WSU Extension

Hortsense

Rhododendron
 
Disease
Armillaria root rot 
Botrytis shoot blight 
Gray blight 
Leaf spot 
Lime-induced chlorosis 
Marginal leaf necrosis 
Physiological leaf spot 
Phytophthora blight 
Phytophthora root rot 
Powdery mildew 
Ramorum leaf and shoot blight (Sudden oak death) 
Rust 
Salt injury 
Sunburn 
Tissue proliferation 
Insect
Aphids 
Azalea bark scale 
Caterpillars 
Lecanium scale 
Rhododendron lace bug 
Rhododendron whitefly 
Root weevils 



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Caption: Rhododendron fungal leaf spot
Photo by: R.S. Byther
  
Rhododendron : Leaf spot
(revision date: 4/11/2018)


Biology
Several fungi cause leaf spots on rhododendron. They often infect through wounds. Infected leaves display dark brown spots which may cover up to half of the leaf surface. These spots may feel bumpy on the surface due to the presence of fungal structures.
Management Options


Non-Chemical Management
  • Avoid overhead watering and maintain good air circulation in plantings.
  • Prevent injury to reduce likelihood of infection.
  • Provide proper culture for plants.
  • Prune out and destroy all infected tissues.
  • Clean up and destroy any fallen leaves to prevent spread of infection.
Select non-chemical management options as your first choice!

Chemical Management

Read label for application timing. Homeowners should not make foliar applications to trees over 10 ft tall. Consult a commercial pesticide applicator for treatment of trees and shrubs over 10 ft. tall.

Listed below are examples of pesticides that are legal in Washington. Always read and follow all label directions.
  • Bonide Copper Fungicide Spray or Dust RTU [Organic]
    Active ingredient: basic copper sulfate  |  EPA reg no: 4-58
  • Monterey Liqui-Cop Copper Fungicidal Garden Spray
    Active ingredient: copper-ammonia complex  |  EPA reg no: 54705-7
  • Soap-Shield Flowable Liquid Copper Fungicide [Organic]
    Active ingredient: copper octanoate  |  EPA reg no: 67702-2-56872
  • This list may not include all products registered for this use.
Images

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Caption: Rhododendron fungal leaf spot
Photo by: R.S. Byther