Hello forum members, I received the following email with the attached photographs of a sufferring Rhododendron. If anyone can provide a diagnosis I am sure the owners would appreciated it. "I have a very old rhododendron tree which, although it produced profilic and beautiful blooms this season obviously has a serious ailment of some kind. I need a diagnosis and advice on treatment,please.I have digital photos which I could transmit to you." Raakel
Mildew? See http://plant-disease.ippc.orst.edu/disease.cfm?RecordID=969 and http://www.scottishrhodos.co.uk/FAQs.html
It seems that powdery mildew is most likely. Here is another link which describes diseases of Rhododendron. If you notice the white powdery bloom that characterizes powdery mildew on some of the leaves, then the diagnosis is more certain. It is possible that you have leaf spot. One of the photographs seems to show more spotting on the leaves, with a slightly different appearance than the close-up photo of the leaf. Please note that the recommendations for control on the link list a number chemicals for treatment. The website is from the United States, and it is very likely that a lot of those chemicals are not registered in Canada. Raakel