Tips for Beginners: Controlling Powdery Mildew
Adapted from the 1996 Plant Disease Control Handbook
Jay W. Pscheidt, editor, Extension plant pathology specialist
Oregon State University

The fungus Microsphaera azalae is found throughout the Pacific Northwest on garden azalea and rhododendron species and hybrids. Microsphaera vaccinii has been found only on wild Rhododendron occidentale growing on the southern Oregon coast. Disease symptoms have not been found on the other Northwest native species, R. albiflorum and R. macrophyllum . These fungi are obligate parasites, which means that the fungus must have live tissue to grow and reproduce, and they produce two different kinds of spores. Asexual conidia are most frequently encountered. Asexual conidia can be thought of as clones that make new powdery mildew colonies once they get to healthy tissue. The asexual conidia spread by wind and produce new colonies which produce more spores. Many infection cycles may occur during a summer. Although the disease develops late in summer, overwintering colonies have been observed throughout the winter on the undersides of leaves. Sexual spores are produced in small, black, spherical structures (cleistothecia). They are produced in fall in great numbers on azaleas but are much less often on evergreen rhododendrons. High humidity favors the disease. Growers recently reported more severe powdery mildew on cultivars, such as Rhododendron 'Virginia Richards', that normally are not attacked. Erysiphe polygone also causes a powdery mildew but has been reported only in California and Virginia.

Symptoms

Symptoms appear to be influenced more by cultivar than by environment. The most common symptoms are diffuse pale yellow spots on the leaf upper surface, usually from 0.25 to 1 inch (0.6 to 2.5 cm) in diameter. On the lower leaf surface are purple to brown, circular, diffuse feathery areas. Sometimes, a sparse fungal growth is with the spots. Other symptoms on the upper leaf surface include large purple-brown spots, or purple-brown areas associated with veins, or purple ring spots, or occasionally green spots as the remainder of the leaf yellows. Some cultivars such as Rhododendron 'Unique' show no symptoms on the top of the leaf but have typical diffuse colored spots or a heavy powdery growth on the leaf's underside. Necrotic, brown, sunken spots are not associated with this disease; they are caused by a number of other fungal and environmental factors. Severe defoliation can occur on some cultivars such as Rhododendron 'Virginia Richards' and species such as R. campylocarpum and R. cinnabarinum . As leaves begin to defoliate, usually in fall or early spring, they can have various patterns of yellow, red, and brown. Other cultivars can tolerate considerable leaf spotting without much defoliation. Most azaleas and some evergreen rhododendrons (such as R. 'Purple Splendour' and R. 'Vulcan's Flame') have the typical white powdery growth on both sides of the leaf usually associated with powdery mildew.

Cultural Control

1. Plant resistant hybrids or species.

2. Remove infected plant parts if practical.

3. The value of removing and destroying fallen leaves is not known. At any rate, it would help only those plants that produce cleistothecia.

4. Reduce relative humidity by adjusting irrigation practices and spacing plants for good air circulation.

5. Avoid planting in heavily shaded areas.

Chemical Control

Begin multiple applications when you first notice the disease on current-year leaves. Early detection and scouting will aid overall control.

1. Bayleton 25 WP at 1 to 2 oz./50 gal. water. (12-hour wait before contact with plant.)

2. Funginex Rose Disease Control at 1 Tbsp./gal. water. Thoroughly cover both leaf surface. (Home packaging available.)

3. Sulfur products such as Safer's Garden Fungicide. Thoroughly cover both leaf surfaces. (Home packaging available.)

4. Terraguard 50 W at 6 to 12 oz./100 gal. water. (24-hour wait before contact with plant.)

USE PESTICIDES SAFELY!

• Wear protective clothing and safety devices as recommended on the label. Bathe or shower after each use.

• Read the label, even if you have used the pesticide before. Follow closely the instructions on the label (and any other directions you have).

• Be cautious when you apply pesticides. Know your legal responsibility as a pesticide applicator. You may be liable for injury or damage resulting from pesticide use.

Further Reading

Gibson, Ken, "Powdery Mildew, The Unknown Garden Intruder," Journal American Rhododendron Society , spring 1992.