Botrytis cinerea is found very commonly as a saprophyte (a micro-organism living on dead organic material), producing large quantities of airborne spores under humid conditions. These spores can infect living plant tissues under certain circumstances. Healthy green tissues are usually only infected through wounds. Some more delicate tissues, such as flowers and ripening fruit, may be attacked even though they are not wounded. Very humid conditions favour both the initial infection and the subsequent spread through the tissues.
The fungus forms black seed-like resting structures (sclerotia) in dead tissues. These can remain in a dormant state for some time, but eventually germinate to produce more spores.