Peony botrytis blight

Botrytis paeoniae & B. cinerea

Peony buds that blacken at pea size and never open, and shoots that keel over at the base in a wet spring. Cut the whole clump to the ground in autumn.

Fungus Serious Airborne sporesRain splashSoil-borne Spreads to neighbors

What you'll see

Flower buds blackening and drying at pea to marble size, hanging on the stem without opening — bud blast.
Young shoots wilting and toppling in spring, with a soft black-brown rot at the base where they meet the crown.
Gray fuzzy mold on the rotted tissue in damp weather.
Brown blotches spreading in from the leaf margins, sometimes with the same gray fuzz beneath.
Open flowers turning brown and slimy after rain and then sticking to the leaves below.

Why it happens

A gray mold specific to peonies, overwintering as hard black bodies in the old stems and in the crown debris left standing over winter. Spores are released in spring exactly as the shoots come through, and infect them at the base and at the developing buds. The generic gray mold does the same on top of it in a wet season.

A cool, wet spring is the whole story. Crowded clumps, a peony planted too deep, heavy mulch piled over the crown, and a bed that was not cut down in autumn all make it far worse.

Treating it

Cut off blasted buds and collapsed shoots at ground level as soon as you see them, and bin them.
Clear away all fallen petals and leaf debris from around the crown through the season.
Pull mulch back off the crown so the soil surface dries.
Thin the surrounding planting to get air moving through the clump.
Do not water overhead, and stake the plant so the flowers are not lying in wet foliage.

Keeping it away

Cut herbaceous peonies to the ground in autumn, take the stems away, and do not compost them. This is the one measure that matters.
Plant peonies with the eyes no more than five centimeters deep and give them full sun and room.
Divide a congested clump; airflow through the middle is what keeps it clean.
Keep mulch clear of the crown, and use a coarse mulch rather than a heavy wet one.
Water at the base in the morning during the spring growth flush.

Who gets it

Herbaceous and tree peonies.