Black Spots on Peony Leaves: Causes and Fixes

Discovering black spots on peony leaves is a frequent issue for many gardeners. These spots are a sign of a fungal disease, often spurred on by specific weather conditions. Fortunately, these problems are manageable and rarely fatal to the plant if addressed correctly. With proper identification and care, you can handle the current problem and take steps to prevent it from returning.

Identifying the Causes of Black Spots

The most common cause of dark spots on peony foliage is Peony Leaf Blotch, also known as peony measles. Caused by the fungus Cladosporium paeoniae, this infection appears in the middle of summer during warm, wet weather. Initially, you might notice small, reddish-brown spots on the upper surface of the leaves. These spots darken and coalesce into larger, glossy, purplish-black blotches.

Botrytis Blight is another fungal issue that presents slightly different symptoms. While it also causes black spots, Botrytis affects the entire plant structure, including stems and flower buds. You might see young shoots suddenly wilt and turn black, or flower buds turn brown and fail to open. A distinguishing feature is a fuzzy gray mold on rotting buds and stems, particularly in damp conditions.

Less common is Phytophthora Blight, which thrives in heavy, wet soil and creates large, leathery, black lesions on leaves, stems, and buds. The dark spots often look watery and can cause new shoots to rot at the base. To differentiate, observe the full range of symptoms. Leaf Blotch is a foliar issue, whereas Botrytis and Phytophthora cause more widespread wilting and stem rot.

Treating Affected Peonies

Once you identify a fungal infection, the first action is sanitation. Carefully prune off all affected leaves, stems, and flower buds using clean, sharp pruning shears. Make your cuts well below the diseased area, inspecting the remaining stem to ensure there are no signs of infection. Do not place this diseased material in your compost bin, as spores can survive and spread. Dispose of the debris by burning, burying, or bagging it.

After removing infected parts, applying a fungicide can help control the disease’s spread on the remaining foliage. Organic options include products containing copper or neem oil. Conventional fungicides should be labeled for use on peonies and for controlling fungal leaf spots. Fungicides are most effective when applied before the disease becomes severe or as a preventative measure, so always follow label instructions.

When treating your peonies, it is best to work on a dry day. Handling plants when the foliage is wet can easily spread fungal spores from one plant to another. A thorough cleanup followed by a properly applied fungicide gives your plant the best chance to recover and limit damage for the season.

Preventing Future Fungal Issues

Long-term prevention centers on creating an environment that is less hospitable to pathogens. Good air circulation is a primary factor, so ensure there is adequate space between peonies when planting or thinning. Overcrowded plants trap humidity, creating the damp conditions that fungi love. Pruning nearby plants can also increase airflow.

Your watering habits also have a significant impact on foliage health. Soil-borne fungal spores can be splashed onto leaves by overhead watering. To avoid this, water your peonies at the base of the plant using a soaker hose or watering can. Watering in the morning is preferable, as it gives any moisture on the leaves time to dry in the sun.

A meticulous fall cleanup is an effective preventative measure. After the first hard frost has killed the foliage, cut all peony stems down to the ground level. Rake up and destroy all of this plant material, as fungal spores can overwinter in the dead leaves and stems. This prevents them from reinfecting new growth next spring. Adding a fresh, two-inch layer of mulch can also create a barrier between the soil and emerging shoots.