Powdery mildew is a well-known nuisance on phlox. The easiest way to prevent powdery mildew on phlox is to plant disease-resistant varieties. Select powdery mildew resistant varieties is another option. Orange Perfection (dark salmon), Starfire (red), ‘Shortwood’ (rosy pink flowers), ‘David’ (white flowers), ‘Katherine’ (lavender blossoms), and ‘Robert Poore’ (reddish purple flowers) possess good resistance to powdery mildew.

Examine your phlox plants once a week. When the first leaf spot is observed, pinch off the infected leaves and begin fungicide sprays to protect healthy tissues. Repeat applications are often necessary throughout the growing season and should be applied according to label instructions. Fungicides do not cure or remove existing powdery mildew infections. Once the majority of leaves have leaf spots, it is too late to treat. Tolerate powdery mildew infections on phlox plants. When planting, space the plants to allow adequate air circulation. Water in the morning so leaves have time to dry. Cut them down in the fall and dispose of infected plant material.

Potassium bicarbonate and horticultural oils are recommended to prevent phlox powdery mildew infections. Potassium bicarbonate is usually least expensively at garden and hardware stores, but is also available at pharmacies and wine-making supply shops.