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What's bugging you? Feb. 1, 2017

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Fire blight

We happen to live in an area that is quite favorable for growing many kinds of fruit trees. As a result, there are acres upon acres of commercial production of mainly apples and peaches, but also pears, apricots and cherries. And because of this we are often tempted to plant our own fruit trees.

I personally favor apple trees with the caveat that successful culture of apples is challenged by many factors including codling moth (wormy apples) and fire blight. Today I will discuss fire blight and its management.

Fire blight is a bacterial disease that is caused by Erwinia amylovora. This disease is native to much of North America. Since the 1950s it has spread into Europe. In addition to apples, fire blight also infects crabapples, pears, firethorns, hawthorn, cotoneaster, serviceberry and some other species of the rose family.

The disease affects blossoms, fruit, shoots and branches. Severe infections can involve the tree trunk and roots. At this point the tree eventually dies. Fire blight typically enters the blossoms and young developing fruit 1-2 weeks following petal fall. Infected flowers and young fruit can have a watery appearance but eventually they shrivel up and desiccate, turning brown or black.

Symptoms of infection are especially apparent on shoots which die back six to fifteen inches, often developing the characteristic "Shepherd's Crook" when the shoot tip wilts. Leaves turn brown and black. When branches are attacked, infections can appear as sunken lesions, cracks in bark, accompanied by bacterial ooze.

Transmission of the infection can be through bees, other visiting insects, birds, rain and wind. Injured wood is particularly vulnerable. Hail storms can cause this damage. Excessive pruning and nitrogen fertilization can promote excessive growth, resulting in an increased risk of bacterial infection. The disease incidence is highest during hot, humid weather.

There is no cure for the disease, only prevention and removal of infected plant parts. If fire blight was present the previous year, it is advisable to spray your trees and susceptible shrubs with either streptomycin or a copper fungicide beginning at bud break and continuing every 5-7 days until complete petal fall. Sometimes it is wise to remove the infected tree or shrub when it appears to be the only one out of many that is infected. That way you would avoid having a source of infection. Likewise, if there are any fire blight-infected wild apples or other affected plants, you might consider their removal as well.

It is also advised that you prune out the diseased shoots, removing infected wood by cutting 6-12 inches below the margin of infected wood. Disinfect your pruning tools between each cut using alcohol, 10 percent solution of household bleach or by spraying with aerosol Lysol®. Looking toward the future, you could also plant fire blight resistant varieties when available.

Writing this column has got me dreaming of apple blossoms and spring. Unfortunately we do have several weeks of winter left before we can get out in our yards and get our hands dirty again.

Jim Leser retired to Cedaredge in 2007 after a career with Texas A&M University Extension in entomology. He is a member of the Cedaredge Tree Board and a Colorado Master Gardener.

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Jim Leser, What's Bugging You
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