Powdery mildew is a well-known nuisance on phlox. The fungus causes an unsightly powdery gray coating on the leaves. The disease tends to start on the lower leaves and work upward. By the end of the summer, lower leaves are usually wilted and dead, and the upper leaves are coated with the fungus.
In addition to detracting from the beauty of this popular flowering plant, the powdery mildew fungus also weakens phlox plants. The fungus has specialized structures that penetrate leaf tissue and absorb nutrients. The overall vigor of the plants is reduced.
High humidity and moderate summer temperatures favor powdery mildew development, as do short periods of leaf wetness. But long periods of wetness inhibit disease development. Plants located in the shade and those that are fertilized heavily tend to have more disease.
As the growing season progresses, the fungus will produce small pepper-like structures on the infected leaf tissue. These structures are resistant to extreme weather conditions and aid in the survival of the fungus from year to year.
Management of powdery mildew is aimed at preventing the conditions that favor the fungus.
- Space plants adequately to allow good air movement throughout the foliage.
- Water plants early in the day so leaves dry quickly.
- Do not over fertilize (especially with nitrogen).
- Remove diseased plant debris at the end of the season to minimize survival of the fungus over the winter.
In addition to these good cultural practices, regular fungicide sprays can be used to protect the leaves of the plants. Spraying will not cure the leaves of the disease, so the fungicide must be applied early and repeatedly according to the directions of the label of the product purchased.
Some phlox cultivars show tolerance to the powdery mildew fungus. Evaluations performed at the University of Vermont and Cornell University found the cultivars with little or no mildew included David (white), Orange Perfection (dark salmon), Prime Minister (white, red eye), and Starfire (red). You also can check seed catalog descriptions for information on other phlox varieties with disease tolerance.
This article originally appeared in the 7/18/2003 issue.