I feel your pain, malorn. It is often hard to say why one specimen gets affected and others close by do not. I have two chinese fringe flower bushes planted close to each other and one gets chlorotic and the other one does not. This requires me to ammend the soil around one more than the around the other but I always wonder if one specimen is a "weaker" plant than the other.

Since knowing exactly the cause of the problem would help you so much, why don't you send a few leaf samples from the one plant to your Agriculture Extension Service for analysis? Use a clear plastic sealed envelope. They can probably tell you exactly what pathogen (or winter burn) is causing the problem in about a few weeks.

Luis

One possibility is root strangulation. If you don't untangle the roots when you plant, the plant may be root bound and eventually the roots will strangle each other. That would explain why one plant could be perfectly healthy and the one next to it dying.

Most rhododendron and azalea plants sold at nurseries and garden centers are sold in containers or have a root-ball that is covered with burlap. These plants have a potentially serious problem when the roots reach the container and start circling inside the pot. They become pot bound or root bound. These roots must be cut so they don't continue to grow and start strangling other roots. Many apparently healthy plants die when the roots start strangling each other. To prevent this when planting, it is necessary to remove the plant from the container and and examine their roots. If the plants appear pot-bound and have a thick, dense mat of fibrous roots along the surface of the root ball, used a knife to make three to six vertical cuts, about 2 inches deep, equally spaced around the sides of the root ball. Then use your hands to gently loosen the roots where cuts were made and pull the roots outward. This process stimulates new root growth and allows water and nutrients to penetrate into the root mass. If the roots are not pot-bound, it is not necessary to slice them with a knife, but it is beneficial to loosen and pull them outward with your hands. When working with roots, make sure the plant is thoroughly watered. Any roots that dry out will die.

Here is a link that might be useful: How to grow rhododendrons and azaleas.