Fire blight is a disease that can cause blight of blossoms and shoots, dieback of branches from cankers and when severe, death of the entire tree. Thankfully several management strategies are available to prevent or eliminate infections before the disease progresses into the main trunk and roots and becomes deadly.
Pathogen and susceptible plants
Fire blight, caused by the bacterium Erwinia amylovora, 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.).
- 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 due to the cool temperatures when trees and shrubs are blooming.
- Young leaves and shoots wilt, turn gray-green, and bend downward forming the characteristic shepherd's crook.
- Leaves and tips of infected shoots turn brown or black. Trees with multiple, infected shoots may appear scorched by fire.
- Brown leaves hang downward and may cling to the blighted twig through the growing season and remain attached to the tree into winter.
- Fruit turns dark and shrivels into a mummy and may cling to the branch for several months.
- 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.
- Cream to light-yellow colored ooze is seen as droplets and found along infected branches, shoots or fruit during humid weather or after a rain.
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 and 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 multiply and sustain themselves on the surface of symptomless leaves, twigs, flowers and immature fruit for a few weeks. They 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 on the flower. 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 progress into the main trunk of the tree and even the roots. At this point the tree will die.
Warm (75 F-82 F), humid (> 60%) or rainy weather in the spring and summer results in repeated disease cycles and increases the severity of the disease. Outbreaks can also occur after damaging events like hail, strong winds, or heavy rain create wounds on susceptible plant parts.
Fruit bearing apples and edible crabapples
For specific management of fire blight in fruit bearing apple trees and edible crabapples consult the publication Pest management for the home apple orchard.
There are no known trees or shrubs in the Rosaceae family that are completely immune to fire blight. Some cultivars are able to defend themselves against the pathogen and limit or slow the spread of disease however. This allows the gardener the opportunity to prune out the infected branches before the infection reaches the main trunk of the tree. Cultivars are often ranked by their ability to resist infection and slow the progression of disease. Since new cultivars are brought to market each year, check with a reputable nursery about the disease resistance characteristics of new cultivars.
Table 1: List of fire blight resistant plants that are hardy to Minnesota
|Most resistant||Somewhat resistant||Less resistant||Least resistant|
|Ornamental Crabapples (Malus)||Adams, Adirondack, Camelot, Lancelot, Tina, Dolgo, David||Candymint, Pink Princess, Red Splendor, Silverdrift||Golden Raindrops, Purple Prince, Red Jade, Spring Snow, SugarTyme||Madonna, Sinai Fire, Snowdrift|
|Cotoneaster (Cotoneaster)||Creeping cotoneaster (C. adpressus) was found to have moderate resistance to fire blight in several studies.|
|Hawthorn (Crataegus)||One European study tested 84 species of Crataegus for resistance to fire blight. The majority were characterized as moderately susceptible. Downy Hawthorn (C. mollis) was one of the most resistant species tested.|
|Mt. ash (Sorbus)||European mountain ash (S. aucuparia) was found to be moderately resistant in some studies but was found to be very susceptible in others. This may be due to genetic variation within the species. More research is needed to learn about the resistance of this species to fire blight.|
Pruning out infection
Twigs and branches infected with fire blight can be pruned out in order to prevent the infection from spreading to the main trunk.
- Prune diseased twigs and branches in late winter when the tree and bacteria are dormant in order to reduce the amount of sucker growth from wounds and eliminate the chance of spreading infections between pruning cuts.
- Make the pruning cut through healthy wood at least 8 inches below the visible symptoms of the fire blight infection.
- If fire blight is seriously damaging a cotoneaster hedge, cut the hedge about six inches above the ground in late winter. If only a few stems are blighted, they can be removed as described above for trees.
- If pruning must be done during the growing season, sterilize pruning tools between each cut by spraying the cutting blade with disinfectant or by dipping the cutting surface in disinfectant for 1 or more minutes. Effective disinfectants include rubbing alcohol, a 10% bleach solution (one part household bleach to nine parts water, equivalent to 1.5 cups bleach to 1 gallon of water) and undiluted Lysol®.
- Burn or bury infected cuttings.
- If the infection reaches the main trunk, the disease cannot be cured and the tree will eventually die. If this happens, it is best to remove the entire tree along with the stump. This will eliminate one source of bacteria for the rest of the susceptible plants in the area.
Other cultural management
Avoid practices that stimulate excessive succulent growth of leaves and shoots such as heavy pruning and excessive nitrogen fertilization. For best fertilization rates submit a soil sample to the soil testing laboratory and follow the recommendations.
Pesticides are typically not necessary to control fire blight in Minnesota and should be used only if fire blight continues to be a yearly problem in your yard. Pesticides with one of the following active ingredients will protect trees from fire blight. Timing of the application can be difficult for landscape ornamental trees however. If blossom blight and early season shoot blight have been a re-occurring problem, products with the active ingredients listed below can be used to protect blossoms. If fire blight has been a problem in the area and a hail storm occurs, the pesticides listed below will help protect the trees from wound infections but only if the product is applied immediately after the storm. Pesticides are completely ineffective against branch cankers.
- Copper-based fungicide
- Streptomycin sulfate antibiotic
Always completely read and follow all instructions on the pesticide label.