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: Armillaria root rot infecting trunk
Photo by: C.R. Foss
  
Rhododendron : Armillaria root rot
(revision date: 4/23/2014)


Biology
Armillaria root rot is a fungal disease transmitted between plants by root contact. Armillaria is often found in newly cleared soils or soils which have been flooded. Symptoms typically include production of smaller-than-normal leaves, leaf yellowing, leaf drop, and dieback of branches. White thread-like masses of the fungus may be found beneath the bark near the crown of infected trees, and/or as shoestring-like rhizomorphs, which are dark strands of the fungus growing on or just beneath the soil surface. Honey-colored mushrooms often grow near the base of infected trees in the fall. Infected trees may also exhibit a dark black line in the infected area encircling the base of the plant. Armillaria is also known as oak root fungus.
Management Options


Non-Chemical Management
  • Remove infected plants, including roots.
  • Air-dry soil from infected site before replanting.
  • Proper irrigation reduces the likelihood of Armillaria infection.
  • Plant only resistant plants in infected areas. A list may be found in the Sunset Western Garden Book, or contact your county Extension agent or WSU Master Gardeners.
Select non-chemical management options as your first choice!

Chemical Management

None recommended

Images

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Caption: Armillaria root rot infecting trunk
Photo by: C.R. Foss
Caption: Armillaria root rot infecting trunk
Photo by: R.S. Byther
Caption: Armillaria rhizomorphs
Photo by: R.S. Byther