Botrytis Blight of Peony

Cut the shoot that flopped at the soil line, slip a bag over a blackened bud before you snip so the gray dust stays in the bag, and in fall cut every herbaceous stalk to the ground and put it in the trash, because the fungus winters as tiny black seed-like sclerotia in that debris. A bottle will not rewind a mushy bud. Water at the base, give the clump sun and about three feet of air, and keep a wet mulch off the red noses in spring.

People call this peony wilt or gray mold once they see the fuzz. The usual fungus is Botrytis paeoniae, a peony specialist, though Botrytis cinerea can blight late flowers the same way. It is not the white flour of powdery mildew, and it is not a dry pea-sized bud that froze or sat too deep. Tree peonies get the same fungus. Do not cut their woody framework to the ground.

Cut the flopped shoot, bag the bud, and bin the fall stalks

Walk the clump on a dry morning, because wet mush puffs spores onto the next stem the moment you shake it. Slip a paper bag over a blackened bud, a collapsing bloom, or a shoot that rotted at the soil line, then cut below the brown and drop the piece in the bag. Bin it. Do not leave those heads on the soil under a crowded Sarah Bernhardt, because fallen petals seed leaf spots and the next wet night starts another round. Clean the blades between plants while an outbreak is on. Skip the job while the foliage is still wet with dew or rain.

In early fall, after the herbaceous peony yellows, cut every leaf and stalk at ground level, rake the old mulch from under the clump, and destroy that debris instead of composting it, because a cold heap will not kill the sclerotia that start next April. Put down a thin, loose inch or two of fresh mulch for winter if you want the crown covered, then pull that mulch back as the red noses push in spring so the new shoots are not sitting in a wet blanket. On a tree peony, cut out only the wilted or blackened stems. The woody frame is next year’s plant, so a ground-level cut would be the wrong cleanup.

Move a shade-sited clump into sun, or thin what crowds it, so the foliage dries after rain, and space clumps about three feet apart if they have grown into one wet mass. Water at the base in the morning. A labeled protectant is last, and only if last year was heavy or the spring stays cool and wet, because a spray coats tissue that is still healthy and cannot rewind a bud that already turned to mush. Start when the tips break ground, then follow the current label for peony and for Botrytis. Skip the brand aisle, and skip a homemade baking-soda mix on rotting tissue. Culture is the job that actually cuts next spring’s spore load.

1. Confirm. A shoot five to eight inches tall that rots at the soil line, or a bud that blackens or browns and then shows gray fuzz in damp air, is this blight. A dry pea-sized bud with no fuzz is not.
2. Bag, then cut infected shoots, buds, and spent blooms on a dry day. Bin the bag. Do not compost it.
3. Fall: cut herbaceous stalks to the ground and destroy the debris. On a tree peony, cut only blighted stems. Keep wet mulch off emerging noses in spring.
4. Sun, space, and water at the base so tissue dries. A labeled protectant as shoots emerge is last, after a heavy year, and only while the plant is still healthy tissue.

A flopped shoot does not mean you should dig the crown

This blight can take most of the flowers in a cool, rainy spring — in a severe year, nine buds in ten fail — and still leave the crown alive, because the fungus is killing soft new tissue, not eating the storage roots. Garden peonies usually store enough in the remaining foliage to come back the next year, so cut the bad stems, keep the rest standing, and do the fall cleanup instead of digging a plant that still has firm, pale roots. A double bloom that leans after rain is often just a heavy flower. Stake that stem. Do not treat every flop as blight.

Dig when the stem at the soil line is dark and leathery, there is no gray fuzz, and the whole plant lifts easily because the roots have rotted. That is a wet-soil job, usually Phytophthora, and it is not this blight. See crown rot if the crown is mush and the hole stays a dish after every storm. Do not replant a peony in that hole until the bed drains. A bottle on a flopped Botrytis shoot will not stand the bud back up, and it will not save a crown that has already gone brown in the ground.

Gray fuzz on a wet bud is this blight

Look at the soil line first. Young shoots rot off when they are five to eight inches tall. The stem looks water-soaked or cankered, then the leafy top wilts and falls over, and in damp weather a gray, velvety mold coats the rot just above the ground and sheds a cloud of spores if you shake it. Small buds turn black and wither. Larger buds turn brown and never open, then the withered head grows the same gray fuzz. Open flowers sometimes brown and collapse under a gray coat. Infected petals that drop onto leaves leave irregular dark brown spots. Late in the year you may find tiny shiny black, loaf-shaped sclerotia just under the skin at the base of a dead stalk.

Cool, rainy weather is the usual season, often as the red noses push through. The first spores come from sclerotia that overwintered on last year’s peony debris. Later spores blow, splash, or ride tools onto new tissue. Spent petals are a favorite food. B. paeoniae lives on peonies, including tree peonies. B. cinerea can hit late flowers from a much wider garden, so a box of strawberries or a grape cluster with gray fuzz is the same genus on a different job. See gray mold or Botrytis for that general blight, not a peony fall cut.

A leathery black stem with no fuzz is a different rot

Phytophthora blight also drops peony shoots in a wet spring, but the tissue is dark, sunken, and leathery, and you do not get the classic gray fuzz. The plant can rot at the crown and pull up with little resistance. That is a drainage job. Cut out what is dead, bin it, and fix the dish of water. Do not run this blight’s bag-and-fall-cut as if it will save roots that have already gone. See crown rot for wet-soil collapse. White cottony webbing and mustard-seed sclerotia at the soil line in hot weather are southern blight, another pull, not this cool-spring gray mold.

A pea-sized bud that stops, looks reddish after a freeze, and never grows a gray coat is bud blast from weather, shade, drought, a plant that is too young or newly divided, or eyes set too deep. Once a Botrytis bud is mush, it will not open, but a dry blasted bud was never this fungus. See how to grow peonies for sun, depth, and why a healthy plant skips a bloom year. White flour you can rub off in August is powdery mildew. It is usually ugly, not a killer, and it is not a spring shoot flop. Glossy purple leaf blotch in midsummer is a different fungus. On a home planting after bloom, bin the leaves in fall and leave the bottle on the shelf.

Gray fuzz on strawberry fruit or a tight grape cluster is gray mold or Botrytis bunch rot. That is the wide-host fungus, not a peony specialist. Do not copper a grape mix onto a peony and skip the fall cut, and do not copy a strawberry harvest onto this clump. Ants on the sticky bud are after nectar. They are not this disease and they do not open the flower for you.

Related reading

• Gray mold or Botrytis — the wide-host gray fuzz on flowers and fruit, not this peony fall cut.
• Powdery mildew — white flour on the leaf in late summer, tissue still firm.
• Crown rot — wet-soil collapse with a leathery black stem and no gray fuzz.
• How to grow peonies — sun, shallow eyes, and a bud that never opens because of site, not blight.