Fireblight is a bacterial disease that affects apple, crabapple, mountain ash and pear trees.
Cherry, peach and other related species are NOT affected by fireblight, a sometime serious disease.
Springtime weather that is warm and rainy encourages fireblight. Bees and other pollinators may introduce the disease to blooming trees during pollination. As a result, symptoms often begin in the blossom, which develop brown, mushy-looking petals.
More commonly, trees with fireblight develop curling, bending and blackening shoots, called shepherd's crooking. Leaves turn yellow, then brown and finally black but remain on the branch.
Manage fireblight with a combination of resistant varieties and pruning. Removing diseased parts of the tree is always recommended. Pruning can be done anytime of the year, but it is best during March when trees and diseases are still dormant. Prune fireblighted trees when symptoms develop in the spring and early summer. If pruning during the growing season, when the disease is active, it is extremely important to sanitize pruning tools BETWEEN EACH AND EVERY CUT.
Chemical control of fireblight is an option, but it's not always effective. Copper sulfate can be used before leaves form in early spring. Another chemical, streptomycin sulfate, can be used when blossoms begin to open. Follow label directions carefully.
For more information, see the following Colorado State Extension fact sheet(s).
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Updated Friday, April 19, 2013