Powdery Mildew on Peonies: Late-Season Identification and Management

My peonies at the Long Island garden have developed powdery mildew on their foliage every August for as long as I have grown them. I have come to accept this as a late-summer feature rather than a crisis. The plants bloom in May and June -- the mildew arrives in August when most of the season's.

My peonies at the Long Island garden have developed powdery mildew on their foliage every August for as long as I have grown them. I have come to accept this as a late-summer feature rather than a crisis. The plants bloom in May and June — the mildew arrives in August when most of the season's work is done. By that point, treating it is largely cosmetic.

That said, the question of whether and when to treat peony powdery mildew is worth answering precisely, because the answer depends on plant age, the severity of infection, and what you are trying to accomplish.

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Pathogen Identification

Per Missouri Botanical Garden, peony powdery mildew is caused by Erysiphe paeoniicola (some sources also list Microsphaera palczewskii). It is distinct from:

• Podosphaera xanthii (cucumber/zucchini powdery mildew)
• Podosphaera pannosa (rose powdery mildew)
• Erysiphe necator (grape powdery mildew)

These pathogens are host-specific. Powdery mildew on your peonies will not spread to your roses, zucchini, or any other plant in the garden.