Answer: It's hard to diagnose rhododendron problems without thoroughly inspecting the plant! If the environment is the same, it still might be that the one with symptoms simply has not established itself. You may want to dig the plant and inspect the roots for signs of distress (mushy roots (root rot) or dried, out dead roots). The only other possible cause for leaf curl is insect feeding, especially Rhododendron Stem Borer. The adults are dark-colored beetles that are about 5/8 inch long. They have long antennae and are pale yellow with two black spots behind the head. In late June to early July, female beetles lay their eggs in new shoots several inches below the bud. The larvae emerge, bore into and down the interior of the twig where they overwinter. The following year, the larvae continue to bore downward where they overwinter in the roots. Adults are present every year, but each insect takes two years to complete a life cycle.
Adult beetles feed on the undersides of leaves along the midveins, causing the leaves to curl. Boring by the larvae causes wilting and eventual dieback of individual canes.
Examine lower branches with curled leaves for the adults and signs of feeding in June and July. Prune and cut open wilted branches to see if larvae are present. Also, check branches for sawdustlike debris that push out through small entrance holes in the bark. If you find evidence, prune and destroy wilting branches. If you experience a similar problem next year, spray the branches with chlorpyrifos.
Best wishes with your PJM's!
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