Question: I see a lot of information about using foliar fungicides to improve soybean health and, thereby, boost yield results in the process. What do you recommend?
Answer: That’s a tough question as there’s not a one-answer-fits-all approach you can take for every situation. Here’s what some of the experts say: X. B. Yang, Iowa State University (ISU) Extension plant pathologist, says most soybean growers see a financial payoff from foliar fungicide use only in the presence of yield-limiting disease pressure. David Holshouser, Virginia Tech Extension agronomist, agrees. His research shows that foliar fungicide use in soybeans results in a return-on-investment only one-third of the time in Virginia.
The reason: it’s difficult to predict whether foliar fungicides will pay-off because the most common ones, the strobilurin products, are preventatives. This means they must be applied before disease develops.
"We’re applying a chemical to prevent a disease that may or may not progress to yield-reducing levels," Holshouser explains. "If there’s no disease we’ve wasted our money."
However, Holshouser adds that, given current soybean prices, farmers need less of a yield boost to make foliar fungicide applications worthwhile.
"If you can average 3 bushels to 4 bushels per acre over all acres (based on April 2013 prices), then a foliar fungicide will likely pay for itself," he says.
Yang’s ISU data from the past six years shows that precipitation levels may also help farmers predict the outcome from foliar fungicide use: "When precipitation was plentiful, more than 50% of the fungicide sprays yielded economical return, and more than 70% of the sprays provided positive yield."
For optimum results, Yang advises farmers to use a foliar fungicide at the R3 growth stage, which is when soybeans start to set pods. Applications at R5 or later provide a financial benefit, he adds.
Holshouser’s recommendation varies slightly from Yang’s. "Our preliminary models appear to be telling us not to spray right at R3, but to wait a few days," he says.
The timing differences may be due to weather factors, notes Holshouser: "In 2012, we received just as much of a yield response from an R5 (beginning seed) application as we did from an R3 (beginning pod) application because weather conditions were just as conducive, if not more so, for disease development."
Foliar fungicide applications at R1 or earlier do not usually provide a return-on-investment. An application for white mold control is the one exception. Yang says an application at R1 is most effective because white mold fungus infects soybean plants through dead flowers on the bean plants.
Anne Dorrance, Ohio State University plant pathologist, advises farmers to consider a foliar fungicide application for fields planted to continuous soybeans and no-till soybeans. These fields tend to have high levels of residue on the soil surface and are at risk for early-season foliar diseases.
In addition, she says, if the soybean variety grown is susceptible to disease, then an application may be warranted. Diseases that respond well to a foliar fungicide application include anthracnose, cercospora leaf blight, frogeye leaf spot, septoria brown spot, soybean rust, pod and stem blight.
Dorrance cautions farmers about spraying soybeans that are growing in dry, hot conditions. Her research indicates foliar fungicide applications can impact yield negatively in those crops and contribute to spider mite flare-ups.