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Fire blight

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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

Fire blight infected blossoms
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.

 

Leaf symptoms

Wilted shoot forming the characteristic hook.
Wilted shoot
  • Young leaves and shoots wilt and turn gray-green. These wilted shoots bend downward forming the characteristic hook.
  • Leaves and infected shoots turn brown or black. Trees with multiple, infected shoots may appear scorched by fire.
  • Brown leaves hang downward. Often, they cling to the blighted twig through the growing season and remain attached to the tree into winter.

Fruit symptoms

  • Fruit turns dark and shrivels into a mummy. This shriveled fruit may cling to the branch for several months.

Bark symptoms

Sapwood under the bark is stained reddish brown below a fire blight canker.
Reddish brown stained sapwood
  • Bark on branch or trunk cankers appear sunken, dark and may be cracked or peeling. If bark is peeled back, brown staining of the sapwood can be seen.
  • Droplets of cream to light-yellow colored ooze is found along infected branches, shoots or fruit during humid weather or after a rain.

Environmental conditions

Fire blight causes the most damage when spring or summer weather is warm, humid or rainy. Temperatures between 75 and 82 F and humidity above 60 percent allow the bacteria to reproduce and spread.

Fire blight outbreaks often happen after events like hail, strong winds or heavy rain. These events can damage the tree and create wounds where the bacteria can enter.

Plants affected by fire blight

Branch killed by fire blight
Branch killed by fire blight

Fire blight is caused by the bacterium Erwinia amylovora. This disease affects over 130 plant species in the Rosaceae family worldwide.

In Minnesota, fire blight is most often seen on apple and crabapple (Malus spp.) and mountain ash (Sorbus spp.). The disease also can occur on raspberry (Rubus spp.), serviceberry (Amelanchier spp.), hawthorn (Crataegus spp.) and cotoneaster (Cotoneaster spp.).

How does fire blight survive and spread?

Fire blight canker on the main trunk of a Mountain Ash tree.
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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Authors: Rebecca Koetter and Michelle Grabowski

Reviewed in 2024

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