Fungicide classes and FRAC codes
How fungicides are grouped by mode of action and FRAC code, contact and systemic action, resistance risk, major groups with examples, and a tank-mix sum.
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Fungicides protect crops from the fungal and oomycete diseases that would otherwise destroy a large share of the world's potatoes, grapes, bananas, wheat and rice. But a fungicide that controls a disease perfectly one year can fail a few years later, because the pathogen has evolved resistance. Choosing fungicides by their mode of action, and rotating and mixing them sensibly, keeps them working.
The Fungicide Resistance Action Committee (FRAC), an industry body, classifies fungicides by mode of action with a code printed on many product labels. Fungicide groups, their properties and resistance management are standard exam topics in plant pathology and plant protection.
Ways to classify fungicides
- By chemistry: inorganic (copper, sulphur) and organic (dithiocarbamates, triazoles, strobilurins and many more).
- By movement:
- Contact (protectant) fungicides stay on the plant surface and stop spores germinating; they must be applied before infection and renewed as new growth appears or rain washes them off. Examples: copper, sulphur, mancozeb, chlorothalonil, captan.
- Systemic (penetrant) fungicides enter the plant. Some move only locally (translaminar, across a leaf), others move upwards in the xylem (acropetal, the commonest kind), and a few, such as phosphonates, move in the phloem as well. Many can stop an infection shortly after it starts (curative or kick-back action).
- By use: seed treatments, soil treatments, foliar sprays, post-harvest dips, and wound protectants.
- By mode of action: the cell process the fungicide disrupts. This is the FRAC system.
Single-site and multi-site fungicides
- Single-site fungicides act on one protein in the pathogen. They are often very effective at low doses and can be systemic, but a single mutation in the target can make the pathogen resistant.
- Multi-site fungicides act on many enzymes at once, usually by reacting with sulphur-containing groups in proteins. Resistance to them is very rare, but they are contact-only and used at higher doses.
The major groups
| FRAC code | Group | Examples | Notes |
|---|---|---|---|
| 1 | methyl benzimidazole carbamates (MBC) | carbendazim, thiophanate-methyl | systemic; tubulin binding; resistance common since the 1970s |
| 3 | demethylation inhibitors (DMI), mainly triazoles | tebuconazole, propiconazole, difenoconazole, prothioconazole | systemic; block sterol synthesis; resistance builds gradually |
| 4 | phenylamides | metalaxyl, mefenoxam | systemic against oomycetes only; high resistance risk |
| 7 | succinate dehydrogenase inhibitors (SDHI) | boscalid, fluxapyroxad, fluopyram | respiration inhibitors; medium to high risk |
| 11 | quinone outside inhibitors (QoI, strobilurins) | azoxystrobin, pyraclostrobin, trifloxystrobin | respiration inhibitors, derived from a natural compound of a mushroom; high risk, often from one mutation (G143A) |
| 9 | anilinopyrimidines | cyprodinil, pyrimethanil | used against Botrytis and apple scab |
| 40 | carboxylic acid amides (CAA) | dimethomorph, mandipropamid | oomycetes |
| M01–M05 | multi-site contact | copper (M01), sulphur (M02), mancozeb and other dithiocarbamates (M03), captan and folpet (M04), chlorothalonil (M05) | protectants; low risk |
| P | host plant defence inducers | acibenzolar-S-methyl | switch on the plant's own defences |
| BM | biologicals with multiple modes of action | plant extracts; Bacillus and Trichoderma products | growing use, often in integrated programmes |
Copper, in the form of Bordeaux mixture (copper sulphate and lime), was the first widely used fungicide, discovered in French vineyards by Pierre-Marie-Alexis Millardet in 1885 against grapevine downy mildew. Several older chemicals, including mancozeb and chlorothalonil, have been withdrawn in the European Union, though they remain in use elsewhere.
Fungicide resistance
Resistance is a heritable reduction in sensitivity in a pathogen population. It can be:
- Qualitative (disruptive): a single mutation gives a large loss of sensitivity, and control fails suddenly, as with benzimidazoles, phenylamides and QoIs.
