Pest
Management Home Page Dr. John F. Baniecki Powdery mildews comprise a large group of fungal diseases that affect ornamentals, vegetable, and cereal and fuit crops as well. In general, the powdery mildews flourish during hot weather when the cooling of night temperatures brings about abundant moisture in the form of dew. They are more severe in shade than in full sun. Powdery mildews of ornamentals commonly found are: Crape-Myrtle mildew (Erysiphe lagerstroemeria), Lilac mildew (Microsphaera alni), and Powdery mildew of rose (Sphaerotheca pannosa var. rosae). Lilac Mildew This disease appears in late summer and fall and is probably the most common one on lilac. The foliage is often completely covered with a thin white coating of mycelium and spores of the fungus; and, upon closer examination, small black dots may be seen on this white coating. These black dots are the sexual fruiting bodies of the fungus (seen with 10X magnifying lens). Powdery Mildew of Rose This disease ranks second to black spot on rose in the eastern United States. It may appear as early as April, as soon as the leaves develop, and becomes well established by mid-May on Dorothy Perkins and other rambler roses, often preventing bloom. The disease is present on hybrid teas and florabundas by June, but it flourishes best on these types in the late summer as the nights become cool. The fungus overwinters in the buds or on the canes and the first symptoms of the disease are slight curling of the leaves followed by the appearance of white powdery growth. Buds may not open at all, or if they do open, the flowers are misshapen. The leaves sometimes become blistered and have a reddish cast under the white fungus growth. This reddish case may become almost black in appearance. Powdery mildew is prevalent on succulent growth brought about by excessive nitrogen application. |
