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, meaning that it is a water mold, requiring free moisture in order to infect. The Phytophthora species that cause root rot of rhododendron are soilborne. As you might guess, the abundantly wet spring and early summer weather we have experienced this year have favored the development of this fungus. Because clay soils hold moisture longer than other soil types, plants growing in clay soils provide more opportunities for Phytophthora to infect. Plants that have experienced stress are more susceptible to infection.
Symptoms of root rot include chlorosis of leaves and a downward roll of the leaves parallel to the midrib. Leaves have the appearance of a cigar roll. They may wilt but usually remain attached to the stems for as long as two weeks after plant death. The roots have a soft rot, usually brown or black. Plants may die in a very short time, with as little as two weeks from the onset of symptoms to plant death. They do not die overnight.
Although there is some resistance available to this root rot, most rhododendron cultivars are susceptible. Established plants diagnosed with Phytophthora root rot can be treated with a fungicide as a drench around plants to saturate the soil. Repeat applications at 3- to 12-week intervals in the spring and fall. With some fungicides, granules can be blended into the soil before planting. Refer to the Illinois Homeowners’ Guide to Pest Management or the Illinois Commercial Landscape & Turfgrass Pest Management Handbook for chemical options. Read labels carefully for rates, warnings, restrictions, and timing.
To avoid this problem in the future, choose your planting site for rhododendrons carefully. The site must be well drained, protected, and have an acidic soil pH. Choose your cultivar to suit your site needs, but try to find one with resistance to Phytophthora, if possible. For more information, consult Dwarf Shrubs for the Midwest by Keith and Giles, University of Illinois Special Publication 60. Phytophthora root rot of rhododendrons is also discussed in Report on Plant Disease No. 664.