I agree with Art. It looks like chlorosis. Chlorosis is caused by too much water or alkaline soil (I am going to ignore the possibily of borers).
You can try giving it some 'Azalea and Rhododendron' food. The 'Azalea and Rhododendron' food has the right nutrients to utilize the iron in the soil. If you add some iron at the same time, all bases are covered.
If you feel you are wateering too much, water less. The borers... still not talking about them.
Daisy
