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Back when fungicides for use in corn and soybeans were just a gleam in the eye of a chemical company marketing manager, a 1990s discovery intrigued European scientists.
They noticed that cereals treated for disease with strobilurin fungicides were much greener than untreated cereals. Digging a bit deeper, the chemical company scientists discovered these disease-fighting compounds also sliced ethylene production.
“Ethylene reduction is part of the reason why plants stay greener longer,” says Fred Below, University of Illinois plant physiologist. Scientists discovered other physiological benefits that may ultimately boost yields.
In the mid-2000s, these qualities struck a perfect storm in the Midwest. Soybean producers feared soybean rust would ravage their fields. They examined fungicides that southern U.S. farmers had used to protect soybeans from fungal diseases like frogeye leaf spot. In corn, shifting agronomic practices (like corn-on-corn and less tillage) that harbored disease were more conducive to fungicide responses.
Meanwhile, skyrocketing commodity prices helped make fungicide applications economically feasible. A 7-bushel-per-acre yield spike can cover a $32- to $34-per-acre fungicide and application cost much better at $6-per-bushel corn prices than it can at $3-per-bushel prices.
Soon, strobilurin fungicides (including Headline, Quadris, and Evito) and strobilurin premixes (including Quilt Xcel, Stratego, Avaris, and Headline AMP) became mainstream agronomic tools. Strobilurin fungicides particularly shine in disease-ravaged corn. University of Kentucky (UK) research showed that a 9.6- to 12-bushel-per-acre yield spike in 2009 occurred in fields with over 5% disease severity.
A three-year University of Tennessee and Pioneer Hi-Bred study showed disease-susceptible hybrids especially benefit from fungicide applications. Average yield gains of 23.5 bushels per acre resulted on disease-susceptible hybrids, compared to 7 bushels per acre for resistant ones. (See chart on page 36).
“There are some very good reasons for using them,” says Paul Vincelli, UK Extension plant pathologist. “I see it over and over again where fungicides show a favorable return where there is significant disease pressure.”
The water gets muddier when strobilurin manufacturers recommend strobilurin fungicide application with no disease present to garner plant physiology benefits.
BASF promoted the physiological impact of its Headline fungicides by trademarking the term Plant Health. In 2009, BASF took Headline's plant physiology benefits a step further when it obtained a federal supplemental label for its Headline products. Besides controlling disease, the supplemental label lists that Headline's benefits include improved plant tolerance to maladies like hail, drought, cold temperatures, ozone damage, and frost. The label also stated that Headline could improve plant use of nitrogen and increase tolerance to bacterial and viral infections.
Syngenta also touts the physiological benefits of its Quilt, Quilt Xcel, and Quadris fungicides under its Plant Performance banner.
Bayer CropScience lists physiological benefits on its label for Stratego and Stratego YLD fungicides, a mix of a strobilurin and triazole chemistry.
“I am a die-hard believer in fungicides on corn,” says Rick Ryan, a Malcolm, Iowa, farmer. “There are other benefits besides yield, like standability and ear retention. Those things are not as tangible as yield, but they are critical in my operation. There is easier harvestability, as I can get more bushels through a combine in a day.”
Others aren't so sure. “For me, a fungicide is a last-ditch effort,” says Ed Winkle, a Martinsville, Ohio, crop consultant and farmer. Good management, such as proper varietal selection, makes a bigger difference, he adds.
Plant physiology benefits are real, says Randy Myers, fungicide product manager for Bayer CropScience.
“We see the things like better nitrogen utilization and lower ethylene production, but the impact on yield from each of those things is often difficult to quantify,” he adds.
It's this difficulty and inconsistency in university trials that give Extension and university plant pathologists pause.
“Strobilurin fungicides have physiological effects in corn resulting in improved growth,” says Vincelli. “But they are just extremely unpredictable. Their use is not a slam dunk.”
In 2011, 12 Extension and university plant pathologists released a 14-state compilation of 2002 to 2009 trials that showed an average 3.66- to 6.21-bushel-per-acre yield spike occurred when strobilurin and premixes containing strobilurin fungicides were compared to untreated acres.
