Fire Blight

Fire blight is a bacterial infection caused by Erwinia amylovota that affects members of the Rosaceae family, namely pear and apple trees. This infection is serious and can cause substantial damage or tree death. In our region last year there were numerous cases of fire blight affecting ornamental pear trees, and we are beginning to see a resurgence of the infection this year.


The first sign of fire blight usually occurs in spring during bloom. Infected blossoms will appear water-soaked and eventually turn brown or black. As the infection spreads, the new leaves and shoots will wilt, turn black, and show the characteristic shepherd’s crook (fig.1). In severe infections bacteria will spread to old growth causing cankers (fig. 2) and killing whole branches or the entire tree. Cankers appear as dark sunken areas in the bark and droplets of bacterial ooze may appear. The infected parts of the tree will appear scorched by fire, which is where the name fire blight came from.

Shepherds Crook Fire Blight
Fig 1. Shepherd’s crook in an ornamental pear, typical of fire blight infection. Note the backwards bend in the shoot. (Click on pictures for full size photos)
Fire Blight Canker
Fig 2. Cankers from fire blight infection as indicated by red squares. Cankers appear as off color sunken areas. (The orange spots on the leaves in the background is a separate fungal infection)
Fireblight in crabapple
Fig 3. Crabapple affected with fire blight.

Life cycle

Fire blight bacteria will overwinter in cankers of the tree, and begin to spread in the spring. The cankers will ooze bacteria, which is spread by splashing water (rain or sprinklers) and also by insects. Bacteria will eventually make its way to the blossoms where it will begin to infect the branches. The infection could also be spread to other openings in the plant such as pruning wounds, insect wounds, hail damage, and other natural openings. Fire blight is spread when temperatures are between 65°F and 85°F during high humidity or rain.

Control of Fire Blight

The best practice to control fire blight is prevention. Planting resistant varieties is the best way to ward off the disease. While no variety is completely immune to fire blight, the most susceptible varieties should be avoided. Wellnitz Tree Care can help you choose a disease resistant tree if you are thinking about a new planting. If you already have a tree that is susceptible, use cultural practices that will decrease the amount of new shoot growth. New shoot growth is most susceptible to infection, therefore practices such as fertilization or heavy pruning should be avoided. Wellnitz Tree Care can also do Cambistat injections on ornamental pear trees to reduce new shoot growth for up to three years. Cambistat will also maintain the shape and appearance of a pruned tree for up to 3 years.

Trees should have infected wood pruned from them, but pruning tools must be sanitized between each cut, so as not to spread the infection. Pruning cuts should be made at least 8 inches below the infected sections. Wellnitz Tree Care can chemically treat infected trees after pruning out infected wood, however it is not a cure for fire blight, only a suppressant and should be applied before infection sets in. The spray must be applied in spring when the tree is in full bloom to prevent the infection of blossoms. In cases where the fire blight infection is severe the most cost efficient treatment may be removal, however a wait and see approach can be taken if there is no risk of infecting nearby trees.

If you think a tree of yours may have fire blight and you are looking for options please contact us. We would love to help you choose the best option for you and your tree(s).


For more information check out these sites:

K-State Extension

Colorado State University Extension

Cornell University

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