How can I control powdery mildew on my garden phlox?
Powdery mildew is a common disease of garden phlox (Phlox paniculata). The fungal disease produces a grayish white coating on the leaves. Infected leaves eventually turn yellow and then brown. Initial symptoms appear on the lower leaves with the disease progressing upward.
Powdery mildew is most commonly found on plants growing in shady areas and in crowded plantings with poor air circulation. Optimal conditions for powdery mildew are cool nights followed by warm days.
Cultural practices can reduce the severity of powdery mildew on garden phlox. The amount of disease inoculum can be reduced by cutting off and removing diseased plant debris in fall. Plants growing in shady locations should be moved to a sunny site. In overcrowded plantings, improve air circulation by digging and dividing perennials.
While cultural practices are helpful, fungicides may be necessary to control powdery mildew on garden phlox. To be effective, fungicides should be applied at the first sign of the disease and repeated on a regular basis.
Selection of powdery mildew resistant varieties is another option. ‘Shortwood’ (rosy pink flowers), ‘David’ (white flowers), ‘Katherine’ (lavender blossoms), and ‘Robert Poore’ (reddish purple flowers) possess good resistance to powdery mildew. (The resistance or susceptibility of garden phlox varieties to powdery mildew varies within the United States. A variety that possesses good resistance to powdery mildew in the Midwest may be susceptible to powdery mildew in other regions of the country.)