Tree Facts: Tree diseases – fire blight

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BY Shauna Kopren

Northwest Area Conservation District

In this issue of Tree Facts we will be discussing fire blight, a bacterial pathogen that affects trees in the rose family including apples, pears and crabapples as well as serviceberries, flowering quinces, cotoneasters, hawthorns, raspberries, mountain ashes and others. The disease infects the flowers of blooming trees, spreading into the branches and eventually killing the tree.

Fire blight symptoms are first seen when infected blossoms appear water-soaked and will wilt rapidly before turning dark brown. As the infection progresses, leaves will wilt, darken but remain attached giving the tree a fire-scorched appearance, which gives it the name “fire-blight.”

The infected twigs will darken and branch tips may bend over. In wet conditions bacterial ooze may exude from infected tissue and fruits, and rather than dropping from the tree, the infected fruit will gradually dry and remain attached. Fire blight cankers on branches and stems are dark discolored areas that are slightly sunken and have a narrow callus ridge along the outer edge that is important in differentiating fire blight cankers from other fungal cankers.

Unfortunately there is no cure for fire blight, so prevention is the best solution for managing this bacterial disease. Planting persistent varieties, which means picking varieties of trees that are less susceptible to pathogens, is the most effective preventative method.

Other preventative methods include pruning all blighted twigs and cankered branches in order to prevent the spread of the disease. Twigs and branches should be pruned 8 to 12 inches below the edge of the visible infection. It is important to note that surface sterilization of all tools should occur after each cut by dipping the tools in household bleach or ethyl alcohol solutions. After pruning, remove the infected branches from the site and destroy them to decrease the chance of spreading the infection to other trees.

Spreading of the disease is also lowered if pruning is delayed until mid-winter however remove infected branches during the summer if infections are in young, vigorous trees; dwarfing trees on highly sensitive rootstocks; the number of infections in older trees is limited and can be easily removed; or it is a dry sunny day with no chance of rain for 48 hours.

Chemical sprays are also a preventive treatment that must be applied prior to the onset of fire blight symptoms however it is important to remember that sprays will have little effect after symptoms of fire blight begin to appear. There are multiple different types of chemicals that can be applied which include streptomycin, aluminum tris, copper sprays and prohexadione-calcium. All of these chemicals have their own specific instructions on spray timing and technique so consultations should be considered.

Please contact Shauna Kopren with questions on fire blight or other related topics at shaunakoprennrs@gmail.com.

Orchard with burnt appearance due to severe fire blight. Photo courtesy of Washington State University Extension
An apple tree showing symptoms of fire blight. Photo courtesy of Michigan State University