Strategic Farming: Let's talk crops! session talks whether foliar corn diseases will continue to threaten MN yields

By Angie Peltier and Ryan Miller, UMN Extension educators, and Dean Malvick, UMN Extension corn and soybean plant pathologist

Curvularia leaf spot in corn.
Photo: Dean Malvick

March 4, 2026’s Strategic Farming: Let’s Talk Crops session discussed whether foliar corn diseases are likely to threaten Minnesota yields in 2026. This webinar series runs through March and registrations are still being accepted: https://z.umn.edu/SF2026

To watch this episode visit:  https://z.umn.edu/SF2026 http://z.umn.edu/StrategicFarmingRecordings.

There is a long list of leaf diseases that can negatively impact a corn crop’s yield potential. The diseases that have been of most recent concern in Minnesota include tar spot, southern rust, Northern corn leaf blight and Goss’s wilt and blight, the diseases that were of focus during this session.

Tar spot

Three things must be present for disease to occur, a virulent strain of a pathogen, a disease-susceptible hybrid and environmental conditions that favor disease development. Tar spot is a disease that has been of interest and concern since its arrival in the US in 2015 (and Minnesota in 2019) due to its capacity to cause severe yield loss. Phyllachora maydis, the fungus that causes tar spot, survives over winter in residue from previously infected crops and has continued to expand its range throughout Minnesota. At some point during the growing season, spores are released as primary inoculum from stroma (fungal structures) in corn residue. However, it is through repeated cycles of infection from spores produced on leaves in-season when both losses to yield potential and field to field spore movement can take place. We are fortunate in that in most of its geographic range in Minnesota, symptom severity has been low enough to not limit yield; southeast Minnesota has been and continues to be the one area of the state where yield has been most severely limited due to tar spot, although hot spots have also occurred near Monticello in the central part of the state. These ‘hot spots’ are likely driven by environmental conditions that are not yet completely understood.

The moisture conditions that favor tar spot are still being characterized. While some studies have shown that relative humidities between 70 and 90% favor tar spot, another study has shown that humidities above 90% favor infection. A series of tar spot observations collected between 2015 and 2022 in Indiana showed that rain in July was negatively correlated with tar spot but rain in June and August was positively correlated with disease. Tar spot is favored by moderate average temperatures in the 60’s or low 70’s °F. Years that favor disease occurrence by mid-July are years that are favorable for yield-limiting tar spot epidemics. Data is being collected and fed into models with the goal of helping to predict tar spot risk; while the current model has often over-predicted tar spot risk in Minnesota, continuous model improvements are likely to make this tool more useful for predicting tar spot risk in years to come.

Tar spot management

Hybrid selection is the foundation of any tar spot management plan, as some hybrids are more resistant to the pathogen than others and resistant hybrids exhibit lower symptom severity (and therefore less tar spot-caused loss of yield potential). Foliar fungicides are both labeled for tar spot and some are quite effective. As not every fungicide product works well against each of the fungal pathogens that cause foliar disease in corn, each year, university-based plant pathologists put together a corn fungicide efficacy table based on field-based fungicide research trials of both single active ingredient fungicide formulations and premix products.

Southern rust: Increasing in prevalence in recent years

Minnesota corn can develop two different rust diseases. However, unlike P. maydis, the tar spot fungus perfectly able to survive the harsh Minnesota winter, the Puccinia species of fungi that cause both common and southern rust in Minnesota corn cannot. Spores of both common and southern rust pathogens need to travel north each growing season to cause disease in Minnesota. While the precise cause of the uptick in southern rust during the 2024 and 2025 growing seasons is unknown, Minnesota saw more southern corn rust than in other recent years. Often occurring later in the growing season than common rust, southern rust is favored by humid conditions, more than 6 hours of leaf wetness and temperatures greater than 80 °F.

