"This fungicide worked for me — I use it every time." Three seasons in a row with the same product against powdery mildew — and in the fourth season, effectiveness has dropped to zero. Not because the product is bad, but because the fungal population underwent natural selection over three seasons and developed resistance. A proven product applied without rotation is not reliability — it is cultivating a resistant population.
Quick Glossary
- Fungicide — a product that kills or suppresses disease-causing fungi; divided into contact types (act on the surface) and systemic types (penetrate plant tissue)
- Bactericide — a product against bacterial plant diseases; most fungicides have no effect on bacteria — this is a separate category
- Resistance — reduced sensitivity of a pathogen population to a product after several generations under its pressure; develops faster with mono-application and at sub-optimal doses
- Rotation — alternating products with different modes of action so the pathogen population cannot adapt to a single mechanism
Why Mode of Action Matters More Than the Product Name
Fungicides are classified by mode of action — FRAC codes (Fungicide Resistance Action Committee). Products with the same FRAC code share the same mode of action and cross-resistance: a population that has become resistant to one product in the group is automatically resistant to all others in the same group.
This is why rotating "tebuconazole — propiconazole — difenoconazole" is not a rotation. All three are triazoles, FRAC code 3, one mode of action. Real rotation requires alternating groups: FRAC 3 (triazoles) → FRAC 7 (SDHI) → FRAC 11 (strobilurins) → repeat.
Knowing the FRAC code of a product matters more than knowing its trade name. One product may be sold under a dozen brands — all of them are the same FRAC code and rotation between them achieves nothing.
Contact vs Systemic: When to Use Each
Contact fungicides (copper, sulphur, mancozeb, chlorothalonil — FRAC M) remain on the surface and act as a protective barrier. They do not treat an infection that has already penetrated. Used preventively or at the very earliest stage of infection. An important advantage: resistance does not develop to multisite contact products (FRAC M) — they do not need to be rotated among themselves, though rotation with systemics is beneficial.
Systemic fungicides penetrate tissue and can stop an infection already underway. Triazoles, SDHI, strobilurins — all systemic. Resistance develops fastest to systemics because they have a single specific site of action.
Combined strategy: contact product for prevention and early applications, systemic when infection is confirmed. During active disease — systemic combined with contact: systemic stops growth, contact protects against new infection.
Bactericides: Separate Logic
Most fungicides have no effect on bacteria. This basic mistake — treating bacterial necrosis with a fungicide — costs a week and allows the disease to spread.
Main bactericide groups for greenhouses:
Copper-based products (copper hydroxide, copper oxychloride, Bordeaux mixture) — have both bactericidal and fungicidal action simultaneously. Contact, preventive. With regular application, copper may accumulate in the substrate and become phytotoxic.
Streptomycin — a broad-spectrum antibiotic against bacteria. Effective but with strict limitations: develops resistance with regular use, restricted or prohibited in some countries for food crops, requires strict observation of pre-harvest intervals.
Phosphonates — when applied systemically, they enhance the plant's own defence against some bacterial and fungal pathogens; not a direct bactericide, but reduce infection severity.
For bacterial disease on food crops — first verify product approval for the specific crop and HACCP requirements. Bactericide residues in the product constitute a food safety violation.
Building a Rotation in Practice
Basic scheme for powdery mildew protection over a season:
- Applications 1–2: FRAC 3 (triazole — tebuconazole or myclobutanil)
- Applications 3–4: FRAC 7 (SDHI — boscalid or fluxapyroxad)
- Applications 5–6: FRAC 11 (strobilurin — azoxystrobin or kresoxim-methyl)
- Then repeat the cycle, or add FRAC M as preventive cover between series
Maximum 2 consecutive applications with the same FRAC code. If reduced effectiveness is observed — change the group immediately; do not increase the dose.
Intervals between applications: follow product label and observe pre-harvest intervals for food crops. For microgreens with a 7–14 day cycle — chemical protection is not used at all: no pre-harvest interval fits within such a cycle.
Three Mistakes That Cost the Most
Rotating trade names instead of FRAC groups. "Acrobat this month, Ridomil next month" — both are FRAC 4 (phenylamides). No rotation, resistance accumulates. Check the FRAC code of every product before planning the programme.
Reducing the dose to "save money" or "avoid burn." Sub-optimal doses are the fastest path to resistance: the pathogen receives enough chemical pressure to select for resistant lines, but not enough to be destroyed. Apply the recommended dose or do not apply at all.
Ignoring pre-harvest intervals on food crops. Cucumber, tomato, lettuce — crops where residues in fruit or leaves are routinely tested. An integrated IPM strategy includes selecting products with appropriate pre-harvest intervals at the planning stage — not after application.
Signs That the Protection Programme Is Correctly Set Up
- A list of FRAC groups used in the season exists, with alternation between series
- No product is repeated more than twice in succession
- The effectiveness of each application is assessed after 7–10 days — if symptoms have not stopped, this signals resistance rather than an insufficient dose
- Pre-harvest intervals for all products are known and observed