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.

Description:

Even though there are several types of powdery mildew fungi, they all produce similar symptoms on plants. Powdery mildews are characterized by spots or patches of white to grayish, talcum powder-like growth. Tiny, pinhead-sized, spherical fruiting structures--first white, then yellow-brown and finally black--may be present singly or in a group. These are the cleistothecia or overwintering bodies of the fungus. The disease is most commonly observed on the upper side of the leaves. It also affects the bottom sides of leaves, young stems, buds, flowers and young fruit. Infected leaves may become distorted, turn yellow with small patches of green, and fall prematurely. Infected buds may fail to open.

Damage:

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.

Life Cycle:

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.

Control:

Several practices will reduce or prevent powdery mildews. Many plants, such as roses, vegetables and Kentucky bluegrass, are developed to be resistant or tolerant to powdery mildew. Use resistant varieties whenever possible.

Once the disease becomes a problem:
•  Avoid late-summer applications of nitrogen fertilizer to limit the production of succulent tissue (which is more susceptible to infection).
• Avoid overhead watering to help reduce the relative humidity or water in the early morning to let the tissue dry as soon as possible.
• Remove and destroy all infected plant parts (leaves, etc.). For infected vegetables and other annuals, remove as much of the plant and its debris in the fall. This decreases the ability of the fungus to survive the winter. Do not compost infected plant debris. Temperatures often are not hot enough to kill the fungus.
• Selectively prune overcrowded plant material to help increase air circulation. This helps reduce relative humidity and infection.
• An alternative nontoxic control for mildew is baking soda combined with a lightweight horticultural oil. Researchers at the University of Rhode Island have confirmed that a combination of 1 tablespoon baking soda plus 2.5 tablespoons oil in 1 gallon of water is effective against powdery mildew on roses. Use of this combination on other crops is still experimental.

Adapted from the Colorado State University Cooperative Extension, 1999