There are more than 1,000 rhododendron species in existence. They are differentiated by size, color and bloom time. Rhododendrons are popular due to their beautiful spring flowers, and are often used as specimen planting or foundation planting in landscape design. Phytophthora--a fungal root rot disease--is difficult to control, and it affects not only rhododendrons but also a variety of other plants.
Phytophthora root rot is a soil-borne fungal disease that is caused by several different Phytophthora pathogens: P. connamoni, P. citriocola and P. cactorium--as stated by Iowa State University. It attacks the root tissue and then travels to the lower part of the main stem of the rhododendron. Roots become brittle and brown in color. This disease is active during extremely wet conditions, and in soil that is heavy and has poor drainage. There are very few rhododendron cultivars that are resistant to the disease. Once the disease is active it cannot be controlled; prevention is the best defense against phytophthora root rot.
Conditions that Promote the Disease
Conditions that promote the occurrence of the disease are wet soil conditions, soil with poor drainage and temperatures of 80 degrees Fahrenheit and higher. Infected plants that are in sites with poor drainage are likely to die, as the disease is easily spread to the main roots and stem of the rhododendron.
Above-ground symptoms are usually minimal. In some instances the only symptom that you will notice is minimal growth activity; however, other symptoms that can be exhibited are drooping leaves, leaves that appear dull and rolled, wilting and even defoliation. Most symptoms of root rot appear below the soil level. You can check for root rot by peeling back a small amount of bark at the bottom or base of the plant. Brown tissue is a sign of phytophthora infection; white tissue is healthy tissue.
When purchasing rhododendrons do not purchase plants whose leaves are pale green, wilted or whose roots are dark and discolored--these are all signs of disease. Plant rhododendrons in well-drained soil and at the same level they were planted when you purchased them. Plant cultivars that are resistant to phytophthora root rot such as Caroline, Martha Isaacson, or Red Head and Professor Hugo de Vrie, as recommended by Carolina State University Extension.
You can limit the spread of the disease from infected plants to healthy plants by soaking the soil around the healthy plants with a fungicide such as Subdue MAXX or Banrot (according to Virginia State University). Apply the fungicide during the summer when there are extremely wet conditions and high temperatures, following the manufacturer's directions. (You will probably have to do this every four weeks.) Chemicals will not save infected plants. Severely infected rhododendrons, as well as the soil around them, should be removed and discarded. Do not put infected plant material in your compost bin.