Powdery mildew is very common in the Pacific Northwest and is caused by members of the fungus genus Microsphaera.  The disease can affect both hybrid and species rhododendrons.  Surprisingly, the symptoms are different for rhodys than for many plants.  Specifically, it is rare to see deformed rhododendron leaves with a gray powder on them.  Some notable exceptions include many azaleas as well as ‘Purple Splendor’ and ‘Vulcans Flame’.

Powdery mildew symptoms in rhododendrons appear first on the leaves, with light green or yellow blotches on the upper surfaces and brown to purple spots or feathery areas on the lower surfaces.  In severe cases, partial or complete defoliation and stem dieback can occur between late summer and early spring.

The Microsphaera fungi seem to be able to infect most rhododendrons, but some are more susceptible to powdery mildew than others.  Some highly susceptible examples include R. cinnabarinum, R. campylocarpum, ‘Elizabeth’, ‘Virginia Richards’, ‘Unique’, the ‘Loderi’ grex and many deciduous azaleas.  Less susceptible plants are R. dengronianum ssp. yakushimanum and its hybrids, rhododendrons with indumentum, and our local native species R. macrophyllum and R. albiflorum.

Powdery mildew flourishes in locations with warm, sunny days and cool, humid nights.  Especially favorable conditions occur in areas where plants are crowded and there is poor air circulation.  Under such conditions, fungus spores can be produced within sixty hours of an initial infection and are then ready to be spread around the garden by means of wind or rain.  Microsphaera spores prefer to germinate on dry leaf surfaces where they quickly infect the plant by sending specialized cells called haustoria into the leaf tissue to absorb nutrients.  Young, soft rhododendron leaves are most easily attacked by powdery mildew although symptoms may not be evident until the leaves mature.  Spores can over-winter on lower leaf surfaces or in buds and will attack new leaves that develop the following spring.

Prevention and treatment of powdery mildew can involve both cultural controls and chemical sprays.

Chemical Controls:

Fungicides will not remove the damage to infected leaves.  However, if there are symptoms on last year’s leaves, you might consider spraying the new spring growth to protect against further infection.  Triflorine (Funginex) and sulphur sprays such as Safer’s Garden Fungicide are registered for use by homeowners.  Newer, potassium bicarbonate (baking soda) based sprays show some promise.  In all cases, full leaf coverage with the spray is essential.  Follow label instructions closely and wear protective goggles and clothing.