Identification
Grey mould is easy to recognise in the field: affected tissue first becomes soft and water-soaked, then turns brown and is covered by a dusty grey, velvety layer of spores that rises in a cloud when touched. Under the microscope, tall, branched, tree-like conidiophores carry clusters of colourless to pale brown oval conidia, like bunches of grapes; the genus name comes from the Greek botrys, grapes.
On dead stems and fruit the fungus forms small, hard, black sclerotia that survive winter. The sexual stage, known as Botryotinia fuckeliana, forms small stalked apothecia from sclerotia but is rarely observed in nature. Laboratory diagnosis can use culture on agar and molecular tests, because related Botrytis species exist on specific hosts.
Habitat and ecology
Botrytis cinerea occurs worldwide wherever plants grow, from fields and orchards to greenhouses and cold stores. It is a necrotroph: it kills host cells with enzymes and toxic metabolites and then feeds on the dead tissue. It often enters through wounds, senescent flowers or petals and can stay latent in young fruit until ripening.
Cool, humid weather, dense canopies, condensation and long periods of leaf wetness favour infection. Its ability to grow at low temperatures explains why it continues to spoil berries, grapes and flowers even during refrigerated storage.
Uses and importance
Grey mould causes major losses in grapes, strawberries, raspberries, tomatoes, cucumbers, lettuce and ornamentals, both before and after harvest. It is a model organism for research on necrotrophic plant pathogens, fungicide resistance and plant defence, and its genome has been sequenced.
On ripe white grapes, alternating misty mornings and dry, sunny afternoons can produce noble rot: the fungus perforates the skin, water evaporates and sugars, acids and aromas concentrate. Wines such as Sauternes, Tokaji Aszú and German Beerenauslese and Trockenbeerenauslese are traditionally made from botrytised grapes. In wet weather the same fungus causes destructive bunch rot instead.
Safety and edibility
Botrytis cinerea is not a human pathogen and is not eaten; mouldy fruit and vegetables should be discarded rather than trimmed, especially soft produce. People who handle large amounts of mouldy plant material, for example in greenhouses or wineries, may react to the airborne spores, so good ventilation and a dust mask help.
For growers, prevention is about humidity and hygiene: good air circulation, open canopies, avoiding wounds and overhead watering, removing infected flowers and debris, harvesting in dry conditions and rapid cooling after harvest. Biological control agents and fungicides are used in integrated programmes, but the fungus readily develops resistance to many fungicide groups, so rotation of modes of action is important.
If you suspect mushroom poisoning, call your local emergency number or poison centre straight away, even before any symptoms appear, and keep the remains of the mushrooms.
Look-alikes
Grey mould can be confused with other storage moulds. Penicillium moulds form blue-green, powdery colonies rather than grey, tall tufts; Rhizopus produces coarse, fast-growing hyphae with black sporangia; Sclerotinia sclerotiorum forms white, cottony mycelium and large black sclerotia. Related species such as Botrytis fabae on beans or Botrytis allii on onions are host-specific.
Around the world
Grey mould is a global problem in fruit, vegetable and flower production and in viticulture. Noble rot wines are produced in a small number of regions where autumn weather regularly alternates fog and sunshine, including Bordeaux, the Tokaj region of Hungary and Slovakia, and the German and Austrian wine regions.