Pathologists studying yield and quality responses to foliar fungicide treatments in soybeans are convinced that the potential benefits could offset some of the costs.

The consensus seems to be that any yield increase or quality improvement attributed to fungicide treatments is not a function of the chemical itself, but of its ability to control a variety of minor diseases, some at sub-threshold levels.

In recent years, Midsouth soybean producers have increased their use of fungicides to control frogeye leaf spot and aerial blight. Now, with the threat of Asian soybean rust, many growers are including fungicides in their 2005 production plans.

Luckily, researchers in the southern U.S. already have several years of data on the pros and cons of fungicide use in soybeans under their collective belts.

“We've had good success controlling frogeye leaf spot and aerial blight with a fungicide treatment at the soybean plant's R3 and R4 growth stages. We're also picking up some foliar diseases with this treatment,” says Cliff Coker, a plant pathologist at the University of Arkansas in Monticello.

Diseases such as pod and stem blight and anthracnose are minor pests that nibble away at yields. While stand-alone treatments of these diseases are simply not economical, Coker says the incidental control of minor diseases through fungicide treatments for other diseases does seem to benefit soybean yield and quality.

“We may be controlling diseases at levels that we aren't aware were damaging yields,” he says. “Over the last two years, even in low disease pressure situations where disease threshold levels didn't call for a fungicide treatment, we've seen a yield response of 4-5 bu./acre. However, the last few years have been cooler and wetter than normal, which favored disease development,” he says.

“It's difficult to separate which minor diseases are causing problems and which aren't,” Coker adds. “I believe we're seeing a cumulative effect of all of these low levels of different diseases, none of which are at threshold type levels.”

In Mississippi, full-scale replicated trials at nine locations over two years have shown Quadris-based fungicide programs have averaged yield responses of 4.5 bu./acre.

Dan Poston, Extension soybean specialist at Delta Research and Extension Center in Stoneville, MS, says, “The best treatment we evaluated was a tankmix of 6.2 oz./acre of Quadris and 2.0 oz./acre of Dimilin, which garnered a 5.5-bu. response on average across 18 trials at nine locations.”

Poston and his Mississippi colleague, plant pathologist Gabe Sciumbato, also documented an average yield response of 4 bu./acre with a 0.75-lb./acre application of Topsin-M, and a 4.5-bu./acre response to a 6.2-oz. application of Quadris.

“We're documenting yield increases anywhere from 4 to 5.5 bu./acre, but strobilurin fungicides need to be applied between the R3 and R4 growth stages to maximize the likelihood of yield enhancement. Quadris-based treatments and Topsin-M on average yielded 2.5 bu. greater when applied at the R3 through R4 growth stages than when they were applied at R5 to R6,” Poston says. “We're basically looking at an early treatment and a later treatment. To get the most bang for your buck, the R3 to R4 growth stages are the times to apply fungicides to control diseases other than soybean rust.”

Earlier foliar fungicide research in the 1970s and early '80s showed inconsistent yield results, with modest yield increases on fairly low-potential soybeans. At that time, foliar fungicides were rarely used by grain producers and were used mostly by seed producers as a method of improving seed quality.

The situation is much different today, Poston says. “We plant earlier, grow a higher percentage of intermediate varieties, have higher yield potentials and have access to new broad-spectrum fungicides like Quadris and Headline that are fairly affordable,” he says.

How does this relate to Asian soybean rust? It means that part of the rust control program cost will likely be offset by yield increases associated with the control of other pathogens.

University of Kentucky Extension Plant Pathologist Don Hershman says the situation in his state is much different than in the Midsouth.

“In the deep South, growers and researchers can almost always identify foliar diseases being controlled by a fungicide treatment. Historically, they have pressure from diseases such as frogeye leaf spot and aerial blight. We don't have that pressure, or if we do it is usually in very low doses. There are also varietal and weather differences between the two regions.”

While soybean producers in Kentucky may have substantially less experience applying fungicides than their counterparts in the Midsouth, Hershman has spent the last two years researching foliar fungicides.

His research was initiated because fungicide manufacturers were marketing product guarantees to soybean producers in his area, says Hershman. The primary treatment being promoted to growers was a tankmix of Quadris fungicide plus Warrior insecticide.

“It's a mixed bag. There are times we see a significant yield increase. Then there are times there is no yield increase, or the yield increase is so slight that it's not economical to treat with a fungicide,” he says. “There is not going to be an automatic yield increase when you apply a tankmix of the two products, but there is definitely something going on with the Quadris and Warrior treatment. We have not seen the same results from either product alone as we have when the two products are mixed together. It's got me scratching my head.”

Attributing the yield increases Hershman has seen to either the fungicide or the insecticide application is difficult, because he's yet to document substantial disease or pest pressure in the fields being studied.

There is the possibility that a greening effect resulting from a Quadris treatment could be slowing defoliation, or that the Warrior treatment could be acting as crop oil because it's an oil-based insecticide. In addition, the fungicide treatment does seem to control sub-threshold levels of stem anthracnose disease.

Whether or not a $23-28 treatment is economical when soybeans are selling for $5/bu. is another question unanswered, according to Hershman.

“After two years of research, we're still at ground zero as to why yields increased. When there is a yield response there doesn't seem to be a clear-cut reason for it. There are no foliar diseases to speak of being controlled, and there are no other pest pressures affecting the yield outcome. Maybe some of these background disease pressures are more important than we realize, Hershman says.

The good news, he adds, is that push for increased fungicide use is providing growers in Kentucky with added experience. “Unlike Southern farmers who are used to spraying soybeans with fungicides, this is really giving our farmers some experience spraying fungicides later in the season, which is what they may be doing with soybean rust control.”

With Asian soybean rust, Coker believes most growers won't have to make more than two in-season fungicide treatments.

He adds, “There are a lot of unknowns because this disease has not shown itself in the U.S. during a production season. We don't know when it may move into our soybean producing areas, and we don't know how it will respond to our climate.”

The South American climate, he says, is more favorable to soybean rust. “They don't have the temperature extremes we do, and this disease organism likes lots of moisture and moderate temperatures in the 70s to mid-80s. Here in the southern U.S. we normally hit temperatures around the 100° mark.”

“The U.S. is the last major soybean-producing area in the world to experience problems with rust,” Coker says. “Also, this is a soybean disease we can manage and it's not going to catch us by surprise.”