Pests of the Week

Beware Tar Spot in Corn and Thistle Caterpillars in Beans

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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A new app called Tarspotter is alerting scientists that many parts of the north-central U.S. are at high risk for tar spot in corn right now. (Photo courtesy Martin Chilvers, Michigan State University)

ROCKVILLE, Md. (DTN) -- If ever there was a crop year to expect the unexpected, it's 2019 -- and pests are no exception.

This week, we're spotlighting two somewhat unusual threats to crops -- a newer corn disease called Tar Spot, and thistle caterpillar -- an insect usually known more for its beautiful butterfly form than economic soybean damage.

Let's dig in.


Tar spot in corn first surfaced in the U.S. in 2015, when farmers and plant pathologists found the black, tar-like speckles on corn leaves in fields in Illinois and Indiana. Tar spot had a banner year in 2018, and infestations spread through counties in six states in the north-central U.S. -- Iowa, Illinois, Indiana, Michigan, Ohio and Wisconsin.

Scientists from the University of Wisconsin and Michigan State University teamed up to produce a tar spot risk-assessing app, called Tarspotter. The app is not public yet, but a large group of industry and academic scientists are using it this year, hoping to validate its models so it can be available for growers to use next year.

So far, the app has put numerous places in the Midwest at high risk for tar spot infestations. Areas such as northern Illinois, northeast Iowa, and most of Wisconsin are experiencing ideal conditions for the disease to develop, said University of Wisconsin Extension plant pathologist Damon Smith.

"The weather is lining up -- cool and humid," Smith said. "It's these cooler temperatures, with monthly averages below-average and above-average rainfall and humidity that favor it."

No-till corn-on-corn fields with a history of the disease will be most at risk, Smith added. "We have found that it does overwinter here in the Midwest on residue," he explained. "It averages around 20% viability on the spores that are out there. It doesn't sound like a great survival rate, but we had so much inoculum last year, that even just 20% survival will be a lot."

Late-planted fields could experience more damage from the disease because infections will have more time to build to economic levels there, he added.

Corn growers should scout their corn early and often for the raised, black spots on the lower leaves of their corn plants. Try not to spray corn too early, Smith cautioned. "You wouldn't want to be spraying V4 to V5 corn, because those leaves won't be on the plants in another month," he said. "But those leaves can be good indicators that the pathogen is active out here and let you know you will need to make a decision closer to tasseling."

Researchers found that fungicides with multiple modes of action were more consistently effective against tar spot in past years than single mode-of-action products, he added.

See more on tar spot in corn management from the Crop Protection Network here:…

See more on the Tarspotter app and current risk for the disease here:…


Some may know the thistle caterpillar better in its lovely adult form: the painted lady butterfly, with its bright orange, black and white wings. The butterflies overwinter in the southern U.S. and Mexico, and this year, they encountered lush conditions, explained South Dakota Extension field crop entomologist Adam Varenhorst.

"In 2019, large amounts of rainfall occurred in the deserts near the U.S.-Mexican border where the painted lady butterflies overwinter," he told growers in a university newsletter. "This resulted in increased host plants being available for developing caterpillars and increased spring populations."

That helps explain why entomologists from across the Midwest -- Kansas, Nebraska, South Dakota, North Dakota, Minnesota and Iowa -- are cautioning growers to watch for defoliation from the thistle caterpillar, the larval stage of the painted lady butterfly.

Thistle caterpillars are nearly as striking as their butterfly form. They resemble little toilet brushes, with small, spiky tuffs lining their bodies, which can range in color from cream to dark grey, with a yellow stripe. They are especially fond of Canadian thistle -- hence their name -- but can defoliate soybean leaves, too. They will produce a fine, web-like structure that pulls some soybean leaves together and provides protection, Varenhorst noted. Inside, they can feed safely on the leaf tissue.

If you find them in your soybean field, scout beyond the field edges, where they tend to congregate. On vegetative soybeans, scientists recommend only treating when you see 30% defoliation, but drop that to 20% on pod-filling soybeans, Bruce Potter, IPM Specialist with the University of Minnesota, recommended in a university pest newsletter.

Potter recommended growers use this defoliation chart from the University of Nebraska:…

He also urged growers to keep an eye on the size of the caterpillars in a field, which indicates how much longer they will be there. "Mature larvae are approximately 1.25 inches long," Potter explained. "Most of the feeding is done by the large larvae; however, if most of the larvae are full grown, or if you see chrysalis in the field, damage may be ending. Make sure insects are still present in the rolled and webbed leaves before you apply an insecticide."

"Larvae are usually concentrated on the upper part of the plant and not that difficult to control. Use a labeled insecticide at a labeled rate," Potter added.

See Potter's newsletter here:… and Varenhorst's article here:…

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Emily Unglesbee