![]() ![]() Incidence of infection increases as relative humidity rises to 90 percent, but it does not occur when leaf surfaces are wet (e.g., in a rain shower). Therefore, the disease is common in crowded plantings where air circulation is poor and in damp, shaded areas. However, the relative humidity of the air does need to be high for spore germination. This is because the fungus does not need the presence of water on the leaf surface for infection to occur. Powdery mildews are severe in warm, dry climates. The severity of the disease depends on many factors: variety of the host plant, age and condition of the plant, and weather conditions during the growing season. Infected leaves may become distorted, turn yellow with small patches of green, and fall prematurely. It also affects the bottom sides of leaves, young stems, buds, flowers and young fruit. The disease is most commonly observed on the upper sides of the leaves. These are the cleistothecia or overwintering bodies of the fungus. Tiny, pinhead-sized, spherical fruiting structures that are first white, later yellow-brown and finally black, may be present singly or in a group. Powdery mildews are characterized by spots or patches of white to grayish, talcum-powderlike growth. SymptomsĮven though there are several types of powdery mildew fungi, they all produce similar symptoms on plant parts. In Colorado, powdery mildews are common on ash, lilac, grape, roses, turfgrass, vegetables (such as cucurbits and peas), euonymus, cherry, apple, crabapple, pear and Virginia creeper, among others. They affect virtually all kinds of plants: cereals and grasses, vegetables, ornamentals, weeds, shrubs, fruit trees, and broad-leaved shade and forest trees. Powdery mildews are one of the most widespread and easily recognized plant diseases. Chemicals are most effective when combined with cultural controls.Plant resistant varieties if available.Once the disease is a problem, avoid late summer applications of nitrogen fertilizer. Succulent tissue is more susceptible to infection.Many plants have been developed to be resistant or tolerant to powdery mildew. ![]() Powdery mildews are severe in warm, dry climates.Powdery mildews are characterized by spots or patches of white to grayish, talcum-powder-like growth.Powdery mildew is one of the most widespread and easily recognized plant diseases. ![]()
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