Humidity pushes crop disease development

Abundant rainfall has put farmers in a position to raise good corn and soybean crops this year, although the stretches of wet, humid weather during the summer have raised the potential for crop diseases.

In addition, many farmers are dealing with crop insect issues, including some invasive species working their way up into Missouri from the South.

Josh Henik, who farms in Linn County in eastern Iowa, says conditions are favorable for some crop diseases.

“I’m just seeing some disease forming, mostly a little bit of northern corn leaf blight in some spots,” Henik says.

He says with wetter conditions in many areas, he and other farmers are doing what they can to combat disease, including scouting fields and applying some treatments.

“We’re gearing up for some fungicide applications,” he says. “I’m surprised we’re not seeing more disease than we’ve had with the weather conditions.”

Iowa State University Extension Plant Pathologist Daren Mueller says wet, humid conditions can support growth of fungal and bacterial pathogens, leading to more crop disease concerns.

He says this year is a change from recent crop years, with summer rains across much of the Midwest in 2025.

“The last couple of years, there have been some wet streaks, but they have been inordinately dry growing seasons, so diseases have not been too problematic,” he says. “It’s been pretty boring years for plant pathologists, which is good for farmers.”

Mueller serves as coordinator for ISU’s Integrated Pest Management program, and he says this year has provided rain but presented different challenges.

“This year has been different,” he says. “We’ve had a lot of rain. We’ve had consistent rain.”

Mueller says the sweltering humidity has also kept fields wetter, raising the potential for disease. The only thing working against crop disease has been the hot temperatures, which can limit some crop diseases.

“If you want to know what’s the ideal condition for disease, it would be like this year, other than the heat,” Mueller says.

Disease ID

Mueller says there has been a lot of bacterial leaf streak in cornfields. He cautions that this can look like gray leaf spot, but it is a bacterial disease. Applying a fungicide can help with fungal diseases like gray leaf spot, but not with bacterial diseases.

Mueller says there has been some tar spot in corn.

“But that is one of the diseases that doesn’t like the heat,” he says.

A big concern is the presence of southern rust in cornfields.

“We’ve already seen a lot of southern rust in Iowa,” he says. “That is one that is more aggressive than tar spot.”

He says many cornfields in Iowa do not have genetics to resist southern rust, which likes heat and humidity.

As for soybeans, Mueller says Iowa and neighboring states have already found some field with white mold.

“It’s like tar spot,” Mueller says. “If it gets too hot, the risks really drop.”

He says sudden death syndrome remains a major concern in soybean fields, although anything farmers can do to manage for that disease has to be done before putting the seeds in the ground.

Invasive insects

Ivair Valmorbida, University of Missouri Extension state crops entomologist, says corn leafhopper is an invasive insect first found in southwest Missouri in 2024. It has since been confirmed in several southern and central Missouri counties, as well as Gentry County in northern Missouri.

“Corn leafhoppers carry corn stunt disease, which can lead to 100% yield loss in corn,” Valmorbida says.

Symptoms include deformed grain, loose kernels and short stalks. Corn leafhoppers are very small insects, about 1/8 of an inch long, so they can be difficult to scout. They are pale yellow to greenish yellow. They can be found in shaded areas of corn, and another indicator is shiny leaves due to honeydew excretions left after feeding.

Corn stunt disease shows up within 30 days of infection, and by then it is too late for farmers to do anything, Valmorbida says. He says some management strategies include planting corn early to reduce the chances of infestation by corn leafhoppers, insecticidal seed treatments that can provide some control up to the V3 growth stage, and considering foliar insecticides if corn leafhoppers are present before the reproductive growth stages.

This year, Valmorbida says Missouri farmers have also been watching for stink bugs in soybeans. The bugs rob yield by feeding on blooms, pods and developing soybeans within the pods. They also threaten corn.

“They can clip corn silks necessary for pollination,” Valmorbida says

Japanese beetles have been a fairly common pest in recent years, and Valmorbida advises scouting fields since the bugs are highly mobile.

Treatment question

There are several factors for farmers to consider when deciding whether and how to treat crops in season, including agronomy and economic considerations.

Mueller says the Crop Protection Network at cropprotectionnetwork.org is a good resource for farmers to help make pest management decisions. The network is a partnership of several land grant universities, including ISU, MU and the University of Illinois.