Corn Foliar Fungicide Modes of Action

• Older fungicides typically only had one active ingredient, but many newer ones have two, or even three, active ingredients with different target sites.
• Fungicide mode of action (MOA) refers to the cellular process inhibited by a fungicide.
• Fungicide target site (or site of action) refers to the specific enzyme involved in a cellular process to which a fungicide binds. It is possible for two fungicides to have the same mode of action but different target sites.
• Target site is the basis for FRAC codes, which are group numbers assigned by the Fungicide Resistance Action Committee that are shown on fungicide product labels.
• In practice, the term “mode of action” is commonly used interchangeably with target site or FRAC group.
• FRAC currently recognizes 12 different known fungicide modes of action. Of these, two are currently utilized in foliar fungicide products used in corn: inhibition of cellular respiration or inhibition of sterol biosynthesis in cell membranes.
• This includes three different FRAC groups (target sites), two of which share the same mode of action:
  • Group 3: Demethylation Inhibitors (DMI)
  • Group 7: Succinate Dehydrogenase Inhibitors (SDHI)
  • Group 11: Quinone Outside Inhibitors (QoI)

• One of the most important benefits of fungicide products with multiple modes of action is pathogen resistance management.
• Fungicides with multiple modes of action can also provide more effective disease control by targeting a broader range of fungal diseases and pathogens and providing more comprehensive protection for the corn crop. 

Mode of Action: Sterol biosynthesis in membranes

Target Site: C14-demethylase in sterol biosynthesis

Mobility: Xylem-mobile

Resistance Risk: Medium

• Group 3 fungicides are referred to as the triazoles, as most of the active ingredients used in corn are in this chemical group.
• These fungicides are effective against many fungal diseases, especially rusts and leaf spots.
• Many current fungicides combine a DMI with a Group 11 fungicide (strobilurin), as well as three-way products that also include a Group 7 fungicide.
• DMI fungicides are locally systemic and xylem-mobile, which means they can spread in the leaf tissue from the site of application and move upward in the plant via the xylem tissue.
• These fungicides typically have around 14 days of residual activity after application.

Mode of Action: Cellular respiration

Target Site: Complex II: succinate-dehydrogenase

Mobility: Locally systemic

Resistance Risk: Medium-high

• The first generation of these fungicides had relatively limited disease spectra. SDHI fungicides with increased spectrum and potency were commercialized beginning in the early 2000s.
• Corn fungicide products that include a SDHI typically also include a Group 3 or Group 11 fungicide, or both.
• SDHI fungicides are locally systemic, capable of moving a short distance from the site of application.
• SDHIs have longer residual activity than other groups.

Mode of Action: Cellular respiration

Target Site: Complex III: cytochrome bc1

Mobility: Locally systemic / translaminar, some are xylem-mobile

Resistance Risk: High

• The QoI fungicides, commonly known as strobilurins, are a relatively new group of fungicides modeled after a naturally occurring fungicidal compound.
• QoI fungicides are active against a broad range of plant pathogens. Most have locally systemic and translaminar mobility in plants, and some are also xylem mobile.
• These fungicides can have 7-21 days of residual activity.

Reference.: Wise, K. 2025. Fungicide Efficacy for Control of Corn Foliar Disease. Crop Protection Network CPN-2011-W.