Recent rains have greatly improved soybean prospects. I have not observed soybean rust in plots but have seen and been contacted about Septoria brown spot. Septoria brown spot is the most common foliar disease of soybeans in Oklahoma.
Plants typically become infected early, during vegetative growth stages in fields where soybeans are cropped after soybeans the previous year. Symptoms appear as small brown spots on lower leaves (Figure 1). Heavily infected leaves turn yellow and drop from the plants.
In my fungicide trials on soybeans where fungicides have been applied at the R3 growth stage (first pods visible at the top four nodes), brown spot is normally already present at significant levels.
According to the literature, brown spot is a cool weather disease that decreases in importance as temperatures rise. It was generally not considered damaging. More recent research in Ohio that used full-season fungicide applications to measure yield loss, showed that brown spot reduced yield by 4 to 6 bushels per acre in high yielding soybeans that averaged 70+ bushels per acre.
The research in Ohio further showed that application of strobulurin (e.g Quadris or Headline) but not triazole (e.g. Folicur or Tilt) applied at R3 effectively controlled brown spot. However, yield responses were only significant in about half of the trials treated only at R3.
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In Oklahoma, application of fungicides has not resulted in statistically significant yield increases. However, in irrigated trials in Bixby and Stillwater where average yields were greater that 60 bu/A yield responses in the 2 to 6 bu/A range have been observed. Both brown spot and Cercospora blight (Figure 2) were present in the trials.
Yield responses were not statistically significant thus I continue to be hesitant about recommending fungicide on soybeans in Oklahoma. More damaging foliar disease such as rust and frogeye leaf spot have not been observed in 2017.