Gadenin日本語Get the appBotrytis blight: the one to know
Gray mold, or botrytis blight, is the main disease of tree peonies everywhere they are grown. It shows in cool, wet spring weather as new shoots that wilt and collapse suddenly, with a brown or black rotted patch at the base of the affected shoot, and as buds that soften, turn brown and never open. In damp conditions a fine gray, dusty fuzz appears on the rotted tissue, which is the fungus producing spores.
It survives the winter on old leaves, spent petals and dead stem stubs lying around the base of the plant, then splashes up onto soft new growth in spring rain. That is why it recurs on the same plants year after year and why the cure is a rake rather than a bottle. Crowded plants, plants under overhanging branches and plants watered over the foliage suffer worst, because the leaves stay wet longest.
Deal with it by cutting and clearing on the day you see it. Remove each affected shoot back into clean, firm wood, disinfecting the blade between every cut, and take out rotted buds and blotched leaves with it. Bag the material and take it off site; a home compost pile rarely gets hot enough to kill the spores and simply returns them to the bed. A plant that loses several shoots this way will recover fully if the remaining foliage stays clean through summer.
A wilted shoot with a firm green base is short of water. A wilted shoot that is soft and brown or black where it joins the old wood is infected, and watering it makes things worse.
Everything else you might see
Leaf blotch produces glossy purple brown patches on leaves and stems from early summer onward, spreading in wet weather. It is disfiguring rather than dangerous; pick off the worst leaves, keep water off the foliage and clear everything at the end of the season. Powdery mildew appears as a white dusting late in the season and is cosmetic at that point, since the plant has done most of its feeding for the year.
Root and crown rot follow bad drainage rather than bad luck. A plant that leafs out thinly, with short pale shoots, and whose base is soft or foul smelling when you scrape the soil back, is sitting too wet. On a young grafted plant this is often fatal. The only fix is to lift in early autumn and replant on a raised, free draining site, and to stop watering in the meantime.
Pests are a short list. Scale insects can build up on old bark as brown limpet like bumps with sooty mold below them; scrub them off with a brush and improve airflow. Ants on the swelling buds are feeding on nectar and do no damage at all. Slugs and snails graze soft new shoots at ground level in spring and can strip a small plant, but they lose interest once the growth hardens. Deer and rabbits browse young plants in some gardens and need a physical guard, not a repellent.
| Symptom | Likely cause | Action |
|---|---|---|
| Shoot wilts, base brown and soft, gray fuzz | Botrytis blight | Cut back into clean wood, disinfect the blade, bag and remove |
| Buds brown and rot without opening | Botrytis in the bud | Remove buds and affected shoots, improve airflow, clear all debris |
| Buds dry, papery and brown after a cold night | Late frost damage, not disease | Remove the buds; row cover on frost nights once buds swell |
| Glossy purple brown blotches on leaves | Leaf blotch | Pick off worst leaves, water at the base, clear up in autumn |
| White dust on leaves late in the season | Powdery mildew | Cosmetic; improve spacing, no treatment needed |
| Thin pale growth, base soft or smelly | Root or crown rot from wet soil | Lift in early autumn, replant raised and free draining |
| Brown limpet bumps on bark, sticky leaves below | Scale insects | Scrub off by hand, thin congested growth for airflow |
| Soft ferny stems from below the base | Suckers from the herbaceous rootstock | Remove flush at their origin on the root, not at soil level |
Sanitation is the control program
Almost all the disease pressure on a tree peony comes from its own debris left in place. Deadhead as the flowers fade and collect the fallen petals; rake every leaf away in autumn and take it off site; keep the crown area clear of mulch piled against the stems. Doing this thoroughly makes a visible difference the following spring, more than any fungicide applied during the season.
Water at the base and never over the foliage, and water in the morning rather than the evening so any splashed leaves dry quickly. Give the plant its full 3 to 4 ft (90 to 120 cm) of space and thin overhanging neighbors so morning sun reaches the base. A tree peony whose leaves stay wet until midday will be first to show botrytis every year, and moving the neighbors is far easier than moving the peony.
Treat your pruners as the main vector, because they are. Disinfect between cuts on an infected plant and between plants, always. Never take cuttings or move a plant that has active botrytis, and never leave the prunings lying at the foot of the shrub while you finish the job.
- Inspect weekly from shoot break to flowering Look at the base of the new shoots, not just the buds. Botrytis starts at the join with the old wood and is much easier to cut out cleanly on the day it appears than a week later.
- Cut infected shoots into clean wood Follow the discoloration down and cut where the exposed tissue is firm and pale. Leaving an inch of rotted tissue behind means the shoot below it goes next.
- Disinfect between every cut Wipe the blades with alcohol or a garden disinfectant before moving to the next shoot or plant. Contaminated tools spread botrytis through a border in an afternoon.
- Bag everything you remove Do not compost infected tree peony material and do not leave it on the soil. Spores from a pile of prunings reinfect the plant they came off.
- Strip the ground bare in autumn Rake off every fallen leaf and the old surface mulch, then re mulch with clean compost kept clear of the stems. This removes most of next spring's spore load.
Telling disease apart from frost and from the graft
Three quite different problems produce dead buds, and the treatment differs completely. Frosted buds are dry, papery and brown, damaged all over on one side of the plant, and appear within days of a clear cold night. Botrytis buds are soft, wet brown and often fuzzy, and appear in wet weather on the shoots with the least air movement. Buds that simply never formed are not a bud problem at all; that is a flowering problem caused by pruning, shade or planting depth.
Similarly, a whole plant that declines slowly over three or four years, with fewer and shorter shoots each spring and no rot anywhere, is usually a grafted plant whose nurse root has exhausted itself while the top never rooted for itself. No treatment helps. The answer is to lift it in early autumn and replant with the graft union 4 to 6 in (10 to 15 cm) below the surface.
Sudden death of a single branch with the rest of the shrub healthy is nearly always mechanical or botrytis damage at that branch's base. Cut it off, examine the cut face, and if the wood is stained brown inside, treat the plant as infected and clear the ground under it thoroughly.
A disease record worth keeping
Note the date you first see a rotted shoot or bud each year, and which plant it was on. Botrytis returns to the same plants at roughly the same point in the season, because the spores overwinter in place. Two or three years of dates turn the disease from a surprise into something you inspect for in the week before it usually arrives.
Count the shoots you remove. Losing two shoots from a mature shrub is an inconvenience; losing eight is a plant whose position needs reconsidering for airflow and light. Only a written count makes that distinction clear a year later, when the plant has leafed out and looks fine again.
Record the spring weather in a line or two, because cool wet springs produce bad botrytis years across the whole garden. Knowing that a bad year was a weather year rather than a decline in the plant stops you from digging up a tree peony that was never in real trouble, which on a slow growing shrub is a very expensive mistake.