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Fire blight

Quick facts

  • In Minnesota, fire blight is most often seen on apple, crabapple and mountain ash trees.
  • Fire blight is a disease that can kill blossoms and shoots and cause dieback of branches from cankers. 
  • Severe fire blight can cause trees to die. 
  • Young leaves and shoots wilt and bend downward forming the shape of a hook.
  • Trees with multiple infected shoots may appear scorched by fire.
  • Prevent infections by planting resistant varieties.
  • To manage fire blight, prune trees to remove infected branches before the disease kills the tree. 

How to identify fire blight

Flower symptoms

  • Infected blossoms
    Flowers and flower clusters appear water-soaked, then droop and shrivel, turning brown or black. 
  • Brown to black flowers remain attached through the growing season. 
  • Flower infections in Minnesota are rare because of the cool temperatures when trees and shrubs are blooming.
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How does fire blight survive and spread?

  • Fire blight canker
    The fire blight bacteria survive winter in cankers on the trunk or branches of infected trees.
  • In spring, during warm, wet weather, bacteria begin to multiply. The bacteria seep out of the canker through natural openings or cracks as a sweet, sticky liquid known as bacterial ooze.
  • Insects such as pollinators are attracted to this sweetness and carry the bacteria to wounds or flowers.
  • Bacteria can also be moved by splashing water from rain or irrigation, or on the hands and tools of gardeners.
  • The fire blight bacteria will live and multiply  on the surface of leaves, twigs, flowers and immature fruit for a few weeks without causing symptoms.
  • Bacteria need a wound or a natural opening to infect the tree.
    • Many infections start when bacteria growing on flowers reach a certain population and enter the flower through natural openings.
    • Young shoots are infected through small wounds caused by insect feeding, wind whipping of branches, blowing sand or other damage.
  • Once the bacteria have infected either blossoms or shoots, the bacteria can move into the branch through the vascular system of the tree. New branch cankers are formed by bacteria moving into branches in this way.
  • In highly susceptible cultivars, the bacteria can move into the main trunk of the tree and even the roots. At this point, the tree will die.

How to manage fire blight

Plant resistant tree varieties

There are no known trees or shrubs in the Rosaceae family that are completely immune to fire blight. Some varieties can defend themselves by limiting or slowing the spread of the disease. This gives the gardener time to prune out the infected branches before the infection reaches the main trunk of the tree.

Varieties are often ranked by their ability to resist infection and slow the progression of disease. Since new varieties are brought to market each year, check with a reputable nursery about the disease resistance characteristics of new cultivars.

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Rebecca Koetter and Michelle Grabowski, Extension educator

Reviewed in 2019

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