Answer: Powdery mildew is a fungal disease which gives the leaves of phlox a dusty appearance. It is common in the fall when night temperatures are in the 60's and humidity is high. Other plants likely to show powdery mildew include roses, lilac, zinnia, chrysanthemum, hollyhock, begonia and vine crops such as cucumbers, gourds, melons, pumpkins and squashes. As the fungal disease progresses the leaves become covered with a powdery web of fungal threads which can be rubbed off. Powdery mildew rarely kills plants and is often best tolerated. Regular and severe loss of leaves, however, can weaken a plant.
Since controlling powdery mildew may require regular spraying, your best solution is to replace plants with varieties that are resistant or more tolerant of the disease. Two phlox varieties with increased resistance include 'Mt. Fuji' and' David'. Both have white flowers. If replacement is out of the question, you have other options. One is to live with the disease since it rarely kills the plant. Another option is to practice simple cultural techniques which reduce the likelihood of infection. Space plants to allow good air circulation and thin out several shoots as new growth occurs in spring to promote an open habit. Clean up and dispose of infected tops each fall. The final option is to spray regularly. One of the least toxic controls is to spray with sulfur at the first signs of the disease. Spray both sides of the leaves and repeat every 7-10 days according to label directions. Spraying will not cure the leaves of the fungal disease but it will protect new leaves from becoming infected. Spraying needs to be done regularly. Be sure to follow package directions.
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