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 sugar content rises. This is very noticeable with strawberries. It can also cause latent infections in primulas, where recent 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.
A note on the latin name: Because the existence of the sexual stage is known, the fungus should be known by the name of the sexual state, Botryotinia fuckeliana; however the name Botrytis cinerea is much better known to gardeners and is retained here.
Botrytis cinerea has a very wide host range, but there a number of closely related species which are much more specific in their host, including those infecting snowdrop (B. galanthina), peony (B. peoniae) and broad bean (B. fabae).