Fire Blight

Hosts

Apple and pear (plus some ornamentals in the Rose family, including quince, crabapple, hawthorn, ornamental pear)

Biology

Fire blight is a bacterial disease caused by Erwinia amylovora.  Susceptibility to infection varies.  For example, Bartlett and Bosc pears and Jonathan, Honeycrisp, Lodi, Rome Beauty, and Transparent apples are all highly susceptible to fire blight.  

The causal bacterium can develop resistance to agricultural antibiotics and complicate control programs; this has happened in Utah County, UT.

The bacteria overwinter within infected twigs and branches in the orchard.  In spring, the bacteria multiply and ooze out of the bark.  They are then spread to open flowers by insects and rain-splash. 

The bacteria colonize the flower stigma and infection only occurs when at least 2 hours of moisture (light rain or dew) to wash the bacteria down into the floral cup.  Infected tissue will be apparent from a few days to a week, depending on temperatures. 

New infections can sometimes occur in summer when bacteria is able to enter leaves or fruit through tiny wounds (caused by hail or insects).

Whether infections are through blossoms or leaves, the bacteria will continue to spread inside the plant tissue, killing flower shoots, twigs, and limbs (depending on tree variety).  Spread slows in hot weather as well as at the end of the season. 

Optimum temperatures for disease development are 70-81°F, with little growth below 50°F or above 95°F. 

Symptoms

Infected blossoms on apple turn brown, and on pears, turn black.  There is often bacterial ooze visible from the pedicel.  Infected terminals (shoot ends) often develop a curled, drooping end, called a “shepherd’s crook”.   The leaves will eventually dry up and hang on to the tree through most of the dormant season. 

Fruit infections on both apple and pear begin with a firm brown rot that quickly includes the whole fruit.  Droplets of ooze may be