Corn Fungicides – Disease Protection and Benefits

Effective corn disease management starts with three questions: Is the crop at meaningful risk of fungal disease, is disease present or likely to develop, and would a fungicide application protect yield potential? The sections below explain how to evaluate those questions by considering the disease triangle, field and hybrid risk factors, scouting observations, fungicide modes of action, and application timing.

The Disease Triangle

An understanding of what a fungicide is, how it protects plants from diseases, and when to consider an application is important prior to using a fungicide.

There are three factors that contribute to disease development, which is commonly known as the disease triangle (Figure 1). These three factors are the pathogen (fungal, bacterial, or viral), a susceptible host plant, and favorable environmental conditions. High moisture situations such as rain, fog, prolonged dew, or overhead irrigation can provide a favorable environment for fungal spore generation and infection. Temperatures also play a role, as some pathogens favor cooler temperatures, while others thrive in warm temperatures. For example, southern rust is favored by humidity and higher temperatures, while northern corn leaf blight favors cool and wet conditions.

How Fungicides Protect a Plant

Fungicides may prevent fungal infection or reduce the impact of diseases caused by fungi; they do not control bacterial or viral diseases. Their activity varies by product. Some fungicides interfere with fungal growth and reproduction, while others form a protective barrier that inhibits spore germination. Certain fungicides can also induce changes in the plant that reduce susceptibility to fungal infection without being directly toxic to the pathogen.

Corn Disease Risk Factor Considerations

Continuous corn fields, fields with a previous history of fungal disease(s), and cultural practices such as no-till or conservation tillage that leave high amounts of residue on the soil surface may increase the risk of infection. Planting susceptible seed products or products with a history of stalk health issues can influence the likelihood of disease development. Portions of a field that border tree lines, creeks or ponds may also be a factor. A proactive fungicide application may help to decrease the likelihood of fungal diseases limiting yield potential.

Why Protecting the Upper Leaf Canopy of Corn is Important

During the grain fill period of corn growth, the developing kernels are the primary sink for the photosynthate produced by the plant. The demand of the developing kernels for photosynthate takes priority and as a result, a corn plant does what it can to produce grain, sometimes at the expense of maintaining the health of other plant parts including the roots and lower stalk.¹

Low-stress conditions during grain fill can help the crop reach its yield potential. In contrast, severe stress may cause kernel abortion or lightweight grain and can predispose plants to stalk rot. The upper canopy is especially important because research indicates that leaves from the ear leaf to the uppermost leaf produce at least 60% of the photosynthate needed for grain fill.¹

Growers have historically reported improved standability and a “greening effect” after fungicide applications, and university trials have examined these observations. In experiments conducted by an Iowa State University research team in 2016 and 2017, an R1 fungicide application was associated with an average 9.3% reduction in stalk lodging and a 4.1 bu/acre increase in corn yield. The researchers also observed a greening effect but cautioned that it could increase grain moisture at harvest. They further noted that using fungicides primarily to achieve greening could increase selection pressure for fungicide-resistant pathogens.^2,3

Classifying Corn Fungicides

Fungicides can be classified in several ways, including by mode of action, movement within the plant (contact or systemic), and timing of activity (preventative or curative). These classifications help explain how a product works, where it moves after application, when it should be applied, and how multiple modes of action can support resistance management.

Modes of Action

The mode of action (MOA) describes what process is impacted in the fungi and is denoted with a letter code. There are eleven fungicide MOA groups classified by the Fungicide Resistance Action Committee (FRAC), with the most commonly used fungicides in corn being respiration inhibitors (C) and sterol biosynthesis inhibitors (G). The eleven MOA classes are further divided into groups which describes the target, or specific site, of the fungicidal activity. Groups 7 (SDHI-fungicides) and 11 (Qol-fungicides) are the most commonly used respiration inhibitors in corn, and Group 3 (DMI-fungicides) is the most common of the sterol biosynthesis inhibitors.⁴ The FRAC code list for 2025 can be found at FRAC Code List as well as comments concerning the risk level for resistance for each FRAC code group.

Preventative or Curative Modes of Action

For a fungicide to be considered curative, it is necessary for the active ingredient to be systemically translocated within the plant, allowing it to inhibit (cure) existing pathogens within the leaf. Curative fungicides may be applied shortly after infection occurs. They are systemic, can be absorbed into the leaf, and translocated through the xylem within the leaf (locally systemic) or the plant (acropetal systemic). In addition to controlling pathogens which have already entered the plant, curative fungicides have longer residual activity as they are protected from wash-off and weathering. Due to their systemic nature, curative fungicides have the potential to reach plant tissues that could otherwise be difficult to reach using spray equipment.⁵

In contrast, preventative (protectant) fungicides are described as having a contact action (but may exhibit some systemic activity), remaining on the surface of the leaf where they can protect the plant against the germination of newly-landed fungal spores. Protective fungicides require application before infection by the pathogen takes place. Translaminar fungicides (locally systemic), such as strobilurin fungicides, redistribute the fungicide from the upper, sprayed leaf surface to the lower, unsprayed surface. Some triazole (protective and early-curative) fungicides may also have translaminar activity.⁵

The three primary fungicide groups used for foliar treatments in corn are as follows:

1. Triazoles (Group 3) inhibit an enzyme involved in sterol production, which is essential for fungal cell membrane development. This disruption causes abnormal fungal growth and can lead to fungal death. Triazoles are generally associated with curative activity, although activity varies by active ingredient. Prothioconazole, an active ingredient in Delaro® 325 SC fungicide and Delaro® Complete fungicide, has both preventative and curative activity.
2. Strobilurins (Group 11) inhibit mitochondrial respiration at complex III, which stops energy production, causing fungal death. These are generally considered to be curative compounds and often have a “plant health” effect.
3. SDHI’s (Group 7) are also respiration inhibitors, blocking electron transport at complex II. They work primarily by stopping spore germination.

