The leaves of 3 of my rhododendrons are covered with a fine yellow powder that can be rubbed off, but spraying with a hose does nothing. The plants, which are scattered throughout a large garden, look healthy otherwise. They receive regular water, have good exposure and are spaced well from other plants. There are other rhododendronss in the yard that are not affected. Any ideas?
We can’t diagnose the problem remotely, but I can tell you a few common rhododendron problems or conditions which sound a bit like what you describe. I consulted a Washington State University Cooperative Extension booklet entitled How to Identify Rhododendron and Azalea Problems, WSU 1984.
If the yellow coating is more like small speckles on the top leaf surfaces, look for small, tarry black spots on the undersides which could be a sign of lacebugs (this may not be a serious enough problem to require control).
If the leaves have light green to yellow diffuse spots randomly distributed around the leaf, and the underside has yellow-orange powdery spore masses, this might be rust (caused by fungus associated with Sitka spruce).
What seems most likely, based on your description, is algae, which coats the leaves with a yellowy greenish powder, and is easily rubbed or scraped off the leaves. Algae does not harm the plant, but may offend one’s sense of aesthetics. The Royal Horticultural Society describes algae on rhododendron leaves.
Excerpt:
Shaded, dense foliage is where algae builds up most on plants, particularly at the base of evergreen hedges, such as holly and yew, or on congested shrubs such as Camellia and laurel. These are areas where water does not quickly dry, creating the damp, shady conditions favoured by algae.
Control:
- Clipping evergreen hedges so that they taper outwards towards the bottom can help improve light access to the base.
- Congested evergreen shrubs should be thinned out at the appropriate time of year to increase air circulation around the leaves. This will allow rain to dry more quickly on wetted foliage and reduce the opportunity for algae to build up.
- Feeding straggly plants in the spring to encourage vigour should also help.