Symptoms
Even though there are several different types of powdery mildew fungi, they all produce similar symptoms on plant parts. Powdery mildews are characterized by the appearance of spots or patches of white to grayish, talcum powder-like growth. Tiny, pinhead-sized, spherical fruiting structures that are first white, later 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 but 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.
Conditions that Favor the Disease
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 climates. This is because the fungus does not need the presence of water on the leaf surface for infection to take place. However, the relative humidity of the air does need to be high for spore germination to take place. 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 does not occur when leaf surfaces are wet (i.e. in a rain shower). Young succulent growth usually is more susceptible than older plant tissues.
About the Fungi
Powdery mildews are host specific, they cannot survive without the proper host plant. For example, the species Uncinula necator, which is responsible for causing powdery mildew of grape and linden, will not attack lilac. Similarly, the powdery mildew, Microsphaea alni, which affects elm, catalpa, lilac and oak, will not attack turfgrass and vice versa.
Powdery mildews produce mycelium (fungal threads) that grow only on the surface of the plant. They never invade the tissues themselves. The fungi obtain their food from the plant 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
Cultural: Several practices will reduce or prevent the development of powdery mildews. Many plants, such as roses, vegetables, and Kentucky bluegrass, are developed to be resistant or tolerant to powdery mildew. Inquire about powdery mildew resistant varieties before a purchase. If resistant varieties are unavailable, do not plant in low shady locations.
Once disease becomes a problem:
1. Avoid late summer applications of nitrogen fertilizer to limit the production of succulent tissue (which is more susceptible to infection).
2. Avoid overhead watering to help reduce the relative humidity.
3. 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.
4. Selectively prune overcrowded plant material to help increase air circulation. This helps reduce relative humidity and infection.
Chemical: If cultural controls fail to prevent disease build-up or if the disease pressure is too great, an application of a fungicide such as chlorothalonil (Daconil 2787) ornamental use only, sulfur, or triforinel (Ortho Funginex) ornamental use only, may be necessary. Chemicals are most effective when combined with cultural controls.
Apply fungicides seven to 14 days intervals to provide continuous protection throughout the growing season. Be sure to follow the instructions on the fungicide label for use on specific plant species, varieties, rates to be used, timing of applications, and waiting periods before harvest.