Azalea leaves can turn yellow naturally when they’re about to drop off. So you shouldn’t freak out at first sight. Check if the leaves have aged and if it’s natural for them to turn yellow. If not, then chances are the leaves aren't receiving the proper nutrients. They might be suffering from chlorosis, which is common among plants when the soil becomes alkaline. As you know, alkaline soil is an Azalea killer. Thus you need to use quick-acting fertilizer to turn the soil acidic. Yellow leaves also hint at possible iron deficiency. So it’s advised to prepare a compost or use fertilizer rich in iron. Once applied, the leaves should turn green over time.