See: Rhododendron-Powdery Mildew. Erysiphe vaccinii (formerly Microsphaera vaccinii) has been found only on wild R. occidentale growing on the southern Oregon coast. It will spread quickly to the rest of the plant if left unchecked. Cause The fungus Erysiphe azaleae (formerly Microsphaera azaleae) is found in western Oregon and Washington and southern British Columbia on garden azalea and rhododendron species and hybrids. Powdery mildew completely covering this leaf. On evergreen rhododendrons, light green or yellowish patches on the top of leaves sometimes accompanied by purple-brown areas on the backside of leaves are signs of powdery mildew. Photo by Pollinator. It often appears in late spring and early summer. With rhododendrons, the powdery mildew can overwinter on the underside of evergreen foliage. They key identifier is that downy mildew has the white powder on the under side of the leaves only whereas powdery mildew has a white powder on both the underside and top of the leaves. Powdery mildew is a common fungal disease that’s easily recognized by the white, powdery spores that cover the leaf surfaces. Powdery mildew is far more common compared to downy mildew but it is important to recognise the difference between the two if you plan to treat either of them. Powdery mildew can also infect new buds, shoots and flowers on rhododendrons, causing new growth to become distorted and stunted. A puzzling aspect of this fungal disease is its varying appearance on different cultivars. I currently have a nasty infection of powdery mildew (PM) on many of my vireyas and decided that something needed to be done. Cure powdery mildew by pruning all of the affected tissue. Particularly virulent infections can be sprayed with a fungicide labeled to treat powdery mildew like Benlate or Bayleton, but this is rarely necessary. Powdery mildew grows best in conditions where you'd least expect it--instead of a cool, moist environment, powdery mildew prefers warm and dry conditions where there is high humidity combined with moderate temperatures. Powdery Mildew. For instance, the rhododendron cultivar 'Unique' shows almost no upper leaf change other than occasional very faint … As the fungi multiply they will cover the leaves in what looks like a powdery coating. Surfing the Web I was surprised to find there is relatively little information about PM on rhododendrons in Australia and even less for vireyas. Lots about PM on other plants (citrus, grapes, strawberries, roses, cucurbits, etc.) Remove affected areas, apply spray treatment, water deeply at the root level and limit overhead watering. 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