- Quantitative (shifting): many small changes lower sensitivity gradually, as with DMIs.
Cross-resistance usually occurs between fungicides with the same FRAC code, so switching between products in the same group does not help.
Managing resistance
- Mix a single-site fungicide with a partner of a different group, often a multi-site protectant.
- Alternate groups between sprays; follow FRAC and label limits on the number of applications per season.
- Apply preventively at full label rates rather than relying on curative action against established disease, which selects for resistance more strongly.
- Reduce disease pressure with resistant varieties, rotation, sanitation and good timing, so fewer sprays are needed.
- Monitor sensitivity in pathogen populations.
Planning a tank mix and a spray programme
A potato grower will spray azoxystrobin (FRAC 11) at 250 g of active ingredient per hectare, mixed with mancozeb (M03) at 1.5 kg per hectare, in 300 L of water per hectare. The products contain 250 g/L azoxystrobin and 75 per cent mancozeb.
- Azoxystrobin product = 250 ÷ 250 = 1.0 L/ha.
- Mancozeb product = 1.5 ÷ 0.75 = 2.0 kg/ha.
- A 400 L tank covers 400 ÷ 300 ≈ 1.33 ha, so it needs 1.33 L of azoxystrobin and 2.67 kg of mancozeb.
Resistance limits. Suppose the programme has 9 sprays and the local guideline allows QoIs in no more than one-third of sprays and never more than three times. QoI sprays allowed = 9 ÷ 3 = 3, applied in mixture and not consecutively; the remaining sprays use other modes of action.
Safe and responsible use
- Follow the label: crop, dose, interval, maximum number of applications and pre-harvest interval.
- Wear protective equipment and calibrate sprayers.
- Keep within maximum residue limits for the target market.
- Protect water and beneficial organisms; copper, for instance, accumulates in soil and is restricted in many places.
Key points
- FRAC codes group fungicides by mode of action; products with the same code share cross-resistance.
- Contact (protectant) fungicides act before infection; systemic fungicides enter the plant, many moving upward in the xylem.
- Single-site groups (1, 4, 11 high risk; 3, 7, 9 medium) are prone to resistance; multi-site groups (M) are low risk.
- Bordeaux mixture (1885) was the first major fungicide.
- Manage resistance by mixing, alternating, limiting applications, preventive use and integrated control.
What the exams ask
- School-leaving and entrance papers: examples of fungicides, contact versus systemic action, and Bordeaux mixture.
- Degree and research-fellowship entrances: FRAC groups and modes of action, resistance types and mechanisms, cross-resistance, and resistance management.
- Professional examinations: designing spray programmes, dose and tank-mix calculations, pre-harvest intervals and residue limits.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why mix a systemic fungicide with mancozeb or copper?
The multi-site partner kills spores that might carry resistance to the single-site fungicide, slowing resistance, and broadens protection.
Can I use a fungicide after I see disease?
Some systemic fungicides have curative action for a short time after infection, but control is always better if sprays start before disease is established. Heavy disease also selects strongly for resistance.
Why do fungicides fail against downy mildews?
Downy mildews are oomycetes, not true fungi, so many fungicides for true fungi do not work on them. They need products such as phenylamides, CAAs, cymoxanil or copper.
Sources
- Fungicide Resistance Action Committee. FRAC Code List 2024: Fungal control agents sorted by cross-resistance pattern and mode of action. FRAC, online.
- Brent, K. J. and Hollomon, D. W. Fungicide Resistance in Crop Pathogens: How Can It Be Managed?, 3rd edition. FRAC Monograph 1, 2007.
- Hollomon, D. W. "Fungicide resistance: facing the challenge." Plant Protection Science 51, 170–176 (2015).
- Lucas, J. A., Hawkins, N. J. and Fraaije, B. A. "The evolution of fungicide resistance." Advances in Applied Microbiology 90, 29–92 (2015).
- Agrios, G. N. Plant Pathology, 5th edition. Elsevier Academic Press, 2005. Chapter 9.
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