📈 Fungicides — Classification and Mode of Action

Fungicides are chemical compounds used to kill or inhibit the growth of fungi and their spores. Within IPDM, fungicides serve as a critical tactical tool when other methods are insufficient to keep diseases below economic thresholds.

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Classification of Fungicides

By Chemical Nature

Group	Examples	Target
Inorganics	Bordeaux mixture, copper oxychloride, sulfur	Broad-spectrum
Dithiocarbamates	Mancozeb, zineb, thiram	Multi-site contact
Benzimidazoles	Carbendazim, thiophanate-methyl	Tubulin synthesis
Triazoles (DMIs)	Propiconazole, tebuconazole, hexaconazole	Ergosterol biosynthesis
Strobilurins (QoIs)	Azoxystrobin, trifloxystrobin	Mitochondrial respiration
Phenylamides	Metalaxyl, mefenoxam	RNA polymerase
Carboxamides (SDHIs)	Fluxapyroxad, boscalid	Succinate dehydrogenase

By Mode of Action

• Contact (protectant) fungicides — remain on the plant surface and prevent spore germination. Must be applied before infection. Examples: mancozeb, chlorothalonil.
• Systemic fungicides — absorbed by the plant and translocated internally. Provide curative and protective action. Examples: carbendazim, propiconazole.
• Translaminar fungicides — penetrate the leaf and redistribute locally but are not truly systemic. Example: azoxystrobin in some formulations.

By Spectrum of Activity

• Broad-spectrum — effective against multiple fungal groups (e.g., mancozeb)
• Narrow-spectrum — target specific groups (e.g., metalaxyl against Oomycetes only)

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Key Modes of Action

Ergosterol Biosynthesis Inhibition (DMIs)

Triazoles block the enzyme C14-demethylase (CYP51), which is essential for ergosterol production. Ergosterol is a vital component of fungal cell membranes. Without it, membrane integrity is lost, leading to cell death.

Mitochondrial Respiration Inhibition

• QoI fungicides (strobilurins) block electron transfer at the Qo site of cytochrome bc1 complex (Complex III)
• SDHI fungicides inhibit Complex II (succinate dehydrogenase) of the electron transport chain
• Both groups disrupt ATP production, starving the fungus of energy

Multi-site Inhibition

Contact fungicides like mancozeb and chlorothalonil act on multiple biochemical targets simultaneously. This makes resistance development unlikely but limits their use to protective applications only.

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FRAC Classification

The Fungicide Resistance Action Committee (FRAC) assigns numerical codes to group fungicides by mode of action. This system is essential for designing anti-resistance strategies:

• FRAC 1 — Benzimidazoles (high resistance risk)
• FRAC 3 — DMI fungicides (medium risk)
• FRAC 7 — SDHIs (medium to high risk)
• FRAC 11 — QoI fungicides (high risk)
• FRAC M — Multi-site inhibitors (low risk)

Understanding fungicide classification and mode of action is fundamental to rational fungicide selection and resistance management in IPDM programs.

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Summary Cheat Sheet

Classification Fast Table

Basis	Main Groups
Nature	Inorganic, dithiocarbamate, benzimidazole, triazole, QoI, SDHI
Action type	Contact, systemic, translaminar
Spectrum	Broad-spectrum, narrow-spectrum

High-Yield Recall

• Triazoles inhibit ergosterol biosynthesis (CYP51).
• QoI and SDHI groups inhibit mitochondrial respiration at different complexes.
• FRAC code rotation is central to resistance management planning.

Exam Traps

• Contact fungicides are mainly protective, not curative.
• Single-site fungicides carry higher resistance risk than multi-site fungicides.
• Same trade name does not guarantee a different FRAC group.

References

2 sources • [1] [2]

[1]

Fungicide Resistance Action Committee (FRAC)

Website

[2]

ICAR plant pathology and fungicide use guidance

Official