Understanding Fungicide Modes of Action and FRAC Groups

Fungicides control plant diseases by inhibiting specific biochemical processes essential to fungal growth and reproduction. Understanding how different fungicide groups work is essential for building effective spray programmes and managing fungicide resistance.

Key Fungicide Groups

FRAC Group M — Multi-Site Protectants

These fungicides inhibit multiple enzymatic processes simultaneously, making resistance development extremely unlikely.

Mancozeb (FRAC M3): Broad-spectrum protectant dithiocarbamate; inhibits multi-site enzyme processes. No documented resistance cases. The backbone of most disease management programmes.

Chlorothalonil (FRAC M5): Broad-spectrum chloronitrile; multi-site activity. Excellent protectant against early blight, Alternaria, Septoria, and many other diseases.

Copper compounds (FRAC M1): Oldest group; still effective protectants for bacterial and fungal diseases. Risk of phytotoxicity at high rates.

FRAC Group 3 — DMI Fungicides (Demethylation Inhibitors)

Inhibit sterol biosynthesis (specifically C14-demethylation). Includes triazoles, imidazoles, and pyrimidines.

Key products: Propiconazole, tebuconazole, epoxiconazole, difenoconazole, cyproconazole

Characteristics: Systemic and curative; absorbed and translocated in the plant; effective after infection has occurred. Resistance is gradual (dose-response shift).

Key resistance concern: Shifting resistance in Zymoseptoria tritici (Septoria) and powdery mildew.

FRAC Group 11 — QoI Fungicides (Strobilurins)

Inhibit mitochondrial respiration at Complex III (Quinone outside Inhibitors — QoIs).

Key products: Azoxystrobin, trifloxystrobin, picoxystrobin, pyraclostrobin

Characteristics: Broad-spectrum; preventive and some curative activity; excellent for plant health effects. High resistance risk — single point mutation (G143A) confers complete resistance.

Key rule: Never use QoIs alone — always combine with a multi-site fungicide (mancozeb) or a DMI.

FRAC Group 7 — SDHI Fungicides (Succinate Dehydrogenase Inhibitors)

Inhibit mitochondrial Complex II.

Key products: Boscalid, fluopyram, fluxapyroxad, penthiopyrad, isopyrazam

Characteristics: Newer group; broad-spectrum; good systemics. Resistance risk is significant and increasing rapidly. Rotate carefully.

FRAC Group 40 — CAA Fungicides (Carboxylic Acid Amides)

Specific to downy mildew and Phytophthora species.

Key products: Dimethomorph, mandipropamid

Characteristics: Systemic; excellent against oomycetes (not true fungi). Must be combined with a multi-site fungicide.

FRAC Group 4 — Phenylamides

Specific to oomycetes (Phytophthora, Peronospora, Plasmopara).

Key products: Metalaxyl-M (mefenoxam), metalaxyl

Resistance alert: Widespread resistance documented globally. NEVER use alone — always combine with mancozeb. Limit to 2 applications per season.

Building a Fungicide Programme

A sound programme follows these principles:

1. •Foundation: Multi-site protectant (mancozeb, chlorothalonil) at every application
2. •Systemic addition: Alternate Group 3, Group 7, and Group 11 (with the multi-site)
3. •Interval: 7–21 days depending on disease pressure and product residual
4. •Curative use: Apply systemic at first signs of disease; protectants alone are insufficient after infection
5. •Season cap: No more than 2–3 consecutive applications from the same FRAC group