It has been said that the two most limiting factors to healthy rhododendron growth in Illinois are clay soils and Phytophthora root rot. Actually, the two usually go hand in hand. Phytophthora is an oomycete fungus, a water mold requiring free moisture to infect. Phytophthora species causing root rot of rhododendron are soil-borne. It seems odd that we might see this disease in a year of drought, but irrigation can also supply too much water and create conditions conducive to infection. Because clay soils hold mois-ture longer than other soil types, plants growing in clay soils provide more opportunities for Phytophthora to infect. Often the top several inches of our lawns is rich topsoil. Unfortunately, many of us have a clay subsoil that is in the root zone of shrubs and trees. If we dig and prepare a nice planting hole (a million-dollar hole) and it is enclosed in clay, drainage problems and this root rot still occur. Site preparation includes ensuring drainage away from the roots. Plants that were under other environmental and site stresses also are more susceptible to infection.
Symptoms of root rot include dull green leaves that may turn yellow and roll. Leaves may wilt but usually remain attached for up to 2 weeks after plant death. Healthy roots are white inside, whereas plants with Phytophthora are reddish brown. Plants may die in as little as 2 weeks from symptom initiation.
Although there is some resistance available to this root rot, most rhododendron cultivars are susceptible. Drs. Hoitink and Schmitthenner at the Ohio State University found the most resistant hybrids in their trial to be Caroline, Professor Hugo de Vries, and Red Head. They reported that Purple Splendor was one cultivar more susceptible to Phytophthora root rot. Keep in mind that even resistant plants let the fungus develop, so good drainage and correct soil pH are extremely important to long-term health for rhododendrons in Illinois. Established plants diagnosed with Phytophthora root rot can be treated with a fun-gicide as a drench around plants to saturate the soil. Applications must be repeated at 3- to 12-week inter-vals in spring and fall. With some fungicides, granules can be blended into the soil before planting. In most cases, fungicides are used to protect plants near one that has died from the disease. Refer to the Home, Yard, and Garden Pest Guide or Illinois Commercial Landscape and Turfgrass Pest Management Handbook for chemical options. Read labels carefully for rates, warnings, restrictions, timing, etc.
To avoid this problem, choose your planting site carefully. It must be well drained and protected and have an acidic soil pH. The pH level could be the most difficult requirement here because most Illinois soils are very alkaline. Choose your cultivar to suit your site needs, but try to find one with resistance to Phytophthora. Consult Dwarf Shrubs for the Midwest, U of I ACES special publication 60/NCR 469, for specific instructions on lowering the pH for rhododendron plants. Did you know that you should not use aluminum sulfate to acidify the soil because aluminum is toxic to rhododendrons and azaleas? Or that iron sulfate is not recommended during the initial soil preparation because it can cause a salt buildup? Before you get your rhododendron bed ready, read SP 60/NCR 469, available in Illinois Extension offices and from ACES Marketing and Distribution, 800-345-6087. Information on Phytophthora root rot of rhododendrons and azaleas is in Report on Plant Disease (RPD) no. 664 on the VISTA Web site or in Illinois Extension offices.