Do you have a photo of a leaf underside? A few yellow leaves overall wouldn't be a concern, but dark spots on leaf undersides sounds more like an insect pest than nutrient issue....one possibility, lace bug.

" It is a bit pricey but I think essential to serious rhododendron gardeners." I don't know. Just to play devil's advocate but I consider myself a fairly serious collection - having bought well over 100 cultivars - and I don't personally see a need for it. I guess I'm lucky? I just don't have many disease issues besides root rot. Which, unless you have a phytosanitary diagnostic laboratory in your house with several ten of thousands of dollars of equipment, isn't something you really need a manual to further interpret. This post was edited by davidrt28 on Mon, Nov 3, 14 at 20:54

You don't say where you are, but if they were mine, it would be winter burn. The leaves that are exposed to sun and wind rather than covered by snow or shaded by other leaves are often damaged by freeze-thaw cycles and dry winds and by the lower levels of moisture in the soil even when it isn't frozen. IME they will fall off when the new leaves open or a bit sooner, and if you look closely you can see new little buds waiting to open.

The problem started about two months ago. The variety is Seaview Sunset. It is about two years old. Weather has been typical San Francisco Bay Area Mediterranean climate. It gets sun in the morning and sits under the shade of a large tree during the middle and late hours of the day. Sunlight has not changed. The damage looks localized to that one limb in the photo. The soil is redwood bark and a little turface, so it is very acidic and does not need modification. It is worth mentioning there is a sister plant to this one and it is doing fine.