Rhododendrons are famous for susceptibility to Phytophthora root rot. They are occasionally host to other root rot pathogens, including Cylindrocladium; but that is mostly in production areas. They also succumb to Botryosphaeria dieback, which can resemble Phytophthora root rot but forms distinct cankers at the base of wilted stems. Botryosphaeria follows drought stress, whereas Phytophthora occurs in wet periods.
Rhododendrons can be spectacular, even in Illinois. They require more planning and work here than in milder climates with loose, acidic soil. Prepare the planting site as recommended by Extension specialists or reputable nurseries. Planting in exposed, poorly drained, or clay sites with a high pH soil invariably leads to slow decline and/or Phytophthora infection.
Phytophthora root rot occurs on rhododendrons throughout Illinois. First symptoms include a dull green cast to the foliage, followed by wilting and lack of new growth. Roots of infected plants are reddish brown. Healthy roots should be white or should at least have white root tips. We usually see Phytophthora root rot in late May or June in Illinois. The fungus thrives in wet or poorly drained sites; so if you are in a wet area of the state, watch for symptoms of this disease.
You will not be able to save infected plants, but proper identification of the problem can help prevent spread to other rhododendrons. Because poorly drained soils allow disease development even in some resistant rhododendrons, proper soil preparation and good drainage are keys to disease control. Chemical drenches are available for commercial growers to help prevent disease spread. Carefully inspect any plant material that you purchase or move to your landscape.
Field work to develop rhododendrons with resistance to this disease has been ongoing for many years. At the Ohio State University, Drs. Hoitink and Schmitthenner performed trials on 336 hybrids (1974). Those most resistant were Caroline, Professor Hugo de Vries, and Red Head. English Roseum was reported as moderately resistant. There were not a great many resistant choices. You can look for resistant hybrids in the nursery, but even with these plants we need to emphasize site and soil preparation. Equally important is the selection of a hybrid hardy in your area.
Phytophthora root rot kills roots, thereby inhibiting water and nutrient absorption. Any factor that similarly injures roots could produce similar symptoms. For more information, consult Report on Plant Disease, no. 664, “Phytophthora root rot or wilt of rhododendron and azaleas in the Midwest,” available in Illinois Extension offices or on the Web at http://www.ag.uiuc.edu/%7Evista/horticul.htm.