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Yield potential still tops hybrid selection list

The specter of Asian rust initially spotlighted fungicide use on soybeans. Now that spotlight is expanding to corn, mirroring a likely increase in corn-on-corn acres in 2007.

The prolific residue that accompanies continuous corn creates a haven for foliar diseases, such as gray leaf spot (GLS), to thrive. Still, remember that fungicides are just one tool for controlling foliar diseases.

"We don't see an advantage to blanketing fungicides across every acre," says Jeremy Groeteke, a research agronomist for Golden Harvest, Waterloo, Nebraska.

Nor is it a given that a hybrid highly resistant or tolerant to foliar disease is your best hybrid pick for corn either grown in rotation or continuously. Disease isn't a problem every year.

"Disease is dictated by Mother Nature," says Groeteke, who spoke along with other Golden Harvest agronomists at a recent Golden Harvest meeting in Council Bluffs, Iowa. In wet and humid years, foliar diseases can thrive. In dry years, they don't.

That's why yield potential still needs to be your top hybrid selection factor.

"Don't necessarily assume that just because hybrids have a great disease score or disease tolerance that they will be the highest yielding hybrid for corn-on-corn," says Groeteke.

Realizing yield potential is a challenge hybrids face year in, year out. That's not the case with disease tolerance or resistance. Some growing seasons have disease-conducive weather where disease resistance or tolerance help hybrids realize yield potential. Others don't.

One constant remains through both kinds of years: Hybrids with mediocre yield potential will yield that way every year.

What to do? In residue-laden fields, pick hybrids with high yield potential and high disease tolerance.

Fungicides are another disease-management tool. In 2006, Golden Harvest researchers evaluated six fungicide treatments on two sets of five hybrids—one set susceptible to GLS and another tolerant to it—at three sites in Iowa, Nebraska, and Illinois.

Researchers tested hybrid sets in continuous corn under no-till or minimum tillage—an environment conducive for disease. Hybrid sets were compared against an untreated fungicide check to assess the impact fungicides had upon hybrid sets.

Results varied. "Depending on the location, there were different responses by fungicides," says Scott Payne, a Golden Harvest Research agronomist stationed in Ames, Iowa. "Not every hybrid responded to a fungicide in the same way. There might have been one hybrid that gained five bushels per acre with a fungicide application, while another hybrid gained 10 bushels per acre."

In other cases, fungicides gave no yield advantage, even with GLS present.

Meanwhile, an average of all study sites and hybrids showed GLS-susceptible hybrids yielded more than the GLS-tolerant hybrids. This led researchers to wonder if differences were due to fungicide applications or to genetic yield potential.

Results of the untreated check showed GLS-susceptible hybrids still outyielded GLS-tolerant hybrids.

The odds of a fungicide payoff increase as corn prices increase.

At $2 per bushel corn, fungicides need 10 bushels per acre to cover product and application costs of $20 per acre. At $4 corn, breakeven levels fall to five bushels per acre.

Still, Payne says this research underscores the importance of matching a top-yielding hybrid with a particular field. "Do that first, and then think about treating disease problems," he says.

It's unlikely that high disease levels will overwhelm disease-tolerant hybrids. Still, it's important to monitor every field, regardless of whether the hybrid plant is GLS tolerant or resistant. Payne advises farmers (or their consultants or agronomists) to start monitoring corn when waist high continuing through the black layer phase.

If disease occurs, fungicide treatment boils down to disease presence and timing. "By the time you see a disease, it's been there two to three weeks," says Groeteke. "It's already hypothetically caused some damage. Is it worth to spend money and time to spray? Yes and no. If it's early in the season during silking and tasseling, you can see benefits. Later in the growing season, more yield potential has been determined, and the less impact you will see."

Severe cases of foliar diseases such as corn rust, northern corn leaf blight and GLS can reduce yields by as much as 20%, says Greg Shaner, Purdue University Extension plant pathologist. That rarely occurs with today's crop genetics. However, it's not uncommon to see infection while scouting.

"Many corn hybrids have what we call a 'partial' resistance to these leaf diseases," he says. "The fungus does infect the plant, and it does produce lesions, but it takes longer for these lesions to develop, and they don't produce as many spores. Therefore, the epidemic doesn't progress that rapidly.

"So if you see two or three lesions per plant in a hybrid cornfield, that doesn't mean the disease is going to become severe enough to justify a treatment with a fungicide," adds Shaner. "That's why I say it is really important to find out as accurately as possible what the degree of susceptibility is to any one of these diseases."

Should fungicide treatments be necessary, timing is critical, says Shaner.

"It is important to get fungicide on very early, when the disease is first showing up," says Shaner. "But, again, for corn hybrids that have partial resistance, a few early lesions doesn't necessarily mean the disease is going to become severe."

Fungicide treatment timing falls between tassel and brown silk. Applicators cannot apply fungicides following brown silk.

"In most places across the U.S., corn diseases can come in right before tassel or after tassel," says Groeteke. "The more that yield potential has been determined, the less impact you will see. There are cutoff points where instead of applying fungicide, you're better off to save your money and go harvest the crop."

The specter of Asian rust initially spotlighted fungicide use on soybeans. Now that spotlight is expanding to corn, mirroring a likely increase in corn-on-corn acres in 2007.

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