Crown rot is a fungal disease that is caused by several fungi that live in the soil. Typically popping up during periods of moist, high heat, it is spread by water and is difficult to control. In fact, you should remove plants affected by crown rot immediately so that it does not spread to other perennial plants. It will also be necessary to treat the soil of the removed plant before replanting.

First be certain that what you have observed is crown rot. There will be telltale, mustard seed-like, sclerotia at the base of the plant if crown rot is present. This excellent fact sheet from Missouri Botanical Garden will help you identify the sclerotia and describes the remedies that should be made to the soil after an affected plant is removed.

Courtesy of NYBG Plant Information Office