Source: JARS V53:No.2:p73:y1999
Tips for Beginners: Controlling Powdery Mildew
Adapted from the 1996 Plant
Disease Control Handbook
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 oxidentale 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, over wintering 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.
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.
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