Botrytis cinerea is in the Ascomycete family Sclerotiniaceae. Like its cousin Sclerotinia sclerotiorum (white mold or rot of bedding plants), it forms sclerotia (hardened, asexual resting structures) in decaying plant matter that has rotted as a consequence of infection and fungal colonization.
Crops like coreopsis, dianthus, heuchera, lavender and rudbeckia can be especially vulnerable to botrytis blight in the spring when they have a full flower canopy. Weather conditions are also an important factor in botrytis blight outbreaks. This is a fungus that prefers cloudy, rainy weather. Showers followed by cool weather are ideal conditions for the gray mold caused by Botrytis cinerea to develop in ornamental nurseries. While diseases caused by Botrytis are not aggressive on mature tissues or woody plants, the fungus rapidly attacks fleshy or juvenile tissues such as flower petals, new shoots or tender growth of bedding plants or other annual crops.
Sources: Michigan State University Extension, University of California, Division of Agriculture and Natural Resources