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gray leaf spot lesions on corn leaf Tom J. Bechman
SPOT LESIONS EARLY: Notice the small gray leaf spot lesions on this leaf. There are enough to warrant monitoring the field closely for possible fungicide treatment later.

Help for making fungicide decisions in corn, soybeans

Agronomist says whether to apply fungicide or not is an important decision.

Writing off fungicide applications in corn and soybeans to save money could be a costly mistake. Instead, Andrew Schmidt suggests that timely scouting can help you decide if fungicides could protect yield potential.

“Know what diseases are in the field,” says Schmidt, an agronomist with WinField United based in Missouri and eastern Kansas. “Scouting for diseases is important in both crops. You’ll also need to pay attention to environmental conditions and attempt to anticipate what diseases might do.

Soybean fungicide decisions

“Field scouting is crucial to getting a handle on possible disease pressure,” Schmidt says.

Look for frogeye leaf spot in soybeans, Schmidt says. It’s showing up more frequently in recent seasons. Caused by a fungus, it first appears as tiny, circular spots on leaves.

According to the Purdue University Corn & Soybean Field Guide, look for circular to angular spots up to a quarter inch in diameter. Spots have tan centers with a reddish-brown or purple outer ring. If let go, frogeye leaf spot can result in premature defoliation.

Darcy Telenkofrogeye leaf spot on soybean leaves


WATCH FOR SYMPTOMS: This is frogeye leaf spot. If you treat, ask for help with fungicide selection. This disease is resistant to some fungicides.

There are other soybean diseases to watch for, including some caused by Septoria organisms, Schmidt says. If you decide to spray, you’re looking at a single application around the R2 to R3 stage. “The R3 stage offers the best return-on-investment potential,” Schmidt says.

If scouting turns up bean leaf beetles or other insects above the threshold, you may add an insecticide with the fungicide, he says. However, it’s important to keep scouting. Because insecticides are only effective for a few weeks, if insects like stinkbugs show up above threshold levels later, you may need a separate insecticide application then.

Fungicides and corn

Ask yourself several questions as you make field-by-field decisions about fungicide for corn, Schmidt says. First, does the hybrid or hybrids in the field have good tolerance or resistance to specific leaf diseases? If so, which ones? For example, maybe a hybrid has good resistance to gray leaf spot but is more susceptible to northern corn leaf blight. Even in the absence of disease pressure, some hybrids may have a record of high response to fungicide applications.

The next question boils down to environmental conditions. What will the weather be like over the next few weeks? Warmer weather favors gray leaf spot, while cooler weather favors northern corn leaf blight.

“In any case, you’re back to scouting and seeing what is in the field,” Schmidt says. “Start scouting early, and note size of lesions and where they’re located. If infection is severe enough and/or the ear leaf is threatened, you may even need to treat hybrids with more resistance.”

You also need to continue scouting. Southern rust must be blown into the area from the South, typically via thunderstorms. It can occur anytime, even into late August or early September. Losses can mount quickly if rust reaches threshold levels and isn’t controlled. Tar spot, a bigger threat in northern areas, can also move in quickly.

“Make sure you or your applicator gets good coverage if you spray fungicides,” Schmidt notes. “An adjuvant such as MasterLock can help with spray coverage. Follow label directions for each fungicide for proper application timing and tankmixes.”

TAGS: Corn Soybeans
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