The edges of the leaves on one of my rhododendrons are turning brown and
crispy. This condition advances toward the midline until the leaf dies. My older, more established rhododendrons are looking fine. All have flowered this spring.
We can’t diagnose the problem via e-mail. However, what you describe could be a couple of things.
It might be marginal leaf necrosis which can be caused by cold damage, and made worse by wind and drought; by drought while the plant is in active growth; by high
amounts of salt in the soil, often due to overuse of soluble fertilizers,
and seen often if plant is located close to the house, where the eaves
prevent rainwater from reaching the soil. Other causes are poor drainage,
planting too deep, root damage, root weevils, nutrient deficiency, or
disease. (Source: How to Identify Rhododendron and Azalea Problems,
Washington State University Cooperative Extension, 1984).
There are descriptions and illustrations of rhododendron problems on the HortSense website of Washington State University Extension.
For comparison purposes, see the lists of diseases and problems affecting
Rhododendrons from University of California, Davis Integrated Pest Management Online. To get a definitive answer, it may be a good idea to take samples of the
affected leaves to a Master Gardener Clinic for diagnosis.