Powdery Mildew on Phlox
My phlox are covered with this white stuff and the leaves are starting to turn yellow and brown. What should I do?
The problem is powdery mildew. This fungal disease attacks many plants but is most common on garden phlox, bee balm, zinnia, lilac and roses.
Make sure the plants receive full sun and good air circulation. This means giving them enough room to grow and spread. Further increase the light and air reaching the center of the plants by thinning out one fourth of the stems as growth begins in spring.
If mildew is still a problem you may want to try using a mixture of 1 Tablespoon of baking soda, 1 teaspoon of lightweight horticulture oil or insecticidal soap in a gallon of water. Cornell University found repeated (weekly) applications of this mix will control mildew.
Or easier yet – replace the mildew susceptible plants with resistant cultivars like Bright Eyes, David, Eva Cullum, Fesselballon, Flamingo, Franz Schubert, Katherine, Look Again, Orange Perfection, Pax, Prime Minister, Sandra, Speed Limit 45, Sternhimmel, Tenor, The King or the closely related species mildew resistant Phlox maculata.