Sprayed a fungicide, a week later grey mould is back. Switched products — same result. The problem is not that the fungicide is weak or wrong. Botrytis returns because the conditions for its development are unchanged: high humidity, stagnant air, dead tissue it feeds on. A fungicide suppresses an active outbreak — but does not remove the environment that caused it.
Quick Glossary
- Botrytis cinerea — a necrotrophic fungus, the causative agent of grey mould; develops on dead and weakened plant tissue, then spreads to living tissue
- Grey mould — a disease that presents as a grey fuzzy coating on affected tissues: leaves, stems, fruit, flowers
- Botrytis spores — microscopic conidia that are constantly present in greenhouse air; germinate under favourable humidity and temperature conditions
- Fungicide — a product that suppresses fungal infection; effective as part of a control system
Why Botrytis Is a Conditions Disease, Not Just an Infection
Botrytis spores are present in the air of every greenhouse at all times. They do not disappear after fungicide application — new spores arrive continuously via air, clothing, and tools. The question is not "are spores present" but "will they germinate."
Spore germination requires three conditions simultaneously: relative humidity above 85–90%, temperature 15–25°C, and an entry point — dead or damaged tissue. Below 85% humidity, or with active air circulation that dries the leaf surface, the spore will not germinate even if temperature is ideal.
The control logic follows from this: removing the conditions for germination matters more than destroying spores. A fungicide kills active mycelium — but if humidity remains at 92% and stagnant air persists, a new wave of infection will develop from spores that arrived after the application.
Where to Look for Botrytis First
Botrytis does not start on a healthy leaf — it starts where dead or weakened tissue exists.
Dead and yellowing lower leaves — the most common starting point. A leaf that has yellowed and is still hanging on the stem or lying on the substrate is an ideal environment for Botrytis. Regular removal of dead leaves is not aesthetic housekeeping — it is a sanitation measure.
Sites of mechanical damage — cuts, broken shoots, tying marks. Botrytis enters easily through a fresh wound, especially at high humidity.
Flowers and petals — when they fall, they rest on leaves and serve as substrate for fungal development. Fallen flower remnants on tomato and cucumber fruit are a classic entry point for the fruit form of Botrytis.
Poorly ventilated zones — corners of the greenhouse, the lower tier of dense plantings, places where leaves overlap each other. Local humidity is higher and air exchange lower there, even if overall greenhouse parameters are normal.
Conditions and How to Control Them
Humidity — the primary factor. Keeping relative humidity below 85% at night is the single most important action for Botrytis prevention. At night, temperature drops and humidity rises — night outbreaks are the most common. Ventilating before closing the greenhouse for the night and heating the air to reduce RH is standard practice in commercial greenhouses.
Air circulation — dries the leaf surface and reduces local humidity even at an overall RH of 85%. Horizontal fans moving air between rows are more effective than exhaust ventilation alone.
VPD — at normal VPD, the plant transpires actively and the leaf surface does not remain wet. Low VPD (high humidity combined with moderate temperature) is the condition under which transpiration slows and the leaf surface stays wet longer.
Sanitation — remove dead leaves, fruit, and plant debris. Do not leave pruned shoots and leaves in the greenhouse even "just for an hour."
Fungicides: How and When to Use Them
Fungicides are part of the system, not the whole system. Key principles of effective use:
Group rotation — Botrytis rapidly develops resistance to fungicides with repeated applications of the same product. Alternate between different chemical groups: for example, alternate SDHI fungicides with DMI or QoI. Never make more than two consecutive applications of the same product.
Preventive vs curative application — preventive application under favourable conditions is more effective than curative application during an active outbreak. During an active outbreak a curative fungicide is necessary, but alongside eliminating the underlying conditions.
Biological fungicides — Bacillus subtilis and Trichoderma products are effective for prevention and mild infections. For an active outbreak — only as a supplement to a chemical fungicide.
Three Mistakes That Cost the Most
Applying fungicide without removing the conditions. A fungicide at 92% humidity with stagnant air is money wasted. A new wave of infection will develop from fresh spores within 3–5 days. First address ventilation and reduce humidity — then apply the treatment.
Not removing affected tissue before application. Grey sporulation on a leaf during fungicide application releases billions of spores throughout the greenhouse. Remove affected plant parts carefully into a bag first, then treat.
Using one fungicide all season. Botrytis resistance to SDHI and QoI fungicides develops within a single season under mono-application. Rotation is not a recommendation — it is a requirement if you want the fungicide to remain effective in the next season.
Signs That Botrytis Is Under Control
- Night humidity is consistently below 85%, VPD is normal, dead leaves are removed on every walkthrough
- No new outbreaks for 10–14 days after the last application
- If new outbreaks appear sooner — conditions have not yet been normalised, and the issue is not the fungicide