If you’ve recently noticed your rhododendron behaving oddly, it may be affected by powdery mildew. This fungal disease attacks broad-leaved evergreen rhododendrons, and has been documented on deciduous azaleas.
This fungus (Microsphaera sp. and others) is typically spread by wind. The symptoms vary, depending mostly on the rhodo cultivar affected rather than on any environmental conditions. Diseased rhodos may even appear normal from a distance. Initially, you might confuse the disease with nutrient stress, or mistake symptoms for the plant’s natural appearance.
On the top leaf surface, some plants may exhibit yellow mottling or spots. Other signs include large purple-brown spots or areas, purple ring-shaped marks, or green spots as the remainder of the leaf yellows. However, certain cultivars, such as ‘Unique,’ will show no symptoms on the top surface of their leaves.
The best way to diagnose the problem is to check the underside of the leaf for feathery purple or brown circular areas. In time, the entire underside may become covered, or sometimes a sparse greyish colouration may be evident.
Depending on the cultivar, the plant may also experience varying degrees of defoliation in fall or early spring (e.g., Rhododendron cinnabarinum is highly susceptible).
Control measures include:
• Sanitizing by raking and destroying fallen leaves throughout the year.
• Providing good air circulation.
• Avoiding shaded, damp planting sites.
• Avoiding overwatering or overfertilizing.
• Selecting resistant hybrids or species, such as R. ‘The Hon. Jean Marie de Montague’ or R. yakushimanum.
• Spraying using sulphur products as directed, on both top and bottom leaf surfaces.
Conway Lum is a Certified Horticultural Technician at Mandeville Garden Centre in Burnaby. Questions can be emailed to him at firstname.lastname@example.org.