Unfortunately, most of the root rots that affect rhododendrons and other ericaceous species are water molds - they travel through the soil with excessive moisture, either from rainfall or irrigation. And there is really nothing you can do to prevent them short of soil sterilization, which is highly impractical. The best you can do is to provide a location with very well drained soil. And provide the other cultural factors the plants prefer - sufficient soil acidity, some protection from hot afternoon sun, a shallow and preferrably raised planting hole, etc.

Thanks! The Rhododendron is already in a raised bed, that didn't work, and azalea on the side of hill. We were planning on digging out big hole and using all new planting material, but some other bushes/trees in the fall. Suggestions?