“In our trials, even where conditions were optimal for disease, if disease didn't develop, we didn't see a yield increase,” says Kiersten Wise, Purdue University Extension plant pathologist. As a rule, it took a 6-bushel-per-acre yield increase to make an application economically feasible, she adds.
“There is also a probability of losing money in spraying fungicides,” says Vincelli. “If corn is $3 per bushel, that raises the pressure for a favorable response even more.”
Industry yield claims are higher.
● BASF officials point to 3,000 on-farm trials from 2004 to 2011 where Headline and Headline AMP increase corn yields an average 12 to 16 bushels per acre.
● Syngenta on-farm trials in 2010 and 2011 showed around a 14-bushel-per-acre yield boost from applying Quilt Xcel at tasseling.
● Bayer tests have shown around a 12- to 15-bushel-per-acre yield increase, depending on disease pressure.
Industry officials say differences are caused by trial size. Industry bases its claims on farm-size strip trials, where untreated portions square off vs. untreated sides. Meanwhile, most university trials are conducted in small plots.
“In a small plot, you have border effect in that the disease doesn't move in from neighboring plots,” says Myers. “You don't have the same inoculum level in these plots that you do in fields.”
Vincelli is open to small-plot criticism. He points to three 2011 UK field studies where three large-scale fields were randomized and replicated. These scientific processes repeat the same trial under the same conditions in order to make sure the same results occur.
Surprisingly, two of the three fields had +20-bushel-per-acre yield responses with little disease pressure.
“One explanation is that these trials are at the tail end of a bell curve,” says Vincelli. “We could test three fields next year and not have it show anything. There also could also be a problem in our assessment in our small-plot trials. I still feel confident in our research base on when yield increases will occur in the absence of disease pressure. We will repeat the (field-size) trials in 2012, though.”
What to do?
There are five situations, though, that all parties agree on to target for odds of a fungicide payoff.
1. Susceptible hybrids. Disease-susceptible hybrids are more prone to strobilurin fungicide responses.
“We don't recommend treating disease-resistant hybrids,” says Wise.
2. Residue-laden fields. “Residue is an inoculum source for diseases,” says Jeff Nagel, an agronomist with Ceres Solutions, West Lafayette, Indiana. Prolific residue from corn-on-corn and no-till harbors more disease inoculum.
3. Intensively managed fields. These include fields like those with high nitrogen or seeding rates and those under irrigation. High-input rates that create fast-growing corn set the stage for a fungicide response.
“The crop canopy is denser, and that creates more stress on the corn,” says Nick Fassler, BASF technical manager.
4. Wet weather. Rainfall and humidity during early grain fill can fuel disease. “What drives disease is rainfall and warmer temperatures,” says Nagel. Fungicides applied during this time can protect yield potential.
5. Fields with sound agronomic strategies in place. If you're missing key inputs, don't expect miracles. A fungicide won't help a cornfield hampered by lack of water or nitrogen. “What a fungicide can do is put the plant in the best position to make use of the resources that are present,” says Myers.
Resistance Rears its Head
Strobilurin fungicides have excellent disease-control attributes, and they can spur physiological benefits in plants. Yet they are prone to the development of resistant fungi.
“Because strobilurin chemistry is now registered on so many crops and used more often, I think the risk may increase regardless of using it only once per year,” says Carl Bradley, University of Illinois (U of I) Extension plant pathologist. “In addition, this chemistry is now being used as seed treatments.”
Frogeye leaf spot resistant to strobilurin fungicides surfaced in western Tennessee in 2010. By 2011, more resistance to this soybean disease was confirmed in new counties in Tennessee, southern Illinois, Kentucky, and Missouri.
Corn fungal diseases that resist strobilurin fungicides will eventually develop, says Paul Vincelli, University of Kentucky Extension plant pathologist. “The more you use them, the more risk there is of resistance,” he says.
