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Fire Blight
Fire blight is caused by the bacterium Erwinia amylovora. This bacterium can attack more than 75 species of trees and shrubs including apple, pear, quince, mountain ash, crabapple, hawthorn, cotoneaster, serviceberry, and pyracantha. The bacterium overwinters on infected plants in darkened, slightly sunken cankers. In the spring, the bacterium is dispersed by insects, rain, wind, and animals.
Symptoms- Twigs, branches, and leaders on trees and shrubs wilt and blacken, especially during flowering.
- Affected twigs and branches may bend over into the shape of shepherd's crook.
- Blackened flower parts remain attached to the tree.
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Cream-colored liquid may ooze out of the cankers and run down the trunk and branches in the spring if conditions are very wet.
Life History
The bacterium is carried from infected tissue or from liquid oozing from the infected tissue to natural openings or wounds in susceptible plants by flower-visiting insects, rain, wind, birds, and various crawling insects. The nectaries and other flower parts, hydathodes and stomates on leaves, and small wounds on succulent twigs and branches all can be sites of initial infection. Succulent plant parts are blackened and killed. The bacterium then moves farther into and girdles branches and the trunk. A slightly sunken,
darkened canker forms in the invaded wood. Close examination will reveal a dark line at the edge of the canker. While plants are most susceptible during flowering and new shoot development, fire blight can continue to spread later in the season.
Favorable Conditions
- Fertilization practices that produce very succulent growth render plants more susceptible to fire blight.
- Moderately high temperatures (70 to 81°F = 21 to 27°C), high relative humidity, and rainfall during flowering provide optimum conditions for fire blight development.
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Injury due to hail or windblown soil opens tissue to infection.
Management
- Grow resistant varieties whenever possible. See a list of resistant cultivars elsewhere in this guide under crabapple, cotoneaster, hawthorn, mountain ash, and pyracantha.
- Do not purchase or plant infected material. Plant only fire blight-free trees and shrubs.
- Remove severely infected plants.
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Once the disease has begun, a three-pronged management scheme must be implemented:
- During the dormant season, closely examine susceptible plants and prune out infected tissues. Look for blackened twigs, branches, and flower parts. Find the sunken, darkened cankers on the wood. Prune when the weather is dry, cutting at least 4 inches below the canker. Disinfest pruning tools between cuts by placing them in 70 percent alcohol or hydrogen dioxide and letting them air-dry.
- During the growing season, do not prune infected tissues. Wait for the following dormant season to prune infected tissue.
- To plants for which copper or copper + mancozeb, phosphite salts, fosetyl-Al, or potassium salts of phosphorus acid is registered, apply it before bud break.