It's powdery mildew. Very common on many varieties of hybrid rhodies in our climate and often overlooked as it does not resemble powdery mildew as presented on most other plants.

There is not a lot one can do about it other than manage and attempt to limit its development. It seems to be more related to specific cultivars rather than just a climate-related occurrence. So mildew resistant choices should play a significant role in selection. There is also some indication that keeping the base of the plants clear of any fallen leaves, etc, will limit the spread of the fungal spores. And make sure there is sufficient air circulation and that the plants are not in excessive shade.

Lace-bug damage is superficially similar and we have that around here also. If it were my plant I would look at the undersides of the affected leaves to see which it was. Maybe using a magnifier, if needed. Otherwise in my experience the mildew is often so aggressive it is actually visible to the naked eye as fungoid looking patches or zones beneath the leaves. Specific fungicidal products can be sprayed on the new growth as a preventative but this has to be done more than once per year, at the right times. Same as with disease susceptible kinds of roses for those other than passionately committed hobbyists it is probably better to do without the problem shrubs than bother with multiple yearly pesticide applications.