Question
I have six rhododendrons planted close to one another in a shady area next to the house. Five of them are doing great, but one has begun to develope yellowish leaves.Any ideas on the cause?
                                        Thank You

Answer
Hi John,
Thanx for your question.  It sounds like you may have spider mites which would eventually attack the other plants.

Spider mites are a major pest of many garden plants. They cause damage by sucking sap from buds and the undersides of leaves. As a result, the green leaf pigment disappears, producing a yellow or bronze stippled appearance. There may be silken webbing on the lower surfaces of the leaves. To diagnose this problem, hold a sheet of white paper underneath an affected leaf or branch and tap sharply. Minute green, red, or yellow specks the size of pepper grains will drop to the paper and begin to crawl around. The pests are easily seen against the white background.  Treat with a miticide.

Another issue could be chlorosis but the other plants would be turning yellow too.

A fairly common ailment of rhododendrons is chlorosis. It is really an iron deficiency of the plant. The leaves turn pale green to yellow. The newest leaves may be completely yellow with only the veins and the tissue right next to the veins, remaining green.  Rhododendrons thrive in acidic soil so use Miracid or another acidic type fertilizer.  

This info comes from Cherie Moyer
Adams County Master Gardener from Emmetsburg.net/gardens/index.htm

Let me know if this works.
Tom