Pilchuck Chapter ARS

Spring gardens rely on rhododendrons for flowers, but also for the strength and beauty of the leaves for eleven more months. Now's the time to check new growth of all rhododendrons for disease symptoms. For about 10 years gardeners have been noticing odd brownish splotches under the leaves of many different rhododendrons. One of the puzzling aspects of this fungal problem is the fact that two different affected rhododendrons vary in appearance. 'Unique,' for instance, shows almost no upper leaf changes, other than occasional very faint lighter yellowish areas, while the underside of the leaves will be completely covered in brown spots. It's possible to walk past a 'Unique' with powdery mildew and not notice any symptoms until the leaf is turned over.

A deep green leaf may begin to show lighter green patches, and these areas will gradually become more yellow. One cultivar, 'Virginia Richards,' gets brownish purple spots on both tops and bottoms of leaves.

This common disease is named 'powdery mildew' despite how little the symptoms resemble the familiar fungal disease often seen on roses and azaleas. The causal agent is apparently Microsphaera azalea. Usually the disease doesn't produce the familiar white powder-like spores, although late in the summer some may become visible. The disease manifests instead as color changes in the leaves, followed by defoliation toward the end of the growing season.

Many rhododendrons, if basically healthy, will coexist with the disease and seem to outgrow or at least survive the symptoms. Last year's leaves, once they have been hit by the disease, will always have it, with symptoms persisting from year to year until the leaves drop off.