botryosphaeria canker on rhododendron

Botryosphaeria canker on rhododendron. Photo: Penn State Department of Plant Pathology & Environmental Microbiology Archives , Penn State University, Bugwood.org

Updated: September 29, 2021

About botryosphaeria canker

  • Botryosphaeria canker is the most common disease of rhododendron in the landscape. A typical symptom of this fungal disease is scattered dying branches on an otherwise healthy plant. Leaves on infected stems droop and roll inward, then turn brown. These leaves often lay flat against the stem and will remain attached.
  • The pathogen can infect all ages of stem tissue through wounds, pruning cuts, and leaf scars. Heat, drought stress, and winter injury can increase disease incidence. Cankers on branches can gradually grow through the wood until the stem becomes girdled.
  • Diseased wood is reddish-brown in appearance.
  • Discolored wood viewed in the longitudinal cross-section often forms a wedge that points toward the center of the stem, and the pith may be darker brown than the surrounding wood.
rhododendron with Botryosphaeria canker
Botryosphaeria canker symptoms on Rhododendron
Photo: David L. Clement, University of Maryland Extension
Botryosphaeria canker wound on stem

Closeup of Botryosphaeria canker symptoms on stem
Photo: David L. Clement, University of Maryland Extension

 

Management

  • Plants should be grown in partial shade, mulched, and kept well-watered during dry periods. 
  • Fungicide treatments are not an effective disease control option.
  • All dying branches should be promptly pruned out in dry weather and all discolored wood should be removed at least 6 inches below the last symptoms of the disease.
  • Plants can be pruned back severely if needed since dormant lateral buds will initiate new growth after pruning.
  • Plants should also be protected from rough treatment during maintenance activities to prevent unnecessary wounds.