To forestall strobilurin fungicide resistance, Bradley advises the following:
● Plant disease-resistant hybrids and varieties. A 2010 U of I trial examined fungicide response in one variety susceptible to frogeye leaf spot and three others resistant to the disease. “We saw a 10- to 11-bushel-per-acre response on the susceptible variety,” says Bradley. No statistically significant differences resulted on the resistant varieties.
● Spray a fungicide only when warranted for disease control. “This limits the selection pressure placed on the fungal pathogen population, thus, reducing the selection of fungicide-resistant pathogens,” says Bradley.
● Apply at recommended rates.
● Rotate fungicide chemistries. For example, if you apply a strobilurin fungicide one time, apply a triazole the next.
Using premixes with multiple modes of action – such as those containing a strobilurin and triazole fungicide – is also an option. Bradley notes that plant pathologists disagree on this one, however. “The argument against multiple mode-of-action fungicides is that some contain reduced rates compared to the stand-alone products, and reduced-rates can lead to a quicker selection of resistant pathogens.”
Early-applied strobilurin fungicides can get corn plants off to a good start, say industry officials.
Compared to conventional tassel-time applications, early applications target the V4 (fourth leaf initiation) and V8 stages (collar of eighth leaf visible).
“Early applications provide plant physiological benefits and early-season disease control,” says Eric Tedford, Syngenta technical brand manager. “Early disease control holds back fungal pathogens and reduces inoculum levels later in the season.”
The program works particularly well in residue-laden fields. “It helps suppress those diseases that come from soilborne inoculum,” says Randy Myers, Bayer CropScience fungicide product manager. “Yield potential is determined between V4 and V6. So if things go bad during this period and good the rest of the season, you aren't realizing full yield potential.” A fungicide can help protect the plant during this critical period, he adds.
Applications can be made when non-selective herbicides like glyphosate are applied, he says. Cost of the fungicide for this purpose generally runs between $6 and $15 per acre. Industry tests show early application yield responses range between 5 and 9 bushels per acre.
As with later applications, though, success hinges upon disease pressure, says Carl Bradley, University of Illinois (U of I) Extension plant pathologist. In 2010, the U of I ran trials at two sites examining several strobilurin fungicide times from V5 (fifth leaf) through R1 (silking) at two sites.
At Urbana, low disease pressure below 2% occurred. No statistically significant difference in disease or yield occurred across all times and treatments. At Monmouth, high disease pressure (66% severity) resulted. Fungicide applications at VT or R1 boosted yields by around 20 bushels per acre. Meanwhile, earlier applications made at V5 or V15 (kernel number determination stage) had no statistically significant impact on yields or disease.
A compilation of land-grant trials in six Corn Belt states by Purdue University plant pathologists Kiersten Wiese and Greg Shaner supplements these findings.
The best yield increase combination that boosted yields 7 bushels per acre occurred with two applications. One was at V6 and one as at R1.
However, economics change with two applications. “You need at least 10 bushels per acre just to break even,” Bradley says.
“If you have decided to use a fungicide, one application at tasseling is the ideal use,” says Paul Vincelli, University of Kentucky Extension plant pathologist.
What About Soybeans?
Like corn, fungicides can also spike soybean yields. And also as in corn, yield increases can be inconsistent.
Industry trials show anywhere from 4- to 8-bushel-per-acre yield spikes. University trials also show favorable responses.
Tom Hoverstad, a University of Minnesota agronomist at the U of M Waseca Extension and Outreach Center, says Center trials show 2- to 4-bushel-per-acre responses in soybeans. “Three out of four times, it has paid,” he says.
Spraying in absence of disease, however, won't result in as clear-cut responses, says Randy Myers, Bayer CropScience fungicide product manager. While corn has a relatively short pollination period of a week to 10 days, soybeans' blossoming and podding period plays out over weeks. Soybeans respond to stress by aborting blossoms and pods, which can curtail yield potential.
A fungicide application can help reduce stress in corn's tight pollination window. With soybeans, though, there's a much wider window to protect.
Location also influences fungicide response consistency in soybeans, adds Myers. Responses in the South are more consistent than those in the North due to higher disease pressure. However, this can fluctuate due to weather, he says.