Iowa Corn, Soybeans: Preventative and Curative Fungicides

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    Corn is starting to tassel and beans are flowering so no wonder everyone is asking about fungicides.  We’ve tried to address some of the questions regarding whether to spray or not in this article.

    There is also a lot of talk about preventative and curative fungicides. Some fungicide labels also use these terms. What exactly do they mean? When is a fungicide “preventative”? When is it “curative”? What should you be applying to protect your crop from disease? Is one better than the other?

    These terms may be used based on the mode of action of the fungicide, or their application timing.

    Firstly, lets remind ourselves the fungicides applied to field crops to manage foliar disease generally belong to three chemical groups:

    1. Demethylation Inhibitor (DMI) fungicides (triazoles)       FRAC Code 3
    2. Succinate Dehydrogenase Inhibitors (SDHI) Fungicides   FRAC Code 7
    3. Quinone Outside Inhibitor (QoI) Fungicides                   FRAC Code 11

    The risk of resistance developing in pathogens to DMI and SDHI fungicides is medium; for QoI fungicides it is high.  In Iowa, both the frogeye leaf spot and Septoria brown spot pathogens are resistant to QoI fungicides so products that contain only a QoI should not be used on soybean.

    Mode of action

    QoI and SDHI fungicides inhibit respiration, which means they stop energy production in the fungus that results in death. If spores are exposed to QoI or SDHI fungicides before they germinate, the spores do not germinate and infection is prevented. Since QoI fungicides usually accumulate in the waxy cuticle that covers the epidermis tissues of the leaf, they do not prevent growth of fungal mycelium that is present inside the leaf tissue.

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    Both QoI and SDHI fungicides should be applied prior to or very early on in infection to be effective. For this reason, they are often said to have “preventative” activity or be classified as “preventative” fungicides.

    DMI fungicides stop sterol production in fungi. Sterols are required to form cell membranes thus DMIs prevent fungal growth and eventually result in death. Fungal spores contain sterol, so a spore exposed to a DMI can still germinate but once the supply of sterols in the spores is depleted, fungal growth stops.

    DMI fungicides are absorbed into the leaf tissue and prevent growth of the pathogen early in the infection process. Thus, a DMI fungicide is often said to have “curative” activity or be classified as “curative” fungicides.. However, to be most effective, DMI fungicides still need to be applied prior to or very early on in infection to be effective.

    Application Timing

    The terms preventative or curative may also refer to when the fungicide is applied. For example, a study evaluating preventative and curative activities of a QoI and three DMI fungicides against rust diseases of ornamental crops. A preventative application was defined as application of the fungicide prior to inoculation; a curative application was defined as application of the fungicide after inoculation.

    This study found the efficacy of the fungicides decreased as the time from application to inoculation (preventative) or inoculation to application (curative) increased. In other words, the fewer days between inoculation and fungicide application the more disease was reduced. Interestingly, both groups of fungicides provided effective disease control when applied before inoculation, and when applied up to 3 days after inoculation (DAI). In other words, both groups of fungicides had preventative and curative activity.


    In summary, the most important thing to remember is that any fungicide is most effective at reducing disease when it is applied before disease development or very early in disease development. Frequent scouting allows observation of the onset of disease. When disease is observed in a field, timely application of a fungicide is important.

    Other factors to consider include susceptibility of the hybrid or variety, growth stage of the crop, weather forecast, cost of the product and application, grain prices, etc. Most research shows that a positive yield response to a fungicide occurs when disease is present.  The greater the disease severity, the greater the yield response.

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