Pencil It Out

March 14, 2009 03:15 PM

John Hoffman doesn't look for extra places to spend money on his soybean crop. But last year the season got late and it was excessively wet through the first third of the growing season. With prices at record highs, a foliar application of fungicide seemed like a safe bet.

Indeed, the move paid off with a 2.2-bu.-per-acre increase compared with the in-field check. "The price of soybeans made the decision easier last year," says Hoffman, who farms in Waterloo, Iowa. "I decided six years ago to start

intensively managing my soybeans; fungicides are part of that equation when the season dictates and when they make sense economically."

Iowa State University plant pathologist X. B. Yang admits he's been on the fence for a long time regarding the practice of applying foliar fungicides preventively. He's been testing various fungicides at the request of chemical companies since 1993 to determine optimal dose and application times.

A closer look. When spray planes became as thick as mosquitoes in 2008, Yang and some of his Iowa State colleagues decided to take a closer look. "We recognized that the soybean price doubling was a driving force in the grower's decision to spray last year, but we weren't sure if it was the only force," he says.

Looking back at the past six years of data from fungicide trials at Iowa State University research farms, Yang discovered some surprises. "Our data suggests an increasing incidence of soybean disease," he notes. Brown spot, frogeye leaf spot and Cercospora leaf blight were the main culprits. Yang says bacterial blight is also contributing to early plant defoliation; however, it is not controlled with fungicides.

While it's possible that diseases, such as frogeye leaf spot, could be blowing northward, Yang thinks the increase in rainfall may also be contributing.

Iowa State University climatologist Elwynn Taylor says the state's annual precipitation days are increasing. One hundred years ago, the annual precipitation was 75 days per year. In 2000, the number of rain days increased to about 100, and since 2004 it has rained more than 120 days per year.

"Increased disease risk from higher precipitations leads to increased use of foliar sprays and seed treatment," Yang says. "Over the past decade, we are seeing diseases start to build up late in the season."

Southern growers have long depended on fungicides to help battle soybean diseases. The uncertainty comes further north, where there's been less history of disease. The debate on routine foliar application is complicated because most studies look only at yield and don't include disease ratings.

Many common diseases of soybeans are not impacted by foliar fungicides. But pathologists are beginning to think that disease complexes made of very low levels of symptomatic individual diseases are shaving off yields.

Looking at yield data from the past six years of studies, Yang says his results clearly show that the use of fungicide as a preventative measure can increase yields in seasons when disease pressure is moderate or high. "In those seasons, many fungicide treatments yielded better and a few treatments increased yield over 10 bu. per acre," he notes.

"Our experiments were aimed at finding effective compounds and determining their optimum spray time in comparison to standard treatments," he says. "Therefore, many fungicide treatments did not increase yields. In seasons with low disease pressure, only a few treatments produced higher yields. Most yield increases were in the 2-bu.- to 3-bu.-per-acre range."

Assess risk. Yang recommends that farmers look at fungicide treatments in terms of assessing risk. "For the last five years, our figures indicate Iowa growers have a 50% chance of gaining an economic return from a properly timed fungicide treatment when soybeans are at $8 per bushel. When disease risk was high, the chance is 66%.

"If you are looking for an insurance policy—that 1 bu. more is enough to justify the expense—the odds go up. Last year, the odds of fungicides being a good insurance policy were in the 80% to 90% range because of elevated prices," he says.

In general, Yang says, each grower needs to study his own field history to assess the potential for disease pressure.

"Fungicide application is best reserved for seasons when foliar diseases are severe. Research indicates that the higher the foliar disease severity, the greater the return from the use of fungicides," he says.

You can e-mail Pam Smith at

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