Why are there curled, droopy leaves on so many Rhododendrons?

   What's happening to large, older Rhododendrons in established landscapes? The Urban Forestry Diagnostic Lab has had a number of inquiries about leaf curling on Rhododendrons, a situation we have also observed on the Umass Amherst Campus and surrounding area. Typically, an entire plant among otherwise healthy plants or several branches on a plant have tightly curled, limp, off-color leaves. Close examination of the affected plants turned up a few plants with black vine weevil, Rhododendron borer or Botryosphaeria cankers playing a role, but in the vast majority of the cases affected shoots had intact though somewhat shriveled, bark with green (but dry) vascular cambium and white sapwood. In many cases, most of the plant had several healthy branches below or lateral to the affected ones.

   So, what's happening? Something is drying out the buds, leaves and branches to which they are attached. Fungal disease and insect feeding damage problems are not playing major roles in this situation. The best explanation has been drawn from several sources and looks like this. Rhododendrons, like many other plants in our area, are still recovering from the drought of 1998 and 1999. They suffered damage as a result of reduced photosynthesis, root damage, depleted energy reserves experienced during the dry conditions. Because of the diminished root systems, these plants were unable to take up the water available during the mild, moist season last year and fully hydrate themselves. Water stored in roots and stems allows these broadleaved evergreens to support themselves during winter when water is not available.