Powdery Mildew on Phlox

Karen Rane, Plant Disease Diagnostician, Department of Botany & Plant Pathology, Purdue University

Garden phlox (Phlox paniculata) is often severely affected by powdery mildew. The white, powdery patches typical of the disease (Figure 1) can be seen now on susceptible phlox cultivars. The disease occurs first on the lower leaves, and appears to “move up” the plant as spores from the initial infections cause disease on younger leaves (Figure 2). In severe cases, virtually all of the foliage can be infected. Plant breeders have developed several garden phlox cultivars that have some resistance to powdery mildew. A partial list of resistant cultivars includes Starfire, David, Orange Perfection, Prime Minister, Bright Eyes, and Rosalinde. Remember that resistance does not mean the plants will not become infected at all, but symptoms will be less severe or the disease will spread more slowly in resistant plants when compared to susceptible ones. When environmental conditions are favorable for disease development (warm temperatures, high humidity, poor air circulation between plants), powdery mildew can spread very quickly. Protectant fungicides may be necessary to keep the disease in check on susceptible plants. Refer to Purdue University Cooperative Extension publication BP-5-W, Powdery Mildew of Ornamentals, for more information on managing powdery mildew.

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Figure 1: White, powdery fungal growth typical of powdery mildew on the leaves of garden phlox.

Figure 2: Severe powdery mildew
infection progressing from lower to upper leaves on this phlox plant.