Answer: It might well be attributable to heat and drought stress. These plants are shallow rooted and need an evenly moist yet well drained soil. As such, they may need watering more than once a week in times of drought plus a generous layer of organic mulch to help keep the soil cool and moist. Note too that the root zone may extend past the drip line of the plant. Having said that, it is also possible that there was an underlying disease or poor rooting problem and the combination of that with the heat/drought killed it. If you suspect a disease, I would not suggest planting another azalea or rhododendron in that spot. In any case, given the choice, it is usually a good idea to wait until the temperatures cool off and milder (and rainier) fall weather starts before planting something else.
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