Powdery Mildew on Roses: Identification, Prevention, and Treatment

Powdery mildew is among the most recognizable diseases on roses. The white to grayish powder on new growth, curled leaves, and distorted buds appears predictably every season in humid climates with fluctuating temperatures. Unlike most fungal diseases, it does not require wet foliage to establish.

Powdery mildew is among the most recognizable diseases on roses. The white to grayish powder on new growth, curled leaves, and distorted buds appears predictably every season in humid climates with fluctuating temperatures. Unlike most fungal diseases, it does not require wet foliage to establish — which is partly why it catches gardeners off guard.

I don't grow hybrid tea or floribunda roses at my Long Island property. I have rosa rugosa along the back fence, which is famously resistant to powdery mildew. This guide draws on Extension research for the rose types that are susceptible.

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The Pathogen

Per Missouri Botanical Garden, rose powdery mildew is caused by Podosphaera pannosa, an obligate biotrophic fungus that infects only plants in the genus Rosa. It is distinct from cucumber powdery mildew (P. xanthii), grape powdery mildew (Erysiphe necator), and the mildews affecting lilacs, peonies, and other ornamentals.

The fungus colonizes living plant tissue, producing white mycelium and conidia (asexual spores) visible as a powdery coating. Spores are released continuously and dispersed by air. Per Cornell Cooperative Extension, P. pannosa produces abundant spores at temperatures between 60–80°F with relative humidity of 40–70% — conditions that are typical of spring and fall in most temperate rose-growing regions.