Both common and southern rust spores cause pustules to form on leaves. Key differences in pustule color and pustule patterns on the leaf can help in distinguishing the two diseases. Common rust is favored by cooler temperatures (below 75 °F), and causes more diffusely scattered, brick-red to brown pustules to form on both leaf surfaces. Southern rust produces small, densely clustered orange to light-brown pustules that occur primarily on the upper surface of leaves. With spores originating from one pustule capable of causing infection and other pustules to form in as few as 7 days, this pathogen has the capacity to develop and spread rapidly and significantly limit corn yield potential. If southern rust develops in states south of Minnesota by mid-July, additional scouting of Minnesota corn fields is required to determine whether an epidemic in need of management is developing. Consulting the corn ipmPIPE southern rust map can keep you abreast of confirmed epidemics in states south of Minnesota.

Southern rust management.

While are differences in susceptibility among hybrids, hybrids adapted to Minnesota have a low level of resistance to southern rust. Routine, weekly scouting of your corn crop will provide you with the information required to make a decision about whether to use a foliar fungicide to protect yield potential. Research has shown that a well-timed fungicide application occurring between tasseling (VT) and the blister growth stages (R2) can help to protect yield potential of leaf tissue that has not yet been infected.

Northern corn leaf blight (NCLB)

In growing seasons that have cool (65-80 °F) temperatures and long periods of leaf wetness (including dew), NCLB can occur and limit yield potential. The fungal pathogen can survive the Minnesota winter in residue from previously infected crops. Lengthening the period of time that infected residue is exposed to the elements can increase degradation of residue (and the pathogen) in the field, reducing the amount of primary inoculum produced. Similar to the tar spot pathogen, spores can also be windblown into a field from neighboring fields, making residue management ineffective in some corn-heavy production areas. Symptoms of NCLB include long lesions that run parallel to leaf veins and taper at each end.

Northern corn leaf blight management

Hybrids vary in their susceptibility to NCLB, with many hybrids showing resistance against specific pathogen races. It is theorized that epidemics of NCLB may partially be the result of deploying resistance genes that do not work against the most prevalent race of the pathogen in a region. Foliar fungicides can be used to manage NCLB and are best timed to occur between VT and R2 when symptoms are prevalent on lower leaves and weather conditions favor additional cycles of infection.

Goss’s leaf blight and wilt

Beginning 10-15 years ago, Goss’s leaf blight and wilt was a significant yield-limiting disease in parts of southern and western Minnesota. Seed companies have focused on incorporating resistance to this disease into Minnesota-adapted corn hybrids since that time, resulting in an overall decline of disease incidence and severity. More recently, there have been renewed reports of Goss’s blight and wilt in west-central Minnesota and South Dakota, suggesting that the pathogen is still out there and capable of causing disease should either seed companies lose focus on breeding resistance or producers lose focus on selecting hybrids with high levels of resistance. Rotating away from corn and effective grass weed management are also useful for Goss’s management as the pathogen can survive and build primary inoculum on green, giant, yellow and bristly foxtail, shattercane and barnyard grass. As this disease is caused by a bacterial pathogen, there is no effective in-season management for Goss’s blight and wilt.

Curvularia leaf spot: The newest corn disease in Minnesota

Found for the first time in Minnesota in 2025, Curvularia leaf spot causes symptoms that appear as small, tan-colored lesions with a brown margin and may be surrounded by a narrow, yellow halo. Curvularia lesions look quite similar to eyespot lesions.

Fungicide trials in southeast Minnesota: 2023-2025

Ryan Miller has run a series of studies (partially sponsored by Wyffels Hybrids) over the 2023 through 2025 growing season in the area of the state that has sustained the greatest tar spot pressure – southeast Minnesota. The goal of these trials was to learn how best to time fungicide applications to manage tar spot. Each year, trials included two hybrids that differed in tar spot susceptibility planted at a density to encourage disease development. Six different fungicide treatment timings were used including an untreated check that received no foliar fungicide, an application at tasseling, one 2-weeks after tasseling, one 5 weeks after tasseling, and two sequential treatments: a tasseling and 2-weeks after tasseling application and a 2 weeks after tasseling followed by a 5 weeks after tasseling application. The fungicide used was 7 oz of Veltyma/acre applied at 20 gallons per acre at 60 psi. The final disease severity was estimated by rating ear leaves at dent (R5) and yield and moisture data was collected.

