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 can stay dormant for some time, but will eventually germinate to produce more spores.
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. paeoniae), onion (B. allii) and broad bean (B. fabae).