Answer: What you describe sounds like powdery mildew, a fungal infection. Powdery mildew generally will not kill the plant; however, repeated infections can reduce plant vigor and predispose the plant to other disease or environmental problems.
The symptoms of powdery mildew include characteristic grayish-white or buff colored, powdery looking patches on leaf surfaces. Viewed under the microscope, one can see the cottony, threadlike mat of fungus mycelium and chains of spores (the source of the powder). These patches may enlarge to cover entire leaf surfaces (both upper and lower). Later in the season, mildewed leaves may become dotted with small dark brown specks, which are the overwintering fruiting structures. Powdery mildew infections can cause a variety of symptoms that range from a light white coating of the leaf surface to leaf distortion, premature yellowing and leaf drop. Infection expression, however is highly dependent upon the age, species and growth rate of the plant, the species of mildew fungus, cultural practices, and the environment. Generally, the tender, young foliage and shoots are most susceptible.
Powdery mildew spores are dispersed by wind and splashing water. Infection is favored during humid, overcast conditions when the days are warm and nights are cool. Powdery mildew, although prevalent throughout the growing season, is generally more severe during the spring and fall. In the landscape, powdery mildew infections are generally observed on plants that have been crowded and on plants in damp, shaded areas.
On very susceptible plants it may be necessary to use fungicides to maintain the aesthetic qualities of the plant. There are several fungicides such as chlorothalonil, copper, sulfur, triadimefon and triforine labeled for powdery mildew control on landscape ornamentals. These products, however, need to be applied at regular intervals through the season. Typically, applications can begin at the first sign of infection and applied as directed by the label thereafter and as the weather conditions warrant. Therefore, it is important that the plants be monitored through the season. However, on extremely susceptible plants it may be necessary to start fungicide applications before budbreak and continue every 2 weeks thereafter.
If you'd rather avoid having to apply fungicides, you can move your plants to a better spot in the garden.
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