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QPlant protectionWhen is it justified to spray a chemical product?
Treatment is justified when the damage from the disease, pest or weed would cost more than the treatment itself. This level is called the economic threshold. The aim is not a "clean" field but keeping pests below this threshold.
How to tell whether the threshold has been reached:
- Walk the field at least once a week, especially during active growth and flowering. Look for yellowing leaves, spots, rot, insects and larvae.
- First identify the cause — pest, disease, or drought and nutrition stress. Yellowing is often not a disease.
- Use simple aids — sticky and pheromone traps, a hand lens or a mobile identification app, weather-based forecasting models.
- Compare with the threshold and record every observation and treatment in a field log.
| Problem | Threshold or trigger for action |
|---|---|
| Cereal aphids | More than 10% of plants infested |
| Common vole | 2 active colonies per decare in autumn, 1 colony per decare in spring |
| Powdery mildew | Risk rises at 15–20 °C and relative humidity above 80% |
| Septoria | Risk rises at 15–20 °C with rainfall above 1 mm for 4 days |
| Fusarium head blight | Rain during flowering at 24–29 °C; if needed, treat at 1–5% flowering |
| Weeds | Actual infestation above the damage threshold; most effective at early weed stages |
If you treat: only with authorised products and according to the label; with selective products, early in the morning or late in the evening, to protect beneficial insects; rotating active substances to prevent resistance.
- Spraying by the calendar or "just in case".
- Treating without identifying the cause — yellowing is often due to drought or nutrition.
- Raising the dose when a product stops working.
- Spraying at midday in flowering crops when bees are flying.
Material: Training Module 1, sections 5.2–5.4 and 6.3; Brochure 2, "Practical plan" and "Key diseases and pests".