Rhododendrons are popular shrubs for growing in your garden. Well-liked in the spring for their bright, colourful flowers, some species of rhododendron retain their deep-green leaves during the winter. Like any plant, though, rhododendrons are susceptible to some diseases that affect their leaves, so it's important to be able to recognise these diseases and know how to prevent them.
According to the American Rhododendron Society, powdery mildew is caused by the fungus Microsphaera azalae, most common in the Pacific Northwest. The most common symptom of this condition is light-yellow spots on the surface of the leaf. In some situations, these spots can be larger and purple or brown. If not treated, powdery mildew can defoliate certain breeds of rhododendron. The application of fungicides such as Benlate help treat and prevent powdery mildew.
Leaf gall, also caused by a fungus (Exobasidium vaccinii), causes swelling in young leaves. These galls are about an inch or so in diameter, and eventually the swollen leaves will shrivel up, turn brown and fall off the plant. The spores spread from one plant to another by the wind, and the best way to prevent leaf gall is to spray your plants with fungicide and hand-remove the galls as they form.
According to the Fraser South Rhododendron Society, leaf spots can be caused by a wide variety of diseases. Certain fungi such as botryis, pestalotia and phyllosticta can cause burns on leaves. Necrotic ring spot, a common virus, causes reddish spots to appear on leaves, though it seems to affect only younger plants of specific breeds of rhododendron, and it doesn't spread from plant to plant. No known cure is available or needed, according to the Fraser South Rhododendron Society.