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: Lecanium scales
Photo by: R.S. Byther
  
Rhododendron : Lecanium scale
(revision date: 3/10/2017)


Biology
Lecanium scale adults are found on the twigs in late spring. At this stage they are usually brown or mottled brown and white bumps on the stems. During the summer and fall, scale nymphs can be found on the foliage and are flat and light brown. The partially developed nymphs overwinter in crevices in the bark.
Management Options


Non-Chemical Management
  • Prune to remove heavily infested branches.
  • Hand removal of adult scales (when practical) can reduce populations to tolerable levels.
  • Keep dust down and plant flowering plants to attract natural predators such as ladybird beetles.
  • Provide proper nutrition for plants. High levels of nitrogen in the foliage may encourage scale reproduction. If necessary, switch to a slow-release formula or one with less nitrogen.
Select non-chemical management options as your first choice!

Chemical Management

Scout for crawlers. Insecticide applications can be effective when applied to control the immature crawlers stage, usually in late spring to early summer. 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 Systemic Insect Control
    Active ingredient: acephate  |  EPA reg no: 239-2461-4
  • ferti-lome Horticultural Oil Spray RTSpray
    Active ingredient: oil/pet distillate  |  EPA reg no: 48813-1-7401
  • Safer Brand Garden Defense Multi-Purpose Spray Conc [Organic]
    Active ingredient: clarified hydrophobic extract of neem oil  |  EPA reg no: 70051-2-42697
  • Safer Brand Insect Killing Soap Conc II [Organic]
    Active ingredient: potassium laurate  |  EPA reg no: 42697-60
  • This list may not include all products registered for this use.
Images

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Caption: Lecanium scales
Photo by: R.S. Byther