Rhododendron: Wilt and Root Rot

Problem
The young leaves are yellowish and wilting. Eventually the whole plant wilts and dies, even though the soil is moist. Dead leaves remain attached to the plant and are rolled along the midrib. The symptoms may develop over a few weeks, or may take many months. Heavy, poorly drained soil favors disease development. The tissue under the bark close to ground level shows a dark discoloration when cut. To check for discoloration, peel back the bark at the bottom of the plant. There is a distinct margin between white healthy wood and dark diseased wood.

Analysis
This plant disease is caused by several different soil-inhabiting fungi, also known as water molds. These fungi (Phytophthora and Pythium species) attack a wide variety of ornamental plants. The fungi destroy the roots, and may work their way up the stem. If they girdle the stem, the plant wilts and dies. Very wet conditions favor the fungi, which are most common in heavy, poorly drained soils. Although azaleas and rhododendrons need constant moisture, they must also have good drainage.

Solution
No chemical control is available. Dry the plant out, following the technique in Water Molds. Improve the drainage of the soil before replanting azaleas or rhododendrons in the same location. (For methods of improving soil drainage, see Soil Structure, Groundwater.) If drainage cannot be improved, plant in beds raised a foot or more above grade. Or plant shrubs that are resistant to wilt and root rot (see Plants Susceptible and Resistant to Phytophthora for a list).