2023 Trial.

Two hybrids (W4196 and W4358), differing in their level of tar spot resistance (with the company “highly recommending the use of a fungicide for the W4196 hybrid) were included in this low tar spot-pressure trial year. In addition to being a drier than normal year, not favoring disease development, a large hailstorm removed 60-70% of the leaves approximately 3 days prior to the VT fungicide application. A hailstorm occurring at this point in the corn growing season was estimated to have cost the crop 40% of its yield potential as corn is incapable of adding leaves during reproductive growth.

This later season hail event provided a case study for testing our current management recommendations to NOT apply a foliar fungicide after a hail event unless treating a known fungal disease problem. There were no disease symptoms of tar spot at the R5 rating in 2023 and yields ranged between 189 and 191 bu/A regardless of fungicide timing treatment. No differences were measured between the untreated plots and any of the single or sequential fungicide timing treatments, stressing the importance of targeting applications when there is a treatable disease at an incidence/severity that warrants control.

2024 Trial.

Disease pressure was relatively low in 2024, with untreated control plots averaging 5% severity in what appeared to be more susceptible hybrid (W4196) and 2.3% in the less susceptible hybrid (W4358). All fungicide timing treatments reduced ear leaf severity at dent when compared with the untreated plots. In the more susceptible hybrid, a tassel-timed fungicide application resulted in an additional 12 bu/A compared with untreated plots. The later timed single and sequential applications, however, did not differ from the untreated control plots. No yield gains were observed compared to the untreated control in the more resistant hybrid regardless of fungicide timing in this low disease pressure year.

2025 Trial.

Two different hybrids were included in the third year of this trial, W5019 (listed by the company as having “high foliar disease tolerance”) and W3286 (for which the company recommends a fungicide application to maintain health) and more frequent precipitation in the 2025 growing season supported tar spot and southern rust. Southern rust pustules were observed as early as mid-July along the field edge, with symptoms spreading to the mid-canopy by mid-August. But few tar spot stroma were observed at this time. On average, a fungicide application prevented ~20% yield loss, with southern rust likely being the more significant yield-limiting disease.

When applied to the more susceptible hybrid, a tasseling application in this high disease pressure year resulted in the numerically highest yields compared to the untreated check, yielding 39 bu/A more with the fungicide application. Results of the tasseling application were statistically similar to the 2 weeks after tasseling and sequential applications. Waiting to time the first application to 2 weeks after tasseling was too late to protect yield potential in this high disease pressure environment. Of additional concern is that the sequential applications and later fungicide timing resulted in an additional 4-5% grain moisture which could add additional drying costs or harvest losses if one waits for the grain to dry down naturally in the field. The more disease tolerant hybrid had ~20 bu/A higher yields in the untreated plots than the more susceptible hybrid. Similarly, the at tasseling, 2 weeks after tasseling, and sequential fungicide applications resulted in statistically similar and higher yields (by 47-65 bu/A) than untreated plots.

It is difficult to determine whether the level of resistance in a hybrid will influence if a fungicide application will be economical in a low disease year. However, with fungicide costs (product plus application costs) running ~$30-45/acre, properly timing a single application is more likely than two sequential applications to be economical.

Audience questions

Malvick and Miller fielded many audience questions, including: What kind of tar spot disease severity at tasseling does one’s crop need to have for yield loss to occur?; What type of severity at tasseling is needed to reach the ‘trigger point’ for a foliar fungicide application?; How much residual activity can one expect with an earlier timed fungicide application?; Are there diseases where once one sees symptoms it is too late to warrant an application?; How well do generic fungicides work against tar spot?; Is there a benefit to applying a fungicide in-furrow with starter fertilizer?

Thanks to the Minnesota Soybean Research & Promotion Council and the Minnesota Corn Research & Promotion Council for their generous support of this program!