Rhododendra, like most plants, are susceptible to developing phytophthora root rot. The pathogen that causes the disease is present in most soils. However, plants are most often affected when they have "wet feet," when the soil drains poorly or the plant is overwatered and the plants roots are sitting in water. Phyophthora root rot acts quickly. Transplanting and root pruning as soon as possible are the best methods to save your plant. But it must be replanted in well-drained soil and watered properly or the disease will strike again.
Dig up the rhododendron plant or remove it from its pot. If you are digging the rhododendron out of the ground, be sure to give the roots a wide berth (roughly 1 1/2 times the diameter of its foliage) to avoid damaging any of its healthy roots.
Wash the soil off of the rhododendrons roots with running water.
Prune away any brown, black, mushy or foul-smelling roots 1 inch into the healthy root tissue. If you have to prune more than one third of the rhododendron's roots, it is unlikely to survive.
Prune any branches with large numbers of wilted and yellowed foliage. Prune back 1 inch into the healthy tissue to be sure you remove all of the disease. If you have to prune more than one third of the rhododendron's foliage, it is unlikely to survive.
Repot or replant the rhododendron in well-drained soil. If the rhododendron is going back into the ground, choose a well-drained spot that is dry a few hours after a heavy rain or raise the existing spot by adding 6 to 8 inches of well-drained garden soil. Refill pots with fresh, well-drained potting soil. Do not cover the top of the plant's roots with more than 1 inch of soil.
Water the rhododendron with 2 to 3 inches of water if it is in the ground. Water potted rhododendron until water comes out of the drainage holes in the pot. Then empty the water from the tray underneath the pot in 30 minutes.