Lurking beneath the soil, in residue, and adrift on the wind currents, a band of disease desperados waits to rob your yields. The prevalence of these diseases varies annually, as many factors influence which may have the biggest impact on yield. Staying informed, scouting fields regularly, and adjusting management strategies accordingly can help you stay ahead of these yield-robbing villains.
Let’s look at the most wanted culprits in corn fields — along with their rap sheets, and how you can bring them to justice.
In many fields, corn disease pathogens sleep in the soil, waiting for the perfect conditions to strike. Foliar diseases attack the leaf surface, reducing photosynthesis and limiting the plant’s productivity. Others weaken the stalk or infect the ear, making harvest a hassle and minimizing crop quality.
Tar Spot
Courtesy of Syngenta
Crime: Tar spot is a significant disease of corn and can be difficult to manage. Yield loss as high as 50 bushels an acre has been observed, according to the Crop Protection Network. Yield impact varies depending on time of infection, hybrid susceptibility, and weather.
The disease causes irregular, raised black structures — called stromata — to form on the leaf surface. Tar spot lesions are embedded in the tissue and cannot be wiped off. In severe cases, these lesions can also form on leaf sheaths, husks, and tassels. Rain and high humidity allow the disease to spread through wind and rain splash.
Multiple infection cycles can occur in the same season. The fungus can also overwinter on corn residue in the Midwest, causing year-after-year problems.
Last Seen: Tar spot favors cool temperatures and high relative humidity. At least seven hours of consistent moisture on the leaf surface allow stromata to develop. Irrigated corn has a higher risk of infection.
How to Stop: The relatively recent rise of tar spot in the U.S. means best management practices are still developing. Fungicides with long residual activity and multiple modes of action applied between VT and R2 growth stages are most effective, according to Purdue University. In addition to fungicides, select hybrids with greater tolerance, especially for corn-on-corn acres. Scout diligently, as the disease can spread rapidly.
Gray Leaf Spot
Courtesy Provided
Crime: Gray leaf spot can cause up to 50% yield loss, according to Purdue University. The fungus can overwinter in corn residue, and spores spread through wind and rain.
Symptoms first appear on lower leaves and may not be noticed for up to two weeks following infection. Symptoms start as small, pinpoint lesions surrounded by yellow halos, and develop into rectangular, brownish-gray spots. Mature lesions can be up to 2 inches long and are restricted by leaf veins, giving them a blocky appearance.
Last Seen: Gray leaf spot favors prolonged warm and humid conditions. Heavy dew and overcast days can increase the risk of infection.
How to Stop: Hybrid selection and residue management through crop rotation and tillage can reduce the risk of yield loss due to gray leaf spot.
Scout fields prior to reproductive stage. Fungicides are most effective when applied between VT and R1 growth stages and disease is present.
Southern Rust
Courtesy of University of Kentucky
Crime: Southern rust can reduce yields by up to 45%, especially in southern U.S. corn-growing regions, according to the Crop Protection Network. It causes raised orange-to-red pustules that rupture the leaf surface and are often surrounded by chlorotic halos. Symptoms typically appear in the middle to upper canopy and spread through secondary infections. Unlike common rust, which has darker, brick-red pustules on upper and lower leaf surfaces and rarely affects yields, southern rust mainly affects the upper leaf surface. The fungus doesn’t overwinter in the U.S., and spreads from tropical areas via wind.
Last Seen: Southern rust favors high relative humidity and temperatures around 80°F. Approximately six hours of consistent leaf wetness are required for infection.
How to Stop: Tracking the spread of southern rust throughout the season can help evaluate risk. Foliar fungicides applied between R1 and R3 growth stages are most beneficial for protecting yield potential, according to the Crop Protection Network. For further control, consider fungicides with long residual action, or subsequent applications.
Fusarium Stalk Rot
Courtesy of Alison Roberston
Crime: Fusarium stalk rot reduces yields through premature plant death and lodging, with symptoms appearing around the R5 (dent) stage. White or pale-pink spore masses may form near stalk nodes. Lower stalks can soften and collapse, and interiors can appear shredded, with tan or pink discoloration. The fungus overwinters in corn residue and spreads via wind and splashing water. Although the fungus can be found in healthy plants, infection can develop under favorable conditions through roots, stalk wounds, and leaf scars.
Last Seen: Fusarium stalk rot favors temperatures from 80°F–100°F. Risk increases when early-season dry weather is followed by a wet midseason.
Additional Crimes: This fungus is also responsible for Fusarium ear rot.
How to Stop: Resistant hybrids, crop rotation, and tillage can help manage Fusarium stalk rot. While no fungicides target this disease, treating foliar diseases may reduce plant stress and lodging risk. If over 10% of the field is infected 60 days before harvest, early harvest is recommended.
Example return on investment (ROI) for a corn fungicide application with a 207 bpa yield. Plug in your numbers at Crop Protection Network’s ROI Calculator.
To Spray Or Not To Spray?
Fungicide applications are often planned far before any evidence of disease is spotted. The logistics of an unplanned fungicide pass paired with the risk of rapid disease spread have led many farmers to be more proactive.
“I just have to plan on doing it,” said Les Anderson, a farmer in Red Wing, Minnesota. “I know in some years it’s going to break even, but in a bad [disease] year, it can make a huge difference.”
But when input prices are high and crop prices are low, the difference between maximizing yield and maximizing profits makes unnecessary fungicide applications harder to justify.
Weather, hybrid selection, and field history are a few factors that influence a fungicide’s return on investment. Those factors paired with robust scouting can allow for better decision-making.
“It can be daunting to think about looking at every acre in every field multiple times a year, but it’s the best way to know what’s present in the field at what levels, and to know if you need to make a management decision about it,” said Kiersten Wise, a plant pathologist at the University of Kentucky.
To aid in prioritization and decision-making, Wise recommends using disease-tracking maps along with local weather and crop progress reports. For corn producers, the Crop Protection Network also offers a fungicide return on investment (ROI) calculator backed by university trial data. For example, with an expected yield of 207 bushels per acre, the probability of breaking even after a fungicide application was similar for high (greater than 5%) and low (1%) disease severity, ranging from 45–53%.
“It’s a good way to assess what the potential profitability would be for an individual farmer based on their expected yield and sale price, and what they’re going to pay for that product,” Wise said.
