Rhododendrons can be spectacular plants, even in Illinois. They just require a little more planning and work here than in milder climates with loose, acidic soil. Prepare the planting site as recommended by Extension horticulturalists or reputable nurseries. Planting in exposed, poorly drained, or clay sites with a high pH soil will invariably lead to slow decline and/or infection by Phytophthora. It is often said that the two biggest problems for rhododendrons in Illinois are clay soil and Phytopthora root rot.
We usually see Phytophthora root rot in late May or June in this state. Because the fungus thrives in wet or poorly drained sites, and because we've had more than enough rain in the past weeks, expect it earlier in 2002. Be ready to identify the symptoms.
Phytophthora-infected roots are reddish brown rather than healthy white. Roots die, and the reduced root mass is unable to absorb enough water or nutrients to support top growth. Often, the top of the plant quickly turns dull green, and foliage rolls. Several Phytophthora species could be involved, some of which do not overwinter in Illinois. Carefully inspect any plant material that you purchase from a nursery that imports plants from the South.
You will not be able to save infected plants, but proper identification of the problem can go a long way in preventing spread to other rhododendrons. Because poorly drained soils allow disease development, even in some resistant rhododendrons, obviously, proper soil preparation and good drainage are keys to disease control. Chemical drenches are available for commercial growers to help prevent disease spread in the planting.
Field work to develop rhododendrons with resistance has been ongoing for many years. At the Ohio State University, Hoitink and Schmitthenner performed trials on 336 hybrids of rhododendendrons (1974). The most resistant hybrids were Caroline, Professor Hugo de Vries, and Red Head. English Roseum was moderately resistant. There were not a great many resistant choices. Even with resistant hybrids, we need to emphasize site and soil preparation. Equally important is the selection of a hybrid hardy in your area.
What else might look like Phytophthora root rot? Any root decline such that as caused by flooding, drought, or mechanical or chemical injury could cause similar symptoms. A fungal disease called Botryosphaeria dieback probably causes the most confusion. That disease forms a canker at the base of a branch. From the canker outward on the branch, the foliage turns off-color and rolls, as with Phytophthora. On the stem, Botryosphaeria forms a canker in which you should see black, pinhead-sized fruiting bodies. The affected branch can be pruned out, and the plant will recover. Botryosphaeria typically follows drought stress, whereas Phytophthora follows wet weather and usually accompanies a growth flush.
For more information, consult Report on Plant Disease (RPD) no. 664, "Phytophthora Root Rot or Wilt of Rhododendron and Azaleas in the Midwest," available in your Extension office or on the Web at the Extension Vista Web site.