin at least break even, spraying when disease is present will further increase your chances of getting your money back.
If brown spot remains in the lowest third of the soybean canopy, it probably will not impact yield. If the disease moves into the middle third of the canopy, brown spot can start to reduce yield. "We saw this at one of our locations in 2010," says Mueller. "At that location, the mean yield response due to a fungicide application at R3 growth stage of the soybean plants was nearly double the responses at other locations. That is, 5 to 6 bushel per acre vs. 2 to 4 bushels per acre."
Word of caution: resistance to fungicides was reported in 2010
A caution when spraying fungicides: scout fields after applying the fungicides and look for applications that do not appear to affect disease severity. In 2010, resistance to the strobilurin fungicides (Headline, Quadris, one of two active ingredients in Stratego YLD, and Quilt) in the fungus that causes frogeye leaf spot was reported in Tennessee, Kentucky and Illinois [University of Illinois].
Funding for this ISU research project was provided by the Iowa Soybean Association, Bayer CropScience, DuPont, Valent and BASF.