Combining Modes of Action

To help slow the potential development of fungicide resistance within a fungal population, it is important to blend effective modes of action whenever possible with each application.

Because different modes of action respond to diseases and growing conditions differently, combining modes of action may help improve performance consistency while reducing the likelihood of resistance. A product with two or more modes of action may offer protection even if a fungus is resistant to one of the important diseases.

Scouting and Corn Disease Thresholds

Scout fields regularly so disease development can be identified early. When lesions are found, assess the planted product’s genetic tolerance, current and forecasted weather, disease severity and location in the canopy, and crop growth stage. Use those observations to determine whether a foliar fungicide should be applied immediately, considered after additional scouting, or not applied.

The generally accepted threshold (excluding common rust) to justify fungicide application is if the field is planted to a susceptible or moderately susceptible seed product, and if lesions appear on the third leaf below the ear or higher on 50% of the plants at tasseling. The best timing for fungicide application is between VT (tasseling) and R2 (blister) growth stages, although some applications can be made as late as R5 (dent).⁶ Be sure to consult specific fungicide product labels for application timings.

Applying Corn Fungicide Proactively May be Beneficial

Applying fungicides proactively may be beneficial for several reasons:

• A proactive application may help protect the plant from disease development before usual symptoms appear. Symptoms take several days to appear after initial infection. The time between initial infection and the appearance of symptoms give a disease such as gray leaf spot (GLS) time to get a foothold and in the process invade plant cells that increase or maintain yield potential. Gray leaf spot has a two-week latent period from infection to visible lesions.
• Fungicides can help reduce the impact of stress events such as disease, hail, drought, and heat. Plants under stress produce the ripening hormone ethylene. When ethylene is produced prematurely, plant growth slows, leaf senescence begins, and kernel abortion can occur, which ultimately and negatively impacts yield potential. Fungicides that contain a strobilurin component inhibit the production of ethylene, which along with other effects, helps keep the plant growing in times of stress, such as drought.
• During periods of rapid growth, the waxy layers on the leaf surface may become thinner, allowing disease an easier entry into leaf tissue.

Importance of Genetic Disease Resistance/Tolerance of Hybrid Corn Seed Products

Each corn product has its own level of resistance to various diseases. One can be highly susceptible to a disease such as southern rust or tar spot, and another highly resistant. If growing conditions are favorable for disease development and a susceptible product is planted, a properly applied foliar fungicide can help reduce the potential impact on yield. If the seed product is highly resistant, a foliar application for the explicit protection against the disease may not be warranted. However, the diseases to which the seed product is vulnerable may have the potential to develop and a well-timed fungicide application can help provide protection against diseases and maintain overall plant health. Fungicide resistance management procedures should be followed.

Corn Disease Management with Delaro® 325 SC and Delaro® Complete Fungicides

Apply Delaro® 325 SC fungicide at 8 to 12 fl oz/acre during the VT to R1 growth stages. Use a minimum carrier volume of 2 gallons per acre (GPA) for aerial application or 10 GPA for ground application. Delaro® 325 SC contains prothioconazole (Group 3) and trifloxystrobin (Group 11). It is labeled to control key fungal diseases, including gray leaf spot, northern corn leaf blight, southern corn leaf blight, rust, eyespot, anthracnose leaf blight, and tar spot. Consult the product label for complete instructions and confirm that the use is registered in your state.

Delaro® Complete fungicide is labeled for control of the same diseases listed above and includes an additional mode of action, fluopyram, a Group 7 product. Delaro® Complete fungicide should be applied at 8 to 12 fl oz/acre at a minimum of 2 GPA aerially or 10 GPA with ground applicators.

Please click on Delaro® 325 SC Fungicide and Delaro® Complete Fungicide to learn more about each product or contact your retailer

Delaro® 325 SC Fungicide and Delaro® Complete Fungicide are not registered for use in all states and may be subject to use restrictions.

The use of adjuvants depends on the fungicide and the timing of application. Fungicide labels should be read to determine if and when adjuvants can be used.

Sources

¹Nielsen, B. 2020. Grain fill stages in corn. Extension Entomology. Pest&Crop newsletter. Purdue University. https://extension.entm.purdue.edu/newsletters/pestandcrop/article/grain-fill-stages-in-corn-3/.
²Robertson, A.E., Serrano, M., Acharya, J., et al. 2020. Effect of foliar fungicides applied at silking on stalk lodging in corn. Online APS Publications. https://apsjournals.apsnet.org/doi/10.1094/PHP-08-19-0049-RS.
³Jackson-Ziems, T. July 2020. Considerations for foliar fungicide in corn. CropWatch. Institute of Agriculture and Natural Resources. University of Nebraska-Lincoln. https://cropwatch.unl.edu/2020/considerations-foliar-fungicide-use-corn.
⁴Fungicide Resistance Action Committee (FRAC). 2025. FRAC Code List 2025. https://www.frac.info/media/ljsi3qrv/frac-code-list-2025.pdf
⁵Teicher, H. 2017. Fungicide mode-of-action: The labcoat guide to pesticides & biopesticides. Bioscience Solutions. https://biocomm.eu/2017/12/17/fungicide-mode-action-labcoat-guide-pesticides-biopesticides/
⁶Byamukama, E. and Strunk, C. 2019. Scouting for corn diseases. iGrow Corn. South Dakota State University Extension. https://extension.sdstate.edu/sites/default/files/2019-09/S-0003-45-Corn.pdf
Web sources verified 04/02/2026. 1211_130337