Fire blight
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An apple tree with fire blight.

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Fire blight is a bacterial infection of trees and shrubs in the rose family, Rosaceae. Apple, crabapple (including ornamentals) and pear trees, as well as serviceberries, cotoneasters, hawthorns, pyracantha and even blackberry and raspberry bushes can be affected. The infection is very destructive and can kill plants if they’re not treated. The bacteria are spread to blooms by insects during spring. If the bloom period is warm, the disease will spread more easily. Wind and rain also encourage spread of the pathogen.

The first symptoms appear during the bloom. But on a large tree, they’re easy to miss. Infected branches will have blooms that appear water soaked, wilt rapidly, then turn brown. As the disease progresses, the leaves wilt and turn brown but do not fall off. Branch ends may bend, resembling a shepherd’s crook. Affected areas may look scorched, hence the name fire blight.

Cankers (depressions in the bark) may form, and you might see a bacterial ooze. These cankers will be dark and slightly sunken with a narrow callus ridge.

While the disease cannot be cured, some management practices can keep your tree healthy even when the bacteria are present.

If you notice symptoms in summer, prune all infected branches at least 8 to 12 inches below the edge of infection. Be very careful to use clean pruners, and sterilize them between each cut, dipping the blades into bleach or alcohol. Once all infected material is removed, carefully monitor the plant for more symptoms and prune as soon as you see more infection. In autumn, once the leaves fall, recheck the tree for any branches that did not drop their leaves. Remove these branches once the tree is dormant, in late November or December.

In spring, during the bloom, you can spray the trees with an anti-bacterial agent. Contact a licensed arborist in late winter if you wish to have your trees sprayed so the application can be properly timed.

While there is no sure prevention of this disease, if you are adding a flowering or fruit tree to your landscape, choose the varieties that are more resistant to fire blight than others. Good maintenance through pruning can reduce the risk of infection.

For more information check out Colorado State University Extension Fact Sheet 2.907 Fire Blight at extension. garden/02907.pdf.

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