Browned calibrachoa roots have been invaded by Thielaviopsis basicola.
Rhizoctonia can attack stems at the soil line, but can also develop into foliar blight under high humidity conditions, as seen here on impatiens.
Rotting within a gerbera flower head is likely to be Botrytis blight.
Progressive wilting, yellowing, and dead lower leaves indicate Pythium root rot in Easter lily.
Small root systems with tip die back may result from Pythium attack.
If a poinsettia, fuchsia or calibrachoa is wilting, check for stunted, blackened roots that indicate Thielaviopsis root rot.
A completely rotted root system indicates Thielaviopsis root rot on pansy plugs.