Beat Back Disease

March 14, 2009 04:01 PM

Beating yield-robbing diseases requires keeping an eagle eye on your crop throughout the growing season. But while scouting is important, disease management involves much more than burning shoe leather walking your fields. Strategic planning begins long before planting time, and it will make your scouting easier and more effective.

"Nowadays, almost all seed is sold with some degree of seed treatment already applied, so you can spend less time thinking about seed treatments," says Farm Journal Field Agronomist Ken Ferrie.

Use that time to think about which types of diseases you're likely to deal with. "If you have noticed seedling blight in past years, remember these are mostly water molds, driven by cool, wet soil conditions," Ferrie says.

"Planting in ideal disease conditions like that can sometimes offset the effect of a fungicide and the resistance that's bred into a hybrid. So don't push the planting date in fields with disease history when they're wet. If you can wait until the soil is drier, you will have fewer seedling diseases," he adds.If you're fighting seedling blight issues, plant hybrids with good early season vigor and rapid emergence, Ferrie adds.

Think about which disease risk factors exist in each of your fields. "We put a lot of weight on evaluating the risk and respecting it," Ferrie says.

Old-crop residue, left on the surface, while valuable for many reasons, increases the risk of disease. "Although a few of the diseases affecting adult corn plants blow in with the wind, 70% to 80% of them are carried over in last year's crop residue," Ferrie says.

Anytime you leave more crop residue on the surface, disease risk goes up. So if you're growing continuous corn, no-till would have the greatest disease risk, followed by strip-till, vertical tillage and moldboard plowing, in that order.

In a corn–soybean rotation, no-till still carries the greatest risk because disease spores can live in corn residue that is left on the surface from two years ago. Strip-till ranks second, followed by full-width tillage. "If you no-till your soybeans but do tillage ahead of corn, your risk factor is in between," Ferrie says.

"If you're growing continuous corn, which carries a higher corn disease risk, visit hybrid test plots," Ferrie advises. "If a continuous corn plot was not sprayed with a fungicide, it will give you a good comparison of hybrids and show you each one's weak links for disease resistance."

Understand what hybrid disease ratings can tell you about managing each field. "With individual hybrids, ratings tend to be excellent for one disease but poor for another," Ferrie says. "See if there is a ‘sweet spot' for certain diseases. If there is, you don't necessarily have to shy away from that hybrid; but you should plan to do additional scouting in that field and possibly apply a fungicide."

If you no-till some of your fields and till others, plan to plant your most disease-susceptible hybrids in the tilled fields, where the atmosphere for disease is less friendly.

Making disease stronger. Ferrie's studies show that growing the same hybrid for more than one year in the same field is a recipe for increased disease and yield loss. It's what the military would call a "force multiplier," making disease organisms in the field more effective than they already are.

"If a hybrid is susceptible to a certain disease, you are selecting for that disease when you plant it two years in a row," Ferrie says. "Rotating hybrids becomes especially important if you leave old-crop residue on the surface. I've seen as much as 60-bu.-per-acre yield loss from failing to rotate."

With many acres to scout, you have to focus your efforts, Ferrie says. Do that by keeping tabs on disease conditions in your area.

That requires understanding the weather conditions that promote various diseases. "Diseases require not only a susceptible hybrid, but also a favorable environment," Ferrie says.
"Anticipate diseases based on weather. If you don't have your own weather station, visit Internet sites that provide weather information. Whether the weather has been hot and dry or cool and wet for the past seven to 10 days tells you what diseases to watch for," he explains.

Carry a scouting manual, and glean tips that may be buried between the lines. "If a manual recommends tillage as a control measure for a certain disease that tells you the disease carries over in crop residue," Ferrie says. "Start looking for symptoms of those diseases at the bottom of a plant and work your way up."

Common rust and southern rust, for example, blow in on the wind. If conditions have been favorable for those diseases, look for them at the top of a corn plant.

Prioritize your scouting.
Once the disease environment is present, prioritize your fields based on hybrid susceptibility and old-crop residue. Scout first where the risk is greatest.
When it's time to treat for disease, consider the price of corn. "In a wet year, such as 2008, we usually get a yield increase of 3 bu. to 5 bu. per acre, even if there's no evidence of disease when we spray," Ferrie explains. "If you have contracted to sell corn for $6 to $7 per bushel that treatment will pay. If you will be selling for $3 per bushel, it won't."

Keep in mind that there may be reasons to spray a fungicide on some fields even if you don't anticipate a yield increase. "Corn disease may not affect yield as much as it affects stalk quality after harvest," Ferrie says. "When you apply a fungicide, you usually get better standability. So you may want to spray part of your acres just to make sure they stand for harvest.
Improved standability could prevent 20 bu. to 30 bu. per acre yield loss by keeping the crop from going down."

Understand the nature of the fungicide you apply because that affects the timing of your treatment. "Some fungicides are preventative in nature, which means they must be applied before a lot of disease damage has occurred," Ferrie says. "Others are curative, which means they can be applied later."

If you think you may be applying fungicides, keep your aerial applicator in the loop. "When we have preseason maps, we can do many things to prepare for timely applications of fungicides," says Scott Schertz of Schertz Aerial Service in Hudson, Ill. "Our process is also more efficient when we have the go-ahead a few days in advance of the requested spray date. Like any other business, we are more successful when things are planned instead of a last-minute reaction."

You can e-mail Darrell Smith at

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