There are about 10 different fungi that can cause leaf spot on rhododendron...and while I haven't seen it here near the coast, powdery mildew is becoming more common on our rhododendrons.

Leaves infected with powdery mildew may become off color and exhibit the white powdery fungus growth associated with the disease, or some rhododendrons lack the white powder and instead show yellow or purplish spots on the upper leaf surface and discolored areas on the lower leaf surface.

My advice without a picture to show us would be to take example leaves exhibiting the symptoms to staff at a reputable nursery (not a box store) or county extension agent for an accurate diagnosis. Funginex sprayed at the appropriate time can be helpful and I've heard Bayleton recommended too since it's systemic and can be easier to apply to large shrubs - but it's expensive so best to know for sure what you are treating.