WSU Extension

Hortsense

Common Diseases
 
Armillaria root rot 
Botrytis blight (Gray mold) 
Cankers 
Crown gall 
Damping-off 
Dead roots 
Dodder 
Downy mildew 
Dwarf mistletoe 
Galls 
Leaf spots and blights 
Nectria cankers 
Phytophthora root rot 
Powdery mildew 
Pseudomonas bacterial canker 
Root rots 
Rusts 
Sclerotinia white mold 
Sudden oak death 
Tubercularia canker 
Verticillium wilt 
Viruses 



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Caption: Begonia powdery mildew
Photo by: R.S. Byther
  
Common Diseases : Powdery mildew
(revision date: 4/30/2013)


Biology
White powdery growth on the surface of leaves and sometimes stems is characteristic of this easily diagnosed fungal disease. Young leaves and shoots may be stunted and distorted. Severe infections can cause leaf drop and death of young shoots. Some powdery mildew pathogens attack many different plant species, but others are specific to a limited host range. Unlike most fungal pathogens, powdery mildew fungi do not require surface moisture for infection, so this disease is often most prevalent during dry weather.
Management Options

Non-Chemical Management
  • Avoid overfertilizing, which encourages susceptible new growth. If necessary, switch to a slow-release or lower-nitrogen formula.
  • Gather and destroy all fallen leaves.
  • Pick off infected leaves and prune severely infected shoots to prevent spread of disease.
  • Plant disease-tolerant or disease-resistant varieties (see lists in PNW Plant Disease Management Handbook).
  • Space plantings and prune to provide good air circulation.
  • Watch for signs of infection during appropriate weather conditions (dry weather with warm days and cool nights).
Select non-chemical management options as your first choice!

Chemical Management

Several fungicides are effective in managing powdery mildew. However, the fungicide must be registered for the host plant. Find the list of registered pesticides for a specific host by finding the fact sheet for powdery mildew on that host. Since the fungus is limited to growing on the surface, fungicide applications after disease is visible can be effective in eradicating the problem.

Images

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Caption: Begonia powdery mildew
Photo by: R.S. Byther
Caption: Powdery mildew on leucothoe
Photo by: C.R. Foss
Caption: Powdery mildew on photinia
Photo by: R.S. Byther
Caption: Powdery mildew on leaves
Photo by: R.S. Byther
Caption: Powdery mildew on rhododendron
Photo by: C.R. Foss
Caption: Powdery mildew on rose stems and buds
Photo by: R.S. Byther
Caption: Powdery mildew on rose leaves
Photo by: R.S. Byther