My phlox and monarda develop powdery mildew each year. The leaves yellow, curl and brown. What can I do?
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 a 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, try growing mildew resistant species and cultivars of these plants. The white cultivar 'David' and the similar species Phlox maculata are mildew resistant garden phlox. Several cultivars of Monarda (beebalm) resist powdery mildew. These include Gardenview Scarlet, Marshall's Delight, Twins, Raspberry Wine, Violet Queen and Aquarius.