Spring 2009

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Fire blight Takes It’s Toll

Many home gardeners have been contacting HGIC about pear and apple trees showing wilt and dieback of shoots, twigs, and small branches. It’s been a tremendous spring in Maryland for the culprit - the devastating bacterial disease known as fire blight (Erwinia amylovora).  This pathogen can kill flowers, fruits, branches, and whole trees.  A wide range of plants are vulnerable including apple (including crabapple), pear (including European, Asian, and ornamental pear like the Bradford pear), serviceberry, cotoneaster, mountain ash, pyracantha, hawthorn, and quince.

Infections start during the bloom period and are promoted by favorable weather conditions (average daily temperature around 60 degrees F. and long period of leaf wetting from dew, fog, or rain).  Infected shoots typically form a “shepherd’s crook” with leaves turning brown rapidly and remaining on the plant.  The dieback is fairly rapid and noticeable.

Now that we are in a period of drier weather it is a good time to prune out infected shoots and branches and continue to prune out blighted shoots as they appear. Make the pruning cut at least 5 inches below the infection symptoms. Pruners, saws, and loppers should be dipped in alcohol or a 10% bleach solution between cuts to disinfect the cutting edges.

"Ugly Stub” Pruning Method

Leave an “ugly stub” when pruning your tree or shrub. That is, do not prune a shoot or branch at the point where it attaches to the next limb, branch, or trunk.  Leave a stub, instead.  The cut ends of the ugly stubs will be colonized with the fire blight bacterium. During cold winter weather (temperature below 40 degrees F.) the ugly stubs are pruned out at the branch collar- the raised ridge where the stub attaches to the tree. This method, developed by the late Dr. Paul Steiner, plant pathologist with the University of Maryland, will help reduce the amount of inoculum available to infect the trees next spring and will help prevent the bacterium from getting into the main branches, trunk, and root system.

Prevention

The key to managing fire blight is selecting resistant cultivars, pruning out infected plant parts, and spraying with liquid copper in spring, just as a little bit of green tissue shows at the ends of buds. If you spray too early it washes off prior to critical infection period.  If you wait too long you may burn plant tissues with the copper.

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