Last summer, Matt Hughes got defensive. With soybean prices at record highs, it made sense to hang on to every bean.
"The late planting season coupled with moist conditions seemed the perfect situation for disease,” Hughes says. "I crunched the numbers and came up with a 2 bu. per acre break-even for one fungicide application. I considered it an insurance policy given the price of soybeans at the time,” he says.
Until a few years ago, foliar fungicides were mostly an input for the most disease-prone southern states. But economic conditions, coupled with claims of plant health effects—longer green leaf retention, delayed plant senescence, improved carbon dioxide assimilation, increased water use efficiency and increased stress tolerance during flowering and pod fill—kicked up a fungicide frenzy.
"The discovery of soybean rust in 2004 started an evolution,” says Randy Myers, Bayer CropScience fungicide product manager. "As growers learned about rust, they also became more aware of other fungal diseases and the products that control them.”
Mississippi State University Extension plant pathologist Tom Allen says that in the South, the timing coincided with growers realizing the economic importance of soybeans. "Managing soybean rust is a different decision than managing other diseases,” Allen says.
"Today, approximately 65% of our soybean acreage receives a routine foliar fungicide application [typically containing a strobilurin] around the R3 growth stage to control the little things that collectively rob us of yield,” he says. "We'll see fungicide use continue to increase this year as more cotton acreage is replaced with soybeans and we have more bean-following-bean acreage.”
The practice is less clear-cut farther north. Foliar diseases, such as brown spot, frogeye leaf spot, Cercospora leaf blight, powdery mildew, pod and stem blight and anthracnose, can sneak in to the Midwest. But University of Kentucky plant pathologist Don Hershman says that while there are exceptions, most of these diseases develop too late in the season to cause measurable yield loss from Kentucky north. Other diseases—such as charcoal rot, soybean cyst nematode, sudden death syndrome and Phytophthora root and stem rot—and viruses are more common, but they are not impacted by foliar fungicides.
"Without disease control being needed, benefits are not predictable and results can and do vary considerably,” Hershman says. "I don't think anyone can predict a pure ‘plant health' response divorced from disease control.”
Here's the rub. There's disagreement among researchers, university Extension specialists, growers and industry on fungicide use. Pesticide applications made in the absence of a specific target pest are contrary to integrated pest management principles. There is already some evidence that indiscriminate use of fungicides has resulted in increased insect and mite activity because native fungi are suppressed.
Still, fungicide use in soybeans dramatically increased, especially in 2008. "Use of Headline [pyraclostrobin] doubled this past year—from 3 million acres to 6 million acres,” says Gary Schmitz, BASF Corporation Midwest technical manager.
"We're still tallying yield studies, but on average, it appears growers realized a 4 bu. to 8 bu. increase,” Schmitz says.
"It isn't all about yield,” says Jamie Eichorn, Syngenta fungicide brand manager. "Seed quality and test weight improvements also contribute to overall return on investment.”
Even Hershman agrees something is happening out there. "In nearly 30 tests in Kentucky, I have seen significantly higher yields in strobilurin-treated plots about 25% of the time,” he says. "But in none of these instances could I tell you where the higher yields are coming from.”
Bayer CropScience's Myers agrees there are no easy answers. "The decision in soybeans is less clear-cut than in corn,” he admits. "Almost any stress can result in a yield reduction in corn, but the nature of soybeans is to compensate. The challenge is, growers want a guarantee that the returns from these products are going to equal their investment.
"These new chemistries with longer spray intervals and increased activity are better tools than the contact fungicides of the past,” Myers says. "I think everyone agrees they are bringing value. What we can't do yet is give a definitive answer to the question of what conditions will guarantee a return at the end of the season.”
For Hughes—who realized a 3 bu. increase from a shot of Stratego (propiconazole plus trifloxystrobin)—the answer is wait and see. He also had an official Headline test plot on Roundup Ready 2 Yield seed beans that had a 3 bu. yield bump. "Disease pressure, weather and the market all have to be considered,” he says.
The following five suggestions from Iowa State University plant pathologist Daren Mueller will help you make better fungicide decisions:
¡Select fungicides based on the disease present. Fungicide activity generally breaks down into two categories: protectant (strobilurin) and curative (triazole). Triazoles are considered to be more effective against soybean rust. Strobilurins are more effective against diseases such as brown spot, Cercospora leaf blight and frogeye leaf spot.
¡Time your spray. Studies show early applications (especially R1) are less effective than R3 applications. Diseases often do not show up until later in the season, so early applications wear off before they can do any good.
¡Assess your field's disease pressure. Data from university and industry shows the greatest yield response from fungicides comes from fields with the most disease pressure.
¡Consider side effects. Some studies suggest early fungicide applications may increase soybean aphids in the Midwest. No one is sure why—tests are ongoing, but farmers should monitor for side effects. Don't delay an
insecticide spray because you want to team it with fungicides and vice versa.
¡Leave a test strip. There's no way to know if the product works if you don't have a check. Make it wide enough so drift doesn't sway the results.
You can e-mail Pam Smith at firstname.lastname@example.org
- December 2008