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: P. ramorum shoot blight symptoms on rhododendron
Photo by: M. Elliott
  
Rhododendron : Ramorum leaf and shoot blight (Sudden oak death)
(revision date: 7/22/2015)


Biology
Ramorum leaf blight is caused by Phytophthora ramorum, the same organism that causes sudden oak death (SOD). In Washington, most cases have been found in nurseries on rhododendrons and camellias, which are highly susceptible to this disease. Some oak species, viburnum, kalmia, and Pieris are also considered highly susceptible, along with native plants such as salal and evergreen huckleberry. On rhododendron, any part of the leaf may be affected; however, symptoms typically appear where water collects. Damage consists of discolored lesions at the tip or near the petiole, often spreading along the midrib. Lesions may move into the shoot, causing shoot dieback. In extreme cases, cankers may develop on branches, but wilting and death of the entire plant is NOT A SYMPTOM of infection by P. ramorum. Cold damage, sunburn, chemical injury, or other fungal diseases may cause similar symptoms; however, lesions caused by abiotic factors such as sunburn tend to have very distinct margins and are often not associated with the leaf petiole or midrib. P. ramorum has a wide and varied host range, so rhododendron is seldom the only species affected. Phytophthora blight is another disease of rhododendrons which may cause symptoms that resemble those of Ramorum leaf and shoot blight, but it is caused by different species of Phytophthora. For more information, see also the Hortsense fact sheet on Rhododendron: Phytophthora blight. IT IS NOT POSSIBLE TO CONCLUSIVELY DIAGNOSE THIS PROBLEM BASED ON VISIBLE SYMPTOMS ONLY. In Washington, Ramorum leaf and shoot blight has so far been found only in association with infected nursery plants and water runoff from infected nursery sites. While this disease has the potential to become a serious and economically significant problem, it is not yet considered to be widespread in Washington in either cultivated or native landscapes. P. ramorum infection is NOT LIKELY in the landscape UNLESS the plant (1) is a highly susceptible species AND (2) was purchased since 2002 AND (3) is showing symptoms associated with P. ramorum infection (or is located near another plant that meets all these criteria). Currently, the only way to confirm a SOD diagnosis is with laboratory tests.
Management Options


Non-Chemical Management
  • If infection with P. ramorum is CONFIRMED by a laboratory test, remove and destroy the infected plant material where practical.
  • Avoid overhead watering.
  • Good sanitation is important. Clean up and dispose of fallen diseased leaves and remove blighted and cankered twigs and branches.
  • Quarantine new plants for 4-6 weeks before adding them to the landscape. This disease can be spread by plant debris, contaminated soil, and contaminated irrigation water or run-off, so keep quarantined plants isolated from healthy plants in the landscape and watch closely for any symptoms of disease.
  • Do not buy any plants that have been lying in standing water or that have disease symptoms including leaf blight or lesions, severe leaf loss, or shoot dieback.
  • Purchase plants from reputable nurseries only. Carefully inspect all plants before purchase, especially those considered highly susceptible to P. ramorum.
Select non-chemical management options as your first choice!

Chemical Management

None recommended

Images

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Caption: P. ramorum shoot blight symptoms on rhododendron
Photo by: M. Elliott
Caption: P. ramorum symptoms on rhodendron leaves
Photo by: M. Elliott