Geography

• Powdery mildew on rhododendron leaves is exacerbated by humid growing locations, affecting plants in coastal areas like the Pacific Northwest, the American south, and the United Kingdom, but the mildew rarely displays its harmful effects in significantly colder climates with frigid winters.
  
  

Symptoms

• Infection with the fungus occurs in the spring or early summer, or may be carried over from the previous growing season. "The most commonly found symptoms are diffuse pale yellow spots on the upper leaf surface. The leaf undersurface will display purple to brown circular, diffuse feathery areas," writes H. Edward Reiley in his book "Success with Rhododendrons and Azaleas" (2004).
  
  

Prevention/Solution

• Some rhododendron species are resistant to powdery mildew. For those that aren't, the condition may be battled by providing significant air circulation to the shrub, and ensuring that the plant receives proper sunshine. Gardeners who prefer to avoid deploying fungicides to prevent powdery mildew may wish to utilize potassium bicarbonate-based products.