Powdery Mildew
caused by Microsphaera, Phyllactinia, and others

Powdery mildew is one of the most widespread and easily recognized plant diseases. Powdery mildews are most severe when the weather is warm and dry, and they affect virtually all kinds of plants: cereals and grasses, vegetables, flowers, weeds, shrubs, fruit trees, and broad-leaved shade and forest trees. Many plants have been developed to be resistant to or tolerant of powdery mildew. Succulent tissue is the most susceptible to infection.

Injury:

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. Powdery mildews are severe in warm, dry weather because, unlike most fungi, powdery mildew require a wet leaf surface for infection to occur. However, the relative humidity of the air does need to be high for spore germination. Therefore, the disease is common in crowded plantings where air circulation is poor and in damp, shaded areas. 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). Young, succulent growth is usually more susceptible than older plant tissues. Powdery mildew can seriously impact yield on flowering crops such as squash, pumpkins, cyclamen and reiger begonia, but on other plants such as lilac and oak, the mildew is unsightly but does not severely harm the plant.

Biology and History:

Powdery mildews are host specific: they cannot survive without the proper host plant. For example, the species Uncinula necator, which causes powdery mildew on grape and linden, does not attack lilac. Similarly, Microsphaea alni affects elm, catalpa, lilac and oak but not turfgrass. Powdery mildews produce mycelium (fungal threads) that grow only on the surface of the plant. They never invade the tissues themselves. The fungi feed by sending haustoria, or root-like structures, into the epidermal (top) cells of the plant. The fungi overwinter on plant debris as cleistothecia or mycelium. In the spring, the cleistothecia produce spores that are moved to susceptible host tissue by splashing raindrops, wind or insects.