2026 Corn Belt disease risks to watch

Corn Belt growers may be relieved that southern rust spores haven’t blown up from the South in severe quantities as they did in 2025, reducing yield significantly. Still, keep the focus on the main corn disease risks — tar spot, gray leaf spot and northern corn leaf blight (NCLB) — along with white mold in soybeans.

As of late June, the southern rust map isn’t showing signs of disease development. The tar spot map, by contrast, continues to intensify.

Tar spot is the biggest concern

Tar spot has become corn’s biggest annual yield threat, followed by gray leaf spot and NCLB, depending on your field’s disease triangle (the interplay of a susceptible host, a virulent pathogen and favorable environment), use of resistant hybrids and foliar fungicide spraying.

As of late June, Nebraska has reported minor tar spot sightings on lower corn leaves in 15 counties, which is a common occurrence for this overwintering disease. It is usually found in irrigated fields initially due to a moist disease environment.

“Tar spot often causes the most concern among corn growers, as corn hybrids currently lack high levels of resistance to tar spot,” says Alison Robertson, Iowa State University plant pathologist. “The other major corn diseases — gray leaf spot and NCLB — are diminishing as more growers plant hybrids that resist these diseases or use preventive foliar fungicides.”

All three diseases overwinter in crop residue, so they pose an annual threat. Robertson isn’t overly concerned with current tar spot sightings, as the heat of summer stops any new infections and slows down any existing infection.

“But once the weather starts cooling off in mid-August, I expect to see tar spot ramping up again,” she says.

Field history and scouting

To understand your risks, it’s best to review different disease triangles, hybrid disease resistance and field disease history.

“The more severe your tar spot was last year or the previous year, the more inoculum that exists in a given field,” Robertson says. “Next, weigh the risks within the rest of the disease triangle as the season develops.”

Tar spot favors cooler temperatures (60 F to 80 F), higher humidity (above 75%), cloudy days and extended hours of nighttime dew. Spores are dispersed by air and rain-splashed residue, including blowing in from neighboring fields. Subsequent infection reduces photosynthesis, cutting yield by 20 to 60 bushels per acre.

Scouting for tar spot is challenging, as infection occurs three weeks before the black tarlike spots (stromata) appear on both sides of corn leaves. Monitoring disease forecasting maps and state plant pathologist/agronomist reports can help with scouting timing.

Planting date and hybrid maturity can affect tar spot risk levels, Robertson says. “Tar spot severity during grain fill in mid- to late-April planting will be reduced compared to mid-May planting,” she says. “The disease has more time to ramp up during the extended grain fill of later-maturing hybrids.”

Foliar fungicide decisions

Note which fields have more-susceptible hybrids, because you’ll want to target those and later-planted fields first with a fungicide application. The same strategy applies if your fields have a recent history of gray leaf spot or NCLB.

Robertson says their research shows the optimum timing for foliar fungicide application against tar spot is to wait until the VT/R1 growth stage. “If gray leaf spot is an issue, we’ve seen better control of it at V12-V14, but the R1 application would give pretty good control of all these leaf diseases,” she says.

Soybean white mold

Knowing your disease triangle by field is important, because sclerotia from previous white mold infection can remain in the soil for five or more years. The pathogen thrives under cooler temperatures (<85 F), high humidity and wet soils during flowering (R1 to R3).

White mold risk increases in a dense canopy. Narrow soybean rows and higher seeding rates (above 150,000 seeds per acre) can speed canopy closure, which traps moisture and humidity. Begin scouting ahead of bloom to monitor for sclerotia germination and the spore release that infects flowers.

Foliar fungicides have proven effective at suppressing white mold when applied from late R1 to R3, when good canopy penetration is achieved.

Current rust potential

Spores must develop in Central America and then blow into southern U.S. cornfields, where they must reproduce before they can blow into the Midwest. As of late June, those conditions have not materialized.

“I didn’t expect the Corn Belt to see two crazy southern rust years in a row,” Robertson says.

If a prolonged warm, wet period occurs in July and rust spores begin moving from the South, it’s worth scouting to assess disease potential. Monitor reports from university plant pathologists and agronomists tracking disease movement on the Crop Protection Network’s disease maps and sign up to receive disease alerts from Corteva Agriscience.

Check out the Crop Disease Forecasting Tool for your area to help determine in-season disease risk potential. This tool by the U.S. Department of Agriculture’s Agricultural Research Service and North Central IPM Center uses site-specific weather data combined with university disease information to show risk possibilities for the location and date(s) you select.