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Q: I garden on the Sunshine Coast and have a great selection of rhodos. There is one that I have had trouble with for the last few years. It is mature at least 15 yrs and has a pale creme flower in early May. Currently it is full of luscious new green growth, looks healthy and blooms are plentiful. However as seasons come the leaves will turn crispy and brown from the outer edges inwards and curl slightly, some will fall, while others won’t, but it will look a sad specimen until next spring when it flowers. There seems to be a very small pale speck on the leaves.
The local nursery said the rhodo was in shock after the removal of a large fir that shaded it and would recover. It still sits in shade of another fir and adjacent other rhodos none of which “went into shock”, nor do their leaves turn brown. I feed and water when necessary and last year I sprayed but to no avail. This has now happened for the last three years.
You may be describing a disease called powdery mildew (Microsphaera spp.), which is very common in the Pacific Northwest. Symptoms vary greatly on evergreen, broadleaf rhododendron cultivars, from large brown areas (resembling burn marks) to reddish spots or faint yellow spots (mimicking a nutrient problem) on the leaf. In some very susceptible cultivars, e.g. Rh. ‘Unique’, plants can shed a lot of leaves, becoming almost naked. This disease can occur as early as January here on the B.C. coast. To confirm this diagnosis, check under the leaves for a whitish-gray cast. This is not to be confused with other powdery mildew which appears more like talcum powder and is very common on roses.
If it does turn out to be powdery mildew, remove and dispose of the infected leaves in the garbage (not in the compost). Increase air circulation around plants to decrease humidity. Keep watering the ground under the plants, but water earlier in the day rather than late in the evening. Apply a good mulch and fertilize with a rhododendron fertilizer as directed. Or remove the susceptible plant and replace with a more resistant cultivar. Sulphur spray applied early in the season from four to six times, especially on the underside of leaves is an effective protectant.
