Controlling Grey Mould in the Greenhouse
Grey mould is a common fungal disease caused by Botrytis cinerea. Look for soft, decaying tissue with fuzzy grey-brown growth. Damp conditions and crowded foliage favour its spread, especially around damaged stems, fading flowers and ripening fruit. Remove affected material promptly, keep foliage as dry as practical and improve ventilation. There is no single humidity setting that guarantees protection.
Key Takeaways
- Damp conditions favour grey mould. A humidity reading alone cannot tell you whether a plant will become infected.
- Remove dead leaves and fading flowers. The fungus can also infect flowers and fruit without an obvious wound.
- Water at compost level when possible. Morning watering gives splashed leaves time to dry.
- Heating may help manage condensation, but it does not cure grey mould or replace ventilation.
- Focus on plant hygiene, spacing and suitable growing conditions. Avoid homemade spray recipes.
Expert Tip
Check the spaces between plants, not just the leaves you can see from the doorway. Fallen petals and damaged fruit can hide there. Remove them carefully and give neighbouring plants room to dry. Ventilate when weather permits without chilling tender plants.
What Is Grey Mould (Botrytis)?
Grey mould is caused by the fungus Botrytis cinerea. It is one of the most widespread plant pathogens in the world. The fungus produces millions of microscopic spores that float through the air. These spores land on plant surfaces and wait for the right conditions.
The fungus often develops on dead or damaged tissue. Flowers and fruit may also become infected without visible wounds. A fuzzy grey-brown coating is a useful clue, but not every brown patch is botrytis. Look at the affected tissue and growing conditions together.
As a greenhouse cools overnight, condensation can form on cold surfaces. Dense foliage stays damp longer, particularly where air movement is poor. Check these sheltered areas during your regular watering round.
How quickly symptoms develop depends on the plant and conditions. Do not wait for a large grey patch before removing rotting fruit or dead growth.
Which Greenhouse Plants Are Most at Risk?
Almost any greenhouse plant can develop grey mould. However, some crops are far more susceptible than others. Fleshy fruits and soft-stemmed plants provide ideal conditions for the fungus to establish.
| Plant | Risk Level | Most Vulnerable Part | Peak Risk Period |
|---|---|---|---|
| Tomatoes | Very High | Fruit, stems at pruning wounds | August to October |
| Cucumbers | Very High | Fruit ends, damaged leaves | July to September |
| Strawberries | Very High | Ripening fruit touching soil | June to August |
| Grapes | High | Bunches (tight clusters trap moisture) | September to October |
| Lettuce | High | Outer leaves near soil level | Year-round under glass |
| Courgettes | High | Flower ends and damaged skin | July to September |
| Peppers | Moderate | Stem joints and ageing fruit | August to October |
| Ornamental plants | Moderate | Fading flower heads and petals | Spring and autumn |
Tomatoes are vulnerable around damaged stems and ripening fruit. When growing grapes, inspect the inside of tight bunches too. Remove affected fruit carefully without shaking mould onto neighbouring plants.
How to Prevent Grey Mould in a Greenhouse
Start with prevention, then act promptly if symptoms appear. Removing infected material and improving conditions can limit further damage. Dead or rotten tissue will not recover.
Ventilate Every Day
Ventilate during suitable weather, including mild winter days. Adjust openings to protect tender plants from cold and strong winds. Automatic vent openers help with temperature control, but they do not respond directly to humidity.
Most wax-cylinder openers respond to temperature. They may remain closed on a cool, damp day. Check the greenhouse yourself rather than assuming an opener has dealt with moisture.
Space Plants Correctly
Overcrowding is one of the main triggers for botrytis. When plants are packed tightly, air cannot circulate between them. Humidity builds in the canopy. Dead leaves and fruit get trapped and go unnoticed.
Follow spacing and training advice for the particular crop. Remove damaged leaves without unnecessarily stripping healthy foliage. When raising seedlings, sow thinly and separate crowded plants as they develop.
Water at Soil Level in the Morning
Direct water towards the compost rather than soaking the foliage. Use a watering can or suitable drip system. Young seedlings may need a fine rose, so water gently and allow good airflow afterwards.
Morning is convenient because splashed leaves have time to dry. If a plant is wilting from dry compost later, water it then. Do not leave it stressed just to follow a timetable.
Remove Dead Material Daily
Walk through your greenhouse every day during the growing season. Pick off yellowing leaves, fading flower petals, and any fruit showing soft spots. Do not drop this material on the greenhouse floor. Bag it and remove it from the greenhouse completely.
Remove rotting fruit and fallen petals before they become a source of further spores. Regular checks help, but cannot guarantee a disease-free crop.
Clean Your Greenhouse Every Year
Clear spent crops and plant debris, then clean glazing, staging and reusable pots. Follow the cleaner label and the greenhouse manufacturer instructions. There is no need to discard all bed soil routinely. Our spring preparation guide covers the wider clean-up.
How to Treat Grey Mould When It Appears
Illustrated reminder of the practical steps below. This is not a diagnostic photograph.
If you spot grey mould, act immediately. The fungus spreads fast and there is no spray that will cure an established infection. Your response should be physical removal followed by environmental correction.
Remove All Infected Material
Remove affected leaves, flowers and fruit carefully. Where a stem is infected, cut back to healthy tissue if the plant can tolerate it. A badly affected plant may need removing entirely. Bag diseased material and follow local disposal guidance; avoid adding it to a cool home compost heap.
Clean tools between affected plants using a suitable disinfectant according to its label. Wash your hands after handling plant waste.
Improve Airflow Immediately
Increase ventilation when conditions allow, and separate crowded plants. Use only electrical equipment suitable for the greenhouse environment. Keep fans and cables clear of water.
Should you use homemade sprays?
We do not recommend milk, bicarbonate or washing-up-liquid recipes for greenhouse grey mould. These are not a substitute for removing affected growth and improving conditions. No fungicides are approved against grey mould for gardeners.
Does Greenhouse Heating Help Prevent Botrytis?
Heating can help manage condensation alongside ventilation. It is not a treatment for an infected plant.
Warm air and cold glazing can still produce condensation. Heating a sealed greenhouse does not remove its moisture. Match any heating to your plants, then ventilate during suitable weather to help damp foliage dry.
Choose heater output for the greenhouse size, insulation, weather and required temperature. A small tube heater is not automatically enough for an entire greenhouse. Follow its clearances and electrical-safety instructions.
Expert Tip: keep watering simple
Aim water at the compost and check that excess can drain. Avoid splashing infected material onto healthy plants. A narrow-spout watering can is often all you need for a small collection.
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A heater option for background warmthUseful for: Local background warmth where the heater output suits the space. Before buying: Check the output and installation instructions. A tube heater does not guarantee dry glazing or prevent botrytis. Price: £75 |
Frequently Asked Questions
Can grey mould spread between greenhouse plants?
Yes, airborne spores can spread between plants. Remove affected material carefully and improve the surrounding growing conditions. Avoid shaking mouldy growth over healthy foliage.
Should I leave greenhouse vents open in winter to prevent botrytis?
Ventilate when weather permits, while protecting plants from cold. There is no fixed two-hour rule for every greenhouse. Adjust openings to the weather and the needs of the plants inside.
Is grey mould harmful to humans?
Avoid breathing in mould dust when clearing affected plants. Handle waste gently and wash your hands afterwards. If extensive mould or a health condition makes clean-up difficult, ask someone suitable to help.
What temperature kills botrytis spores?
Do not overheat a planted greenhouse to try to kill spores. You can damage the crop without solving the underlying problem. Remove affected growth and improve moisture management instead.
Can I use fungicide on grey mould in a greenhouse?
No fungicides are approved against grey mould for gardeners. Use the practical hygiene and growing-condition measures described above. Do not substitute a professional-use product or a homemade spray.
