One of the more common problems affecting rhododendrons is that of root rot according to the Connecticut Agricultural Experiment Station. Cause by the fungus Phytophthora, this disease can be difficult to treat. Taking some precautions in cultivar and site selection, however, can improve your chances of avoiding the problem altogether.
An especially wet spring or summer can leave your rhododendron susceptible to root rot, since the Phytophthora fungus thrives in warm, wet soil. Heavy soils with high clay content are also a breeding ground for the disease. Make sure that your planting location is well drained. Resist the urge to water your rhododendrons if you see wilted foliage--a common symptom of root rot--because this can compound the problem.
Plants suffering from Phytophthora root rot often suffer from slow growth. Their foliage will have a wilted appearance and will sometimes be off-color. Roots of an affected plant may be reddish-brown and the color will extend up into the main stem and some branches as the fungus advances. With an infected plant, there will be a distinct boundary between the discolored, diseased tissue on the lower branches and the healthy, white tissue of the plant. If you are still unsure, peel back a bit of bark on the lower branches near the soil line to check for dark discoloration.
One non-chemical approach to treatment is to lift the rhododendron from its site, improve the soil aeration and drainage, and replant. Heavily compacted or clay soils can be amended with sand or organic matter, which improves airflow and decreases water retention. The Iowa State University Extension Service recommends pruning out infected branches, and completely removing severely diseased rhododendrons along with their surrounding soil. This can control the spread of the disease to surrounding plants. It may also be helpful to observe how water from unintended sources, such as run-off from a house or driveway, may be affecting the soil in question. Drains may be necessary to redirect such water and keep your rhododendron from being inundated too frequently.
In more severe cases, it may be necessary to use a fungicide to bring the Phytophthora fungus under control. Look for a chemical designed to battle this particular problem and drench the soil around the plant according to product directions. Two common choices are metalaxyl and fosteyl-A1. The fungicide, however, may not rid the infected plant of the fungus, but will help control its spread to other plants and curb future damage to existing plants. For a small plant, soak about 10 square feet of ground around its base. For larger plants, increase the treatment area accordingly.
Replanting in Problem Areas
If you have lost plants to Phytophthora root rot in a particular spot before, treat that area before risking new plants in the same soil. You may be able to use the non-chemical approach of soil amendment and improved drainage. If the fungus was present in abundance, however, drenching the bare soil in fungicide before replanting can improve your chances of success with future shrubs.