Powdery mildew, caused by more than 1,000 fungus varieties, shares the same general characteristics when attacking trees, plants and bushes. A serious problem if untreated, the fungal infections can cause severely stunted growth or even death to a young tree, plant or bush. In recent years, tree and bush cultivars have been developed that offer some disease resistance to powdery mildew.
The fungus appears on foliage, stems and even twigs. The mildew grows as a light coating that has a velvet-like appearance in shades of white and gray. If the gardener lightly rubs the leaf of the infected tree, plant or bush with her fingers, the coating of mildew will easily rub off. In its early stages the mildew looks almost like the tree or bush simply has a light coating of dust, but as it advances the appearance becomes more identifiable as a fungus.
Trees, bushes and other plants affected with powdery mildew will suffer severely stunted or distorted foliage. Leaves often curl and crinkle under the onslaught of the mildew fungus. The leaves begin to turn yellow and will often drop. Blossoms and buds will either drop or fail to open. The fungus removes valuable nutrients from the infected foliage, according to Cornell University.
The mildew fungi are usually species specific. Most will only appear on certain trees, shrubs or other plants and will not spread to other species. The fungi actively produce spores, which the wind carries to other species so the mildew can set up residence on new foliage and reproduce. Most mildew fungi types will easily survive the winter to arise again in the spring to infect new foliage. The mildew fungus varieties prefer humid, cool weather in the spring and fall.
Determine the type of tree or bush that suffers from the powdery mildew before purchasing a fungicide because many fungicides are species specific. Propiconazole, sulfur, triforine and myclobutanil fungicides work well at treating mildew. Spray the fungicide when the mildew is first noticed for best control. Powdery mildew that appears late in the season on maple and oak trees generally requires no treatment before the winter months set in, according to Iowa State University.
Rake up all fallen leaves and promptly dispose of them to limit the spread of the powdery mildew. Mildew often develops on plants, bushes and trees that are not adequately spaced, so space when planting so each species receives ample air circulation. If the branches and foliage are severely infected with powdery mildew in the early spring, prune away the infected areas.