Root rot of azaleas and rhododendrons
Azalea root rot
Photo: U of MN Plant Disease Clinic
Root rot is a serious disease affecting azaleas and rhododendrons. The fungus Phytophthora cinnamomi is often associated with root rot, although other fungi may also be involved. Phytophthora cinnamomi survives as thick-walled spores in infected roots, lower stems, plant debris, and in the soil. The fungus can persist for many years, but it does not overwinter well in the northern United States in the soil alone. During cool, wet weather, the fungus produces mobile spores, which can swim short distances. Spores are also transported to nearby roots by water movement through the soil.
Fine roots become infected first and turn brown. Infection may advance into all parts of the root system, spreading toward the crown of the plant. There it can girdle the plant and move into the stem. Infected tissues in the stem turn dark brown while young plants may also develop brown visible cankers near the base of the stem.
The first noticeable symptoms of root rot are drooping and yellowing leaves that roll downward parallel to the midrib. It is important to note that some rhododendrons will roll their leaves due to cold weather or extreme drought. In these cases, the affected leaves will recover. Large plants may show aboveground symptoms of mild leaf yellowing, branch dieback, and/or stunted growth. A plant may survive with only a few uninfected roots, until the plant is stressed. It is then overcome by the pathogen and killed.
To prevent root rot, plant healthy stock on a favorable site. Avoid planting in poorly drained soil, on the exposed south side of a building, or in windy sites. Azaleas and rhododendrons grow best in an acid soil. A soil test before planting will indicate whether soil amendments are needed. For more information on planting and caring for these shrubs, see the MN Extension Service Publication, Azaleas and Rhododendrons for Minnesota (AG-FS-2386).
Mulch, properly fertilize, and water during dry periods to keep plants growing vigorously. Moisture extremes cause rhododendrons to be more susceptible to P. cinnamomi. Remove and destroy infected plants. Do not replant azaleas or rhododendrons in the same hole or next to the area where an infected plant was removed.
Coyier, Duane L. and Roane, Martha K. 1986. Phytophthora Root Rot, pp. 4-7 In: Compendium of Rhododendron and Azalea Diseases. APS Press, St. Paul MN. 65 pp.
Moe, Susan. 1984. Azaleas and Rhododendrons for Minnesota (AG-FS-2386). Minnesota Extension Service, University of Minnesota, St. Paul MN. 2 pp.
Sinclair, W.A., Lyon, H.H., and Johnson, W.T. 1987. Diseases Caused by Phytophthora cinnamomi, pp. 288- 290 In: Diseases of Trees and Shrubs. Comstock Publ. Assoc., Cornell U. Press, Ithaca NY. 574 pp.
Chad Behrendt, Crystal Floyd