Fungal Infection Affects Leaves Of Rhododendrons

Q: I've heard there's a rhododendron problem affecting leaves - a new powdery mildew?

A: Gardeners and growers of rhododendrons and azaleas throughout the maritime Northwest have been noticing leaf symptoms for several years, from a fungal infection caused by "powdery mildew."

Symptoms differ from the common mildew often encountered on deciduous azaleas in the fall. Common powdery mildew looks uniformly grayish white and talc-like, and it comes off the stem, bud, or leaf when touched firmly. This other fungus will not rub off.

Symptoms of powdery mildew will be noticeable first primarily as pale yellow spots on the upper leaf surface, with feathery purple or brown spots underneath. The spots underneath will have blurry edges and grayish centers. Without looking under the leaf, you might see the diseased areas as a symptom of poor nutrition (a need nitrogen or iron.)

Lots of variation in symptoms and susceptibility is being reported. Two commonly planted cultivars, "Unique" and "Virginia Richards" are being affected in Western Washington and often severely. On "Virginia Richards," the fungal infection shows up as purplish circles on the upper leaf surface and brownish-gray underneath. On "Unique" you may not see any obvious symptoms on the upper leaf surface. Turn it over to reveal a light powdery-looking gray surface spotted with slightly darker gray spots. The whole under leaf surface may appear coated with the disease.

Plants may be able to tolerate the defoliation that often results from this mildew problem, or they may decline. The disease shows up in areas with high humidity and in heavy shade, but it seems to be more associated with which cultivar of rhododendron is grown than with environment.

If you can, remove affected plant parts. (This may be impractical on a severely affected plant.) Prune the plant to allow good circulation. Monitor plants to check the problem, and if you choose to treat with a fungicide, treat as new leaves are opening. Use a fungicide containing sulfur, a fungal spray such as Funginex, or Bayleton. Thoroughly cover both sides of all leaf surfaces.

Read all label directions carefully and wear a hat, gloves and goggles as well as long-sleeved shirts and long pants when applying pesticides. The label may also indicate further protective clothing required.

This disease goes through several life cycles a year and may need to be treated more than once as the season progresses. Again not all rhododendrons in all gardens will show signs of this disease; some species and cultivars appear to be resistant.

In addition to "Unique" and "Virginia Richards" mentioned as likely to be affected by the disease, other cultivars reported susceptible by growers in Washington are "Cheer," "Crest," "Loderi King George," "Taurus" and "Vulcan's Flame."

Complete lists of susceptible and resistant plants aren't available yet. Some plants reported resistant by growers in British Columbia were "Mrs. G.W. Leak" and "The Hon. Jean Marie De Montague," but this may not hold true in Western Washington.

Gardening runs Friday in Scene and Sunday in Home/Real Estate. It is prepared by Mary Robson, Area Horticulture Agent; Holly Kennell, Washington State University/King County Cooperative Extension agent, Susan Miller, integrated pest management specialist, and volunteer Master Gardeners.