Fire blight

Plants affected 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 can also occur on raspberry (Rubus spp.), serviceberry (Amelanchier spp.), hawthorn (Crataegus spp.), and cotoneaster (Cotoneaster spp.).

How to identify fire blight

Flower symptoms

• 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 less common than in other production regions because of the cool temperatures when trees and shrubs are blooming.

Leaf symptoms

• 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

• Bark on branch or trunk cankers appear sunken, dark, and may be cracked or peeling.
• If the bark is peeled back, brown staining of the sapwood can be seen.
• Droplets of cream to light-yellow ooze are found along infected branches, shoots, or fruit during humid weather or after rain.

How fire blight survives and spreads

Because fire blight is caused by a bacterium, it enters the plant through natural openings like the stigma (top of female flower part), stomata, or wounds caused by hail, for example.

Fire blight causes the most damage when spring or summer weather is warm, humid, or rainy. Warm weather, above 65 F, combined with high humidity or a wetting event allows the bacteria to reproduce and spread.

Bacteria reproduction and spread

• 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, through 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.

Entry and spread of infections

• 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.