Rhododendron leaves will often curl and droop in response to cold weather. Unfortunately however, if you see droopy leaves during the growing season, fungal root rot, often from Phytophthera is usually the cause.  Our very wet spring may have aggravated the the situation.

To diagnose the problem first check the roots.  If Phytophthera root rot is the problem, the smaller roots will be missing and the larger roots will be discolored.  If the disease has spread to the stems, when you scrape away some of the bark and you should see reddish brown discoloration, this article has a picture, Phytophera Root Rot in Trees and Shrubs.

Unfortunately, Phytophthera spores are able to survive unfavorable periods up to several years.  Infected plants serve as a reservoir for the disease.

To control Phytophthera prevention is the key and the key to prevention is drainage.  The fungus thrives in saturated soil.  When planting rhododendrons, first select resistant varieties.  Most rhododendrons are highly susceptible to Phytophthera, but OSU lists some resistant cultivars in the article, Rhododendron -Phytophera Root Rot,

Hybrid cultivars that are resistant include 'Caroline', 'Serenade', 'Martha Isaacson', 'Pink Trumpet', 'Red Head', and 'Professor Hugo de Vries'. Rhododendron species that are resistant include R. davidsonianum, R. delavayi, R. glomeralatum, R. hyperethrum, R. lapponicum, R. occidentale, R. quinquefolium, R. sanctum, R. pseudochrysanthum, R. simsii, and R. websterianum.

Select a location for new plants outside the area where the infected plants were growing.  Make sure the soil has very good drainage, raised beds are an excellent idea.  Check the pH of the soil, and add soil acidifiers (usually sulfur) if it is too high. Rhododendrons like acidic soil (low pH) and fungus doesn't!  Make sure the soil has sufficient organic matter.  Rhododendron roots are shallow and fibrous.  This article from the Rhododendron Society, Rhododendron Culture and Care, has excellent information on growing these beautiful plants.