Advice
RHS Help & Advice
Grey mould (Botrytis cinerea)
Hosts
This very common fungal disease affects a wide range of plants. It is particularly frequent as a fruit rot on strawberries and raspberries, and it can also infect and kill the stems on raspberries and gooseberries. It frequently rots grapes, tomatoes and cucumbers under glass and ornamentals are also prone to infection, particularly primulas and cyclamen. Most plants can occasionally develop infections in stems, leaves or flowers, especially if wounded, and ripe fruits are more commonly attacked than unripe ones.
Symptoms
The symptoms vary according to the humidity and which part of the plant is attacked. Under very humid conditions, a fast spreading rot develops, covered in a fuzzy grey layer of fungus - hence the name. When humidity is low, plant parts may still rot and shrivel, but without the external grey fungus growth. On tomato fruit, infections may be limited to small, pale spots, the so-called 'ghost spots'. Similar small, discoloured spots may occur on cyclamen flowers.
Biology
The disease is caused by a fungus called Botrytis cinerea. This fungus is extremely common because as well as infecting live plant tissues, it also grows on dead, decaying plant material.
When humidity is high, the grey fuzzy growth produces large numbers of airborne spores. These spores can usually only infect healthy plants through wounds. These wounds can be due to mechanical damage, or dead tissues killed by other diseases, for example infections of powdery mildew in grapes and downy mildew in lettuce. The spores can directly infect tissues that are particularly delicate, such as flowers or ripening fruit.
The fungus also has the ability to remain latent (dormant) in developing plant tissues, only breaking out and causing a rot at a later stage. This is known in primulas, where recent research has shown that the fungus is actually carried in the seed. It develops in the tissues of the growing plant, but only breaks out and causes damage as the plant matures.
In soft fruit, the fungus adopts a different strategy, by infecting the styles of the ovaries at flowering, but then remaining latent in the green fruit, only breaking out and causing a rot as the sugar content rises when the fruit begins to ripen. Botrytis is also able to form small, black, seed-like structures called sclerotia. These can survive for many months in the soil before germinating to release spores, probably in response to changes in temperature.
Control
There are now no fungicides available to home gardeners for specific control of grey mould. The use of sulphur dust on vines to prevent powdery mildew will incidentally reduce grey mould on this crop. Myclobutanil (e.g. Systhane Fungus Fighter) and penconazole (Scotts Fungus Clear) are approved for other diseases on ornamental plants and may give some incidental control.
Under glass, it is very important to remove damaged and dead leaves from plants, to reduce spore production. All dead plant material should be cleared up and destroyed and hard surfaces should be regularly cleaned with a disinfectant such as Jeyes Fluid or Armillatox. Air circulation should be maintained to avoid high humidity and reduce spore production. On soft fruit little can be done to prevent infection other than prompt removal of infected material to limit disease spread.