As far as your Mother in Law's azalea goes it could very well be powdery mildew. It is pretty common and usually does not do much harm to the plant, as it merely exists on the plant, and does not act like some of the more parasitical fungi. You do need to watch for other problems that may kick the plant while it is down however such as spider mites or such.

Unfortunately you did not give too much information on what is bothering your plants. There could be all sorts of reasons of what is going on.
Do the leaves have any discoloration? (Black or brown spots or patches, yellowing ect...) That could be a possible fungi attack. Is there any webby material on the stems or under the leaves? The critter that would be responsible for that would be spider mites. Are the leaves curling up before falling off? A possible vascular disease if they are curling under and on the sides. If they are curling up into a gnarly shape it could be aphids. Has there been anything chewing on them? Beetles or caterpillars. Have there been any changes in the immediate area surrounding the plants, such as new plants or landscaping that may be in competition with the azaleas? It could be something such as a new side walk or driveway that has compacted the soil. Has the shade in the area become denser? How deep is the mulch at the base of the plant?
If any of these sound like what might be going on, let us know and we can go from there.

Mike

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Mike Maier
aka
The Plant Doc