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WSU Tree Fruit

Bacterial Diseases

Fire Blight
Fireblight symptoms on pear
Fireblight symptoms on pear
  • Causative agent: Erwinia amylovora
  • Crops affected: Apple & Pear
  • Disease description: Bacteria overwinter in tree cankers from infections that occurred during the previous year.  Bacteria will ooze from cankers a few weeks before bloom, and can be carried by rain or insects to blooms, other parts of the tree and adjacent trees. Warm, wet weather during bloom time promotes the disease.
  • Management: Its important to monitor for cankers and remove them all and destroy debris during the dormant season to keep the disease from spreading. Fire blight has been controlled with antibiotics, but these chemicals are now in the process of being phased out. Research is being conducted on alternative control methods.
  • Resources:
Pseudomonas Bacterial Canker
  • Causative agent: Pseudomonas syringae
  • Crops affected: Cherry
  • Disease description: Bacterial canker is favored by cool, wet weather. The bacteria overwinter in tree cankers, buds and other host tissues. Dark cankered areas on trunks and branches may develop and expand in early spring. The infected tissues may produce gum, although gumming can also be caused by other factors. The cankers often girdle twigs and branches, causing dieback above the lesion. Leaves on girdled twigs often yellow and fall by late summer.
    Infected buds may be killed or leaf infections may occur as the new growth emerges resulting in collapse of leaves. Infection can be spread by wind, rain, insects, pruning tools, or by planting or grafting with infected stock. The disease may spread throughout the entire tree (systemic infection) with or without visible symptoms.
  • Management:
    • The primary focus should be on cultural controls first. Avoid injury to the tree and tissues.
    • Burn (cauterizing) or cut out cankers on branches or trunks. Cauterizing should be done in the spring prior to bloom. Check cauterized areas for continued bacterial activity 15-20 days later.
    • Control weeds, which may be a source of bacteria.
    • Prune out and destroy infected tissues during dry weather. Make cuts well below visible canker and sterilize tools frequently. Do not remove cankers at the same time as regular pruning.
    • Remove severely infected trees.
    • If this is supplemented with a chemical control, make one application in October during the fall prior to fall rains. Then make a second application in early January. Products containing copper may have limited efficacy due to resistance.
  • Resources: