I have powdery mildew all over my itoh peony. After it blooms should I just cut it to the ground? Do I need to spray it once I do this(with what?)? Will cutting it in June effect next years growth? Does it matter what time of year I cut it to the ground? I have never had this problem before. It is 6 years old in full sun and I do not water it. There are a lot of tulips near it in the spring -but they have also been there for 6 years with no issue.
Thanks for contacting Toronto Master Gardeners. Powdery mildew is a common problem with peonies. With this fungal infection, a white to grey dusty growth appears on the upper surface of the leaves. In severe cases, leaves may appear distorted and cause defoliation. Powdery mildew occurs following warm days and cool nights in the fall and spring. It doesn’t usually kill plants but can weaken them. Generally the best way to deal with powdery mildew on peonies is to prevent it. Choose Itoh peonies that mildew resistant such as: Pea Green, Yellow Charm, and Morning Lilac. Grow them in the full sun. Water the plants early in the day rather than at night time. This will allow the water to evaporate preventing the spread of the fungal spores. Ensure that the plant is spaced well with good air circulation
Generally as soon as you see an infected leaf, it is best to remove it and dispose of it in the garbage. To avoid spreading the disease to other peonies, disinfect your pruning shears with bleach or alcohol. You can treat powdery mildew in its early stages with fungicide. Since the disease is quite extensive on your plant, it is best to cut it down in the fall before the first frost and discard all of the plant debris in the garbage. As the plant grows in the early spring next year, treat it with a fungicide. If you notice the powdery mildew returning promptly remove the leaves and continue with the fungicide.
Good luck with your peony.