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Corn+Soybean Digest

Pathologist: Be Wary Of Fungicide Yield Boost Claims

Farmers who apply fungicide to corn and soybeans because they believe the chemical boosts yields could be treating their crops with little more than a placebo, Greg Shaner, Purdue University Extension plant pathologist, says.

After two years of field trials to determine whether fungicide provides a yield response in healthy crops, the data are inconclusive, says Shaner.

"It's almost a coin toss as to whether producers will get their money back in increased crop yields from fungicide treatments," Shaner says. "Producers probably should only use a fungicide if they have reason to think they will have disease pressure."

Fungicides are used to control such fungal diseases as gray leaf spot and northern corn leaf blight in corn, and Asian soybean rust and frogeye leaf spot in soybeans. The diseases attack leaves – plant parts that are critical in normal development and grain production. Fungicide applications usually are made at the onset of disease.

In recent years, some chemical manufacturers have claimed that fungicide applications enhance corn and soybean yields even when no disease is present. Shaner and other Purdue researchers have not been able to either prove or disprove those claims.

The Purdue trials were conducted in small plots and large fields at five Purdue agricultural research centers across Indiana. Researchers applied up to 30 different fungicides in replicated trials at some locations. Results were mixed among and between corn and soybean plots, Shaner says.

With corn, the Purdue trials indicate "about a 50-50" positive to neutral yield response following fungicide treatments, Shaner says.

"In soybeans, in some cases, there's a yield response," he says. "That is, the yield increase is more than enough to pay for the fungicide application."

Fungicide treatments run $18-23/acre, including application costs.

"When you look at the data from a lot of different trials from many states, the yield response may be as high as 20 bu./acre, but it may be a fraction of a bushel," Shaner says. "On the negative side, there have been instances where yields have been 6 or 7 bu./acre lower.

"In any individual trial, sometimes those yield responses are statistically significant, which means that, based on the analysis of the data, it looks like we can ascribe the positive effect to the fungicide. In other cases, you can have a pretty big yield difference between a sprayed and unsprayed crop, and yet the conclusion is that it doesn't have anything to do with the fungicide – it's just chance variation."

Among the various classes of fungicides, strobilurins are the type believed by some to provide a yield increase. Shaner says the way strobilurins work could explain why some crops appear to benefit from the applications.

"Strobilurins inhibit fungi by shutting down an enzyme that's involved in respiration, and that's lethal for the fungus," he says. "There's maybe a little bit of evidence that when a plant is sprayed with these fungicides, a similar enzyme involved in respiration might be slightly inhibited. The effect of that inhibiting action would be that the plant isn't giving off as much CO2 during respiration. So by conserving carbon, this could mean there's more carbon to go into the grain."

Farmers who plan to treat their 2008 crops with fungicide likely already have purchased product or hired professionals to do the work, Shaner says.

"Most fungicide on corn is going on by aerial application because there aren't that many ground rigs that can get over the top of tasseled corn," he says. "We don't have that many aerial applicators in the state, so what I think happens is that contracts to treat fields are being set up in February. Obviously at that point, a grower hasn't got a clue what the disease pressure is going to be the following summer."

Shaner encourages producers to carefully weigh their fungicide-use plans against the disease resistance traits of the corn hybrids and soybean varieties they plant. Seasonal weather patterns also are a factor, he says.

"If a hybrid or variety has pretty good resistance, then I think the chance it will benefit from the fungicide isn't very great," Shaner says. "Conversely, if a hybrid or variety is susceptible to one or more leaf diseases and weather is favorable for disease development, then it is more likely that a fungicide application will be economically justified.

"This whole business of fungicides on corn and beans is pretty new.
There's a lot that needs to be learned about decision aids, thresholds and weather-based disease forecasts that can help growers decide if a fungicide would be valuable or not."

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