There are many fungal genuses, species and races that cause powdery mildew. The disease affects many herbaceous plants such as yarrow, cone flowers, phlox and roses.
White powdery patches develop on leaves, young stems, buds, flowers and even fruit. Patches may enlarge until the entire area is covered. With time, these patches become mealy or felt like and turn gray to tan. Leaves may stunt, curl, become chlorotic and drop early. Flower buds maybe deformed.
Some species of powdery mildew fungi infect only a few closely related hosts, whereas others have a broad host range. Powdery mildew fungi overwinter on plant tissue and dormant buds. Spores are released in damp spring weather and move to uninfected tissue in water or wind. The fungus growing on the surface of leaves gives the foliage a white, powdery look. The disease cycle can continue throughout the growing season as long as days are warm and dry followed by cooler, damp (but NOT wet) nights.
Plant in areas that are proper for the plant species. Place plants in sunny locations with good air movement. Do not crowd plants (allow for good air circulation). Dense, shady, or damp/humid areas favor disease development. Remove and destroy dead foliage in the planting beds in the fall to decrease the level of primary inoculum the next spring. Maintain healthy plants, but avoid excessive fertilization and watering as these practices encourage succulent new growth, which is more susceptible to powdery mildew. Resistant varieties continue to offer the best source of disease control. If infections occur early in the growing season, fungicides can be used to reduce infection, but treatment must be started as soon as the first symptoms are seen. Once the disease becomes widespread, it cannot be controlled in that year. Chemical control is not recommended for infections that take place late in the growing season.