It's hard to guess without a photo, but two possibilities -

Rhododendron powdery mildew which rarely exhibits the same symptoms as powdery mildew on other plants.

And, rhododendrons do occasionally get rust. I do hope you will either post a picture, or take an example leaf to a full service nursery (not a box store) or your county extension agent before treating....treating the wrong problem isn't helpful and using a chemical control against drought or cultural damage isn't either :)

Here is a link that might be useful: Rhododendron powdery mildew

Another possibility -- it's normal.

Can you post a photo?

I think jean001 is referring to indumentum. Indumentum is a fuzzy coating, usually just on the back. It does not include dryness or yellowing of the edges. Those are symptoms of other problems or old leaves ready to fall off. To expand on morz8's reply:

RUST:

Small bright yellow spots is a symptom of Azalea Rust, Puccininastrum vaccinii, a fungal disease. Orange-red pustules containing spores form on the lower surface of the leaf. This rust is only sporadically severe and typically infects deciduous azaleas. Control by avoiding those hybrids and species which are very susceptible to rust. Good air circulation is helpful. Triadimefon seems to help, but may not be registered for use on rhododendrons.

SCALE INSECTS:

Cottony masses on underside of leaves are a symptom of egg masses of the Cottony Azalea Scale, Pulvinaria floccifera. They are up to 1/2 inch long. Control is the same as with other scale insects.

WHITE FLIES:

Yellow mottling on the upper surface of leaves and black sooty mold and transparent insects on the bottom are symptoms of Azalea Whitefly (Pealius azaleae.) and Rhododendron Whitefly (Dialeurodes chittendeni.). These may also cause the following symptoms:

Small white spots on the underside of leaves and small white flies on under-surface of leaves is also an indication in infestation of Azalea Whitefly (Pealius azaleae.) and Rhododendron Whitefly (Dialeurodes chittendeni.). They are more prevalent on certain varieties and on plants grown in protected areas. These small white flying insects look like an aphid with wings and suck on the underside of foliage, leaving white spots where it has been. Heavy infestations cause the margins of terminal leaves to cup. These infested leaves will eventually turn yellowish and appear wilted. The lower leaves become covered with honeydew, followed by sooty mold (a black coating). To check for the presence of whiteflies, shake the terminals of white azaleas to flush out adult whiteflies which look like tiny white moths. Examine the lower surfaces of leaves for the presence of nymphs, which are flat, yellowish green, and resemble scale insects. All stages occur on the under sides of leaves. This whitefly is usually limited to varieties of the snow azalea, Rhododendron mucronatum. If the infestation is light, little or no plant symptoms are evident, and if beneficial insects are present, spray the undersides of leaves with insecticidal soap or a horticultural oil at the 2%summer rate. If the infestation is heavy use a registered residual insecticide such as Malathion, Diazinon or Orthene. Dick Murcott had a simple remedy to control the numbers of white flies. He would hang pieces of stiff plastic or 12" square metal sheets painted with a bright yellow/orange paint and then covered with petroleum jelly or any clear, sticky material. The white flies will fly to the colored material and get stuck in the sticky stuff!

MILDEW:

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.

Two of my rhododendrons and both my pieris have this. I think it's lace bugs. The pieris are prone to them when they get too much sun and maybe the rhododendrons are the same, I dunno.

Photo of lace bug damage on a rhododendron:

{{gwi:388790}}

It was said: "I think jean001 is referring to indumentum. Indumentum is a fuzzy coating, usually just on the back. It does not include dryness or yellowing of the edges. "

Yes, I was referring to indumentum.

As for the leaf edges drying, that could be something else entirely.

If you can post photos? They would help us help you.

"sickly the last 2 or so years. Leaves are Dry and leathery on top with yellowing on the edges and black spots on the bottom" - didn't sound like the poster was seeing normal indumentum to me.

Spanky, yes, rhododendrons and azaleas planted in sun are more prone to lace bug damage than those given some shade.