Grey mould (Botrytis) on vegetables: recognise and avoid

Grey mould is the fungus almost everyone has seen without naming it: the furry greyish brown coating on a rotting strawberry, on a bruised tomato, or at the cut where a side shoot was pinched out.

The pathogen Botrytis cinerea is present virtually everywhere. It only becomes a problem when three things come together: persistently wet plant surfaces, cool air and a point of entry. Those are exactly the levers you can pull in a garden.

In short

Grey mould appears as a greyish brown, dusting fungal mat on rotting tissue.
It needs moisture on the plant and almost always enters through wounds or dead parts.
Especially affected: strawberries, tomatoes, lettuce, beans, courgettes and all propagation stock.
The most important measure is air: ventilate, thin out, keep leaves dry.
Remove affected parts immediately and put them in household waste, not on the compost.

What is Botrytis?

Botrytis cinerea is a weakness parasite with a very broad host range — more than 200 plant species. The fungus persists as resting bodies in plant debris in the soil and, given moisture, produces vast numbers of spores that travel on any air movement.

For infection it needs a film of water on the tissue lasting several hours. That is why cool, damp nights with dew, dense stands and unventilated greenhouses are the classic starting point. The fungus does best at 15 to 20 °C — high summer heat slows it down.

How do I recognise grey mould?

The coating: a greyish brown, velvety fungal mat that visibly dusts when touched. That is unmistakable.
Fruit: soft, watery rotten patches, often starting from a bruise or from the calyx.
Stems: on tomatoes, brown sunken patches at leaf scars and side-shoot wounds. The shoot above wilts.
Lettuce: rot at the root collar, and the whole head lifts off.
Seedlings: young plants topple — here Botrytis is often involved alongside Pythium.

How serious is the damage?

Outdoors it usually stays at individual fruits, particularly after rainy spells. Under glass and in propagation areas, however, grey mould can cost you an entire batch: from an infected stem the shoot dies back above it, and before long the spores are everywhere.

Infection in stored produce is especially annoying. A single affected fruit in a crate is enough to take its neighbours with it — which is why ruthless sorting matters when you put things into store.

Prevention: air and dryness

Ventilate, ventilate, ventilate. Open the greenhouse in the morning even on cool days so the night moisture can escape. This is the single most effective measure.
Water at the base. Water to the roots, not over the leaves. Water in the morning so everything has dried by evening.
Thin the stand. Keep spacings, remove lower leaves on tomatoes, do not let strawberries grow into a mat.
Cut cleanly. Pinch out side shoots in dry weather, ideally in the morning, so the wound dries fast. Clean cuts heal better than ragged tears.
Mulch strawberries. Straw under the fruit prevents soil contact and keeps them dry.
Tidy up. Remove dead leaves and fruit remains consistently — they are the fungus launch pad.

What to do once it is there

There is no good remedy for grey mould in a home garden, and that matters less than it sounds: change the conditions and you get it under control.

Remove affected fruit, leaves and shoots immediately — carefully, so you do not release half a cloud of spores into the stand. Best to cut them straight into a bag.
Dispose of them in household waste. A home compost heap rarely gets hot enough to kill the resting bodies.
Then ventilate and thin out, so the conditions are not immediately right again.
Wipe tools after cutting affected plants, before moving to healthy ones.

Tip

For greenhouse tomatoes it pays to close the door in the evening but leave the ridge vent open. It stays a little warmer at night while the damp air can still escape — exactly the combination Botrytis dislikes.

Caution

Do not remove affected parts in wet weather, and never by tearing them off. Both spread spores and open fresh wounds. A dry morning, a sharp knife, and the cuttings straight into a bag.

Frequently asked questions

Can I still eat affected fruit if I cut the patch out?

With soft fruit and vegetables, better not. The fungal network reaches further than you can see, and Botrytis can produce metabolic products. With firm produce such as squash you can cut generously around it.

Is grey mould on grapes the same fungus?

Yes. Under particular conditions it is even welcome there, as noble rot for sweet wines. There is no such special case in the vegetable garden.

Do baking soda or milk help as they do against mildew?

No. Those remedies work against powdery mildew on the leaf surface, not against a fungus that enters the tissue through wounds.

Why does it always hit my propagation area?

Because everything comes together there: dense spacing, high humidity under the cover, and soft tissue. Taking the cover off earlier, spacing plants out and ventilating in the morning usually solves it.

Our young plants are raised with plenty of space and daily ventilation — so they reach your garden sturdy and without a load of fungal spores.