I planted some peony bulbs last year and they grew nicely until they reached about 10 inches high. One was in the ground, and the other is planted in a medium sized pot outside. The one in the ground is now dead, and the other one is not looking good. It gets dark spots on the leaves, and then the leaves die. Can you help?
Without additional details, it is difficult to say what may be wrong with your peonies. The Penn State Extension has information on different diseases that can affect peony plants. What you describe sounds somewhat like peony leaf blotch or measles, as shown in Iowa State University’s Plant Pathology webpage on peony diseases. Here is an excerpt:
“Peony leaf blotch is also known as measles or stem spot. Warm, humid weather provides optimal conditions for infection by the causal fungus, Cladosporium paeoniae.
The leaf spots are glossy and purplish-brown on the upper sides of leaves. On the lower sides, spots are chestnut-brown. Infection is generally more pronounced at the margins of outer leaves. Leaves may become slightly distorted as they continue growing.
Fungal infections on young stems first appear as elongated, reddish-brown streaks. As plant growth continues, infected tissue near the crown may darken and become depressed. Stems on the upper portion of the plant may show individual, raised spots.
To manage peony leaf blotch, cut the stems at ground level in the fall or early spring. Rake the area before new shoots appear. Fungicides are available to help control the disease, but must be used in combination with other management practices. Also, providing good air circulation and avoiding wetting the leaves when watering can help reduce disease severity.”
There are other possibilities, including peony blight, also known as Botrytis blight. The Royal Horticultural Society discusses this problem:
“Peonies collapse at soil level and the stem bases are covered in grey mould. In a severe attack the leaves are also affected and the plant may be killed or so badly weakened it fails to sprout again next spring. Infections also occur frequently behind the flower buds just before they open.
This is a disease that affects both herbaceous and tree peonies. It is caused by a fungus (Botrytis paeoniae) related to grey mould (Botrytis cinerea), which may also attack peonies in a similar way.
Wilt is encouraged by high humidity which builds up around dense clumps of peonies. Increase the circulation of air by thinning out overcrowded shoots. Also avoid over-feeding, especially with nitrogen-rich fertilisers, which encourages lush, disease-prone growth.
Cut out all infected stems well below soil level, as soon as you notice them. Don’t put infected material in the compost bin but burn it or put it in the dustbin, preferably in a sealed bag. If whole plants are badly affected lift and destroy them in their entirety along with the soil surrounding the roots. This total destruction is essential as the fungus can produce black resting bodies (sclerotia), which survive for long periods in the soil ready to re-infect new peonies.
There are no fungicides available to amateur gardeners at present.”
I recommend taking plant samples to your local county extension agent for diagnosis.