Be Weather Wary When Scouting

Insects got off easy this winter and persistent wet conditions this spring mean diseases have a head start. 

“This season, more than most, 55-mph drive-by scouting just isn’t going to work,” says Erich Eller, crop consultant and owner of ForeFront Ag Solutions in Indiana. “One reason is you might find more insect issues if you have a lot of replanting or late planting on your farm.”

The potential for bigger, stronger flushes of insects means you have to be ready to act. 

“If you look at states all around we’re hearing about large flights of armyworm,” says Dean Grossnickle, Syngenta agronomic services representative. “Illinois, Minnesota and Wisconsin have all put out bulletins about them.”

Armyworms are flying in from southern states with rain and warmer temperatures. They attack corn leaves, sometimes defoliating them. Scout through July and consider treatment if more than 50% of plants show fresh feeding and larvae are 1¼" long.

“Corn rootworm might not be a bigger hatch than normal, but in my area we’re three to five weeks behind so any pressure on that small of plants will intensify damage,” Eller says.

Corn rootworm pose a double threat by attacking the crop in two stages, larvae and beetle. Watch for root damage from larvae through July, which can lead to lodging. Consider treatment if you see eight or more rootworms per plant after washing roots or two rootworms per plant if you don’t wash roots. Adult beetles attack the top of the plant from July through September. If silks are clipped to ½" or less during pollination it can impact the number of kernels that develop. Consider treatment if silks are clipped to ½" or shorter and fewer than 50% of plants are pollinated.

“European corn borer is making a comeback as more farmers try conventional corn to cut costs,” says Kelley Timon, Ohio State University Extension specialist and associate professor. 

The pest of old tunnels into leaves, stalks, tassels, ears and ear shanks, causing standability issues and dropped ears. With the potential for three generations, corn borer attack from June to October. There’s no defined threshold for the pest. You’ll need to look at yield potential, number of borers per plant, and cost and effectiveness of treatment. Hybrids with Bt proteins provide protection from this pest, but scouting is still important because corn borer’s resistance to the protein is growing in many states.

Finally, watch for bean leaf beetle in soybeans because the mild winter increases the likelihood of a surge in populations. The beetle defoliates plants and has two generations per year—so you need to scout for them all season. In soybeans, consider treatment if there’s 15% defoliation from bloom to pod fill and if there’s 25% defoliation with active feeding from full pod to harvest. 

“The best advice I can give about insects is look around—if you can’t see insects or damage it’s hard to justify foliar insecticide,” says Christian Krupke, Purdue University professor of entomology. “Foliar insects rely on contact, so you can only kill what you can see—if a corn borer is in the stalk an insecticide won’t kill them.”

Expect diseases to spread faster, especially if early season stress has left plants more vulnerable.

“Looking at weather and disease pressure and considering the mild winter we had, conditions might favor more gray leaf spot and northern corn leaf blight,” says Melissa Bell, Mycogen commercial agronomist.

Gray leaf spot (GLS) emerges from silking to maturity and prefers moderate to warm temperatures, above 75° F. Look for yellow to tan spots on leaves that turn into gray rectangle-shaped lesions as the disease progresses. Fields with a history of the disease are especially susceptible.

Unlike GLS, northern corn leaf blight (NCLB) needs moderate to cool temperatures, less than 75° F. If you see cigar-shaped gray to green lesions from silking to the end of the season it’s likely NCLB. The disease overwinters in corn residue so corn-on-corn fields and fields with a history of NCLB are most susceptible.

Anthracnose stalk rot in corn appears at pollination and is most visible just before maturity. The disease occurs when the plant is under stress from insects or weather and the pathogen is present. Check stalks for shiny black blotches and note the inner stalk could also be black—but isn’t always. The stalk will crush easily and could cause the plant to fall. Anthracnose can also affect soybeans. The leaves, pods and stems will show irregular brown spots, pods will have fewer or reduced seeds per pod and the plant might defoliate.

If it stays wet and turns warm, look for frogeye leaf spot in soybeans—especially in southern states. If infected, you’ll see round brown or gray lesions on leaves with reddish margins. New leaves are most likely to show signs of infection, and the disease is more likely to be found in fields with a history of frogeye.

Finally, look for white mold in soybeans. It appears after the flowering stage in cool, wet weather especially in narrow rows and where it has been before. If present, plant tops will turn gray, wilt and die and hard black fungal masses will develop on or inside pods and stems.

Scouting is vital to the health of your crop—make it count this year by considering weather’s impact.

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