Lace Bug

• The rhododendron lace bag attacks over 120 rhododendron varieties, sucking sap from the leaves and damaging them to such an extent that white blotches appear. "The upper surfaces become mottled with white spots, and many times the leaves will dry and shrivel. The undersurfaces are also discolored," according to the Integrated Pest Management program of North Carolina State University.
  
  

White Fly and Weevils

• The white fly appears as it sounds -- a small, white, flying insect, it typically sucks on the underside of a rhododendron leaf, turning the foliage white in spots. In other instances, white spots could potentially be nesting larvae or pupae of the rhododendron weevil. Both of these pests may be controlled with Orthene.
  
  

Diseases

• Several different diseases may produce white spots on rhododendron leaves, including a petal disease called Petal Blight. "On colored flowers the spots are white, and on white flowers the spots are brown. The spots quickly enlarge and become soft and watery," according to the Clemson University Cooperative Extension Service. Another condition called Leaf Gall produces a white, powdery substance.