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A deep green leaf may begin to show lighter green patches, and these areas will gradually become more yellow. One cultivar, 'Virginia Richards,' gets brownish purple spots on both tops and bottoms of leaves.
This common disease is named 'powdery mildew' despite how little the symptoms resemble the familiar fungal disease often seen on roses and azaleas. The causal agent is apparently Microsphaera azalea. Usually the disease doesn't produce the familiar white powder-like spores, although late in the summer some may become visible. The disease manifests instead as color changes in the leaves, followed by defoliation toward the end of the growing season.
Many rhododendrons, if basically healthy, will coexist with the disease and seem to outgrow or at least survive the symptoms. Last year's leaves, once they have been hit by the disease, will always have it, with symptoms persisting from year to year until the leaves drop off.
Keep rhododendrons healthy to help them manage this problem. The disease is worse in areas with wet, mild winters (yes, that describes the Puget Sound basin.) Be sure that the soil drains well, because rhododendrons require moist soil but cannot tolerate soggy conditions. There's no direct apparent link between bad drainage and powdery mildew, but it's necessary to correct drainage to avoid root rots which will place more stress on a plant that may be defoliating from powdery mildew. Do not over-fertilize the plants, which might cause susceptible soft growth.
Cleaning up fallen leaves under plants may help reduce some of the disease pressure, but can't eliminate it because the fungal organism also lives through winter on leaves that remain on the shrub. If it's practical, remove the affected leaves. However, in my garden this practice would mean having no leaves left on 'Unique.' Prune the plants to improve air circulation if they are in a crowded, shaded planting. In my own garden, I'm watching and waiting to see if the affected plants will continue to resist worsening problems.
If you notice symptoms on last year's leaves, consider protecting the new growth with a fungicide. Apply it now to the new growth as it expands, before symptoms appear on this spring's leaves. Fungicides won't get rid of the existing problem on old leaves. A new fungicide, 'Remedy,' which is a potassium bicarbonate (made by Bonide Company), is registered for the problem. Thorough leaf coverage is necessary with all fungicides. Fungicides containing sulfur (such as Safer Garden Fungicide RTU) are also registered. Others are Funginex (sold as Ortho RosePride Funginex Rose and Shrub Disease Control Concentrate.) Be very careful to read all label instructions, and wear protective goggles and gloves. Funginex can be corrosive to eyes.
Rhododendrons vary in their susceptibility to the disease, though many more are affected than are resistant. No trials have been done to determine precisely which rhododendrons grow best under the pressure of powdery mildew. Growers around the world responded to a survey in 1995, with mixed results. One common report is that rhododendrons that have been hybridized with Rhododendron cinnabarinum as one of the parent plants do get the disease severely. Two of those are 'Elizabeth' and 'Lady Chamberlain.' The Cornish Cross hybrids, including 'Virginia Richards,' 'Seta' and 'Mrs. G.W. Leak' seem vulnerable also. In my garden, 'Mrs. G.W. Leak' an old-fashioned pink, resisted infection for decades until 1999 when it began to show symptoms.
Local gardeners are reporting good success in disease resistance with all the 'Yak' rhododendrons, such as R. yakushimanum. In my garden 'Yak Princess' has stayed clean. So has the tall, tree-like species Rhododendron augustinii.
I would appreciate your reports on what you see in your rhododendron plantings, and will share the information. For an informal survey, please e-mail or drop a line to tell me which rhododendrons are doing well, and which are not! E-mail Mary Robson (Ret.). Or write:
| Mary Robson Area Horticulture Agent 500 7th Avenue SW Suite 200 Renton WA 98055-2983 |
Hortsense: Managing plant problems with Integrated Pest Management
