Phytophthora root and stem rot of rhododendron is initiated in wet weather. The causal fungus is a water mold and needs water to infect. Recently, we have received several samples of rhododendrons with this disease. Although we had a long stretch of drought in much of Illinois, recent rains brought as much as 8 inches of water in a short time, causing flooding and waterlogged soils. Details about this disease are discussed in issue no. 4 of this newsletter.
Although Phytophthora root rot is most common on rhododendron, the fungus can also cause a top dieback. In warm, wet conditions, the fungus can splash onto foliage, causing brown blotches. Infection may move into the petiole and then to the stem. This dieback may occur without the root rot.
If you are doing garden renovations this fall, try to provide good drainage in beds containing rhododendrons. Add organic matter and adjust the soil pH to between 5.5 and 6.0 for ideal growth in Illinois. Remove old debris that can harbor this fungus over winter. Water the soil around the plant--rather than the foliag--to reduce foliar infection.
Details on growing rhododendrons can be found in Large Flowering Shrubs for the Midwest, University of Illinois special publication no. 74. Details about the disease can be found in Report on Plant Disease, no. 664, "Phytophthora Root Rot or Wilt of Rhododendron and Azaleas in the Midwest."