Identification
Gray mold is easy to recognize in the field: affected tissue first becomes soft and water-soaked, then turns brown and is covered by a dusty gray, velvety layer of spores that rises in a cloud when touched. Under the microscope, tall, branched, tree-like conidiophores carry clusters of colorless 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 favor 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
Gray mold 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 defense, 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 botrytized 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; moldy fruit and vegetables should be discarded rather than trimmed, especially soft produce. People who handle large amounts of moldy 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 programs, but the fungus readily develops resistance to many fungicide groups, so rotation of modes of action is important.
If you suspect mushroom poisoning, call Poison Help at 1-800-222-1222 or 911 in an emergency straight away, even before symptoms appear, and keep the remains of the mushrooms.
Look-alikes
Gray mold can be confused with other storage molds. Penicillium molds form blue-green, powdery colonies rather than gray, 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.
In the United States
Gray mold is one of the most familiar plant diseases in the United States, affecting strawberries, grapes, flowers and many greenhouse crops, and it is a common cause of losses after harvest.
In vineyards it usually means bunch rot, but under the right conditions it becomes noble rot, which concentrates the sugars of grapes used for some sweet wines. American growers manage it with canopy management, good airflow, sanitation and careful timing of harvest.