Grey mould

What is grey mould?

Grey mould is a disease caused by the fungus Botrytis cinerea. It normally enters through a wound or infects plants under stress, but will infect healthy plants as well, especially under humid conditions. It can be expected at any time of year.

It is common on apples, grapes, strawberries, blackberries, raspberries, gooseberries, beans, cucumber, courgettes, lettuce and tomatoes. It is also a problem for plants grown under glass, where conditions can be humid and overcrowded. It will infect Chrysanthemum, Cyclamen, Pelargonium, Primula and, in fact, most ornamental plants.

Symptoms

You may see the following symptoms:

• Under humid conditions fuzzy grey-brown mould grows on decaying buds, leaves, flowers or fruit
• If humidity is low, infections may be contained within discrete spots, but if it is high they can spread rapidly
• Above-ground parts of many plants, particularly buds and flowers, shrivel and die
• Small black seed-like structures form in infected material (these are often overlooked)
• On soft fruit, particularly gooseberries, Botrytis infection kills branches, but the fuzzy grey mould is seldom evident
• On strawberries, grapes and sometimes other fruits, Botrytis infection leads to a soft brown rot, often as the fruit is ripening

Control

Non-chemical control

• Hygiene is very important, especially under glass. Remove dead and dying leaves, buds and flowers promptly 
• Do not leave dead plant material lying around
• Reduce humidity by improving ventilation and do not overcrowd plants

Chemical control

No fungicides are approved for use against grey mould by gardeners.

Biology

Botrytis cinerea is an ubiquitous fungus, whose airborne spores are always present. It thrives as a saprophyte (a micro-organism living on dead organic material), but can also infect living plants under certain conditions. On green plant parts a wound or other stress is usually needed for infection, but on flowers and fruits it can infect without wounds, particularly under humid conditions.

It is also able to cause latent infections where the plant is infected but symptoms of the disease may not be produced for a considerable time. In some fruit crops the fungus enters flowers and colonises the developing fruit, but does not break out and cause a rot until the fruit begins to ripen and the sugar content rises. This is very noticeable with strawberries. It can also cause latent infections in primulas, where research has shown the fungus is seed-borne and develops with the plant, only breaking out after considerable growth has occurred.

The fungus forms black, seed-like resting structures (sclerotia) in dead plant tissue which can carry the fungus through periods when host plants are scarce. These germinate to generate the sexual structures which in turn release a second, sexual, spore. These can initiate infections, but most of the damage is done by spread of the airborne, asexual spores (conidia) released from the fuzzy grey fungal growth.

Botrytis cinerea has a very wide host range, but there a number of closely related species which are much more specific to their host, including those infecting snowdrop (B. galanthina), peony (B. peoniae), onion (B. allii) and broad bean (B. fabae).

See also...

Broad bean chocolate spot
Cyclamen grey mould
Greenhouses: choosing
Greenhouse: ventilation and shading
Snowdrop grey mould