Paeonia Karl Rosenfield

Hi dim, as usual Spruce is spot on with his advice. But here's an extract from the RHS Pests and Diseases Book on the subject:

"PEONY WILT

Symptoms: Shoots of infected herbaceous peonies wilt and turn brown at the stem base. IN damp conditions, fuzzy grey fungal growth may develop at the base of the stem. The affected stem withers and dies, shortly followed by other stems on the plant.
Sclerotia [shiny black fungal resting bodies] form on and within the infected stems. Occasionally the fungus attacks higher up the stem, causing similar symptoms. Wilt is most common in herbaceous peonies, but tree species may also be affected.

Cause: The fungus Botrytis Paeoniae, which produces numerous spores that are carried on air currents and by water splash. The sclerotia are usually responsible for wilt infections, which appear early in the season, subsequent infections may be due to spore spread.

Control: Cut out infected stems promptly, cutting right back into healthy growth and belwo soil level if necessary. If the stems are not removed, the sclerotia fall into the soil where they can persist for several years. Remove all debris at the end of the season, as this may harbour sclerotia. If a plant has been severely damaged by the infections, scrape off the uppermost few millimetres of soil in the Autumn in case it contains sclerotia - this must be done with care so as to avoid injuring the roots."

I've got 9 different lots of Paeonies, including Karl Rosenfield which is a good plant, and so far they have never been affected by wilt. But if one was I would dig it out and quarantine it, dose the soil around with Jeyes fluid, and try to get the Paeonie back to health.

Your client is not going to like having a sick plant on display so I'd remove it and, since you've spent good money on it, put it in a large pot and try to kill off the wilt fungus.