You should call your county agent. Each state has different laws and recommendations. Your climate is much different than mine.

If you are in Portland you could ask Kathy Van Veen. She is near Reed College. If you are near Eugene, you should visit Greer Gardens. I believe it is on Good Pasture Road.

A grayish white, powdery coating or fuzzy white growth on upper or lower surfaces is Powdery Mildew (Microsphaera azaleae). This is more prevalent on deciduous azaleas and sometimes it affects the lower surface more. Entire leaves can be covered. In late summer and fall, small black specks may be found in the white areas. Powdery mildew is more severe on shaded plants. It is favored by the high humidity found in crowded plantings and damp locations. The disease is more severe during periods of cool, moist weather. These fungi produce spores on the surface of the infected leaves which are spread by wind currents to surrounding leaf tissue. These fungi over-winter in the bud scales for initiation of infection next season. There are a number of ways to manage this disease.

* Do not overwater or overfertilize plants, as the fungus prefers succulent new growth.
* Hand-pick and destroy mildewed leaves to control small amounts of infection.
* Hose diseased plants with water when practical. This can help remove fungus and prevent new infections.
* Prune and space plantings to allow good air circulation. Do not plant in extremely shaded or damp areas.
* Rake and destroy fallen leaves year-round to reduce infection source. Do not compost diseased materials.

Chemical control is possible. Begin applications when you first notice the disease on current-year leaves. If disease appears late summer, applications are not necessary on deciduous azaleas. Do not apply sulfur products when temperature is over 85F or within a few weeks of an oil spray.

* Black Leaf Sulfur Dust
* Bonide Lime Sulfur Spray
* Bonide Remedy
* Monterey Fungi-Fighter
* Ortho RosePride Funginex
* Ortho RosePride Orthenex
* Safer Flower, Fruit & Vegetable Garden Fungicide
* triadimefon (Bayleton, Strike)
* Spectracide IMMUNOX

This fungal disease can weaken the plant. Spray when you first see the disease and then again in 10 days. Chemicals will not control the fungi that has already become established. For more information see the section following on 'rhododendron powdery mildew'. The symptoms are different, but the organism and control are the same.

Some other possibilities are:

1) Thrips are characterized by a silvery white discoloration on the leaf's upper surface and silvery on the bottom with small black frass specks. The damage is similar to that from lace bugs. Thrips are a problem in warm and dry climates like California and New Zealand. Most of the standard insecticides and insecticidal soaps can be used. Control for thrips whether an insecticide or insecticidal soap is a contact control and must be applied on the adults. This is most likely done in May in California when they are visible. Locally, biological controls may also be available. It is best to remove infected flowers.

2) Whitish specks on the upper surface of leaves and dark spots varnish-like on the bottom are symptoms of Rhododendron Lace Bugs, Stephanitis rhododendri, and Azalea Lace Bugs, Stephanitis pyrioides, small insects with transparent wings on under-surface of leaves.. This insect hatches early in spring as the new foliage begins to mature and its numbers may build to damaging levels with successive generations. Lace bugs reach their peak in late summer and do their worst in sunny, exposed sites. Spiders are important predators of lace bugs and since they shy away from sunny, hot places, plant your azaleas where there is some shade. Insecticidal soap, horticultural oil, or a systemic insecticide may spare your azaleas the damage if applied in spring when the first hatchlings are noticed. Care must be taken to spray the lower surfaces of the leaves where the lace bugs live. Moving a plant to an area with more shade may alleviate the problem.

Here is a link that might be useful: How to grow rhododendrons and azaleas.