Answer: Spider mites are a possible culprit. They are hard to see -- try holding a sheet of paper below a yellowed branch and tap it. The tiny mites should appear as dark specks against the white paper. Mites usually attack plants that are suffering from moisture stress. Hosing off the plants every few days will help keep mites in check. Also, be sure the plants are watered regularly and deeply if nature doesn't provide. Azaleas don't like to have their roots sitting in water, but they equally dislike drying out. They also need acidic soil in order to thrive. You may want to test your soil to check the pH. Too high a pH can cause an overall yellowing of foliage.
Fungal twig canker is another possibility. Canker diseases infect stems and twigs to creating sunken, dying lesions on the bark. Check the branches that are turning yellow for lesions. Once a branch begins dying back, there is no cure. Just prune out and destroy all affected branches. Sanitize pruners between cuts with rubbing alcohol to prevent spread of the disease.
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