Purple spots on peonies mar the appearance of this fragrant garden shrub. The color change is caused by a fungal disease known as leaf spot, leaf blotch, red spot or peony measles, one of several leaf diseases affecting peony plants. If not controlled, the disease may occur on your peonies in subsequent seasons.
Peonies with purple spots on the leaves have contracted a fungal disease called leaf spot. Caused by the fungus cladosporium, this disease affects leaves, stems and petioles or leafstalks. It does not affect peony flowers. Leaf spot will not seriously harm the plant so it is considered a cosmetic disease.
Affected plants develop small red or purple dots on the top surface of the leaves, often right before the peony blooms. The spots then migrate to the underside of the leaf. As the disease matures, the spots on the leaf underside turn dull chestnut brown, while those on the top surface of the leaf turn deep purple and have a shiny appearance.
Once peonies develop leaf spot, you cannot reverse the foliage discoloration. Fungicide can halt the infection so no new leaves are damaged. Use a fungicide containing mancozeb, maneb, thiophanate-methyl or copper. To improve the appearance of the plant, prune off infected foliage and dispose of it with your garbage. Composting it allows the spores to spread. If your plant was infected in the current season, remove all infected growth at the end of the season and pretreat your peony plant next spring with a fungicide.
Not all types of peonies contract leaf spot. Many newer varieties show resistance to this disease and are marketed as such. If your peonies regularly contract leaf spot, they are probably an older variety. Replacing them with a new variety saves you trouble and unsightly plants.