Here is a photo of my rhodo associated to the point I made on the discussion forum: Image link:

The photo shows areas of necrotic tissue. There can be a number of causes for this. Winter sun/wind/cold damage is probably the most common. When the leaves roll tightly in response to cold temperatures, the area along the leaf midrib is left exposed and causes areas of leaf tissue to dry out severely and die. Caterpillars can also be a cause as can some fungal infections. All in all, the leaves look quite healthy so physiological winter damage seems the most likely cause.

" 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

I'm in Brooklyn, and I've never had any luck with evergreen rhododendrons or laurels. After a couple of years they'd turn black and die. When I put in a couple of deciduous rhodies, they were fine. They're the "Lights" type -- one gold and fragrant, the other white hasn't flowered yet but should be fragrant too. Eve