Answer: It's really difficult to diagnose a plant problem without seeing it, but I will offer some possibilities for the yellow spots on your rhododendron leaves. Sunscald or heat damage can show up as brown blotches, with yellowing tissue all around. These are blotches, however, not spots. Chemical injury (from overspray or drift) can show up as yellow spots with brown centers. It can take a year or two for these spots to be noticed. Rust, a fungal disease, begins as light green to yellow spots on the upper surface with rusty colored spots on the undersurface of the leaf. Powdery mildew, another fungal infection, can begin with yellowing spots of irregular size and progress to yellow and then brown spots.
For a positive identification of the problem and recommended controls, why not take a sample of the leaf problem to your local Cooperative Extension office or Master Gardener Clinic? Or visit the Washington State University website and download their publication on Rhododendrons (http://caheinfo.wsu.edu).
Best wishes with your rhododendron.
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