My rhododendrons have been developing brown spots on their leaves. The spot may begin anywhere on the leaf, but typically occurs in the middle. It begins as a roughly oval or circular yellowed area, which quickly turns brown and brittle, with a well-defined edge. The undersides of the leaves show no signs of spores--the yellowing begins on the top of the leaf and often cannot be seen from the underside when it first begins. The brown spot may but does not always spread to cover large areas of the leaf. The leaves do not curl or show signs of wilting; large, well-defined areas of the leaf simply die and turn brown.

I don't know what's causing it. I thought it might have been sunburn, but it can occur anywhere on the rhodendron, whether shaded or not. It also appears to spread, although I can't be sure. If it's environmental, it may be that the environmental irritant is still present. I took pictures of some of the leaves; see the links below.

http://www.geocities.com/leah.crosby/leaves1.html

http://www.geocities.com/leah.crosby/leaves2.html

Does anyone have an idea on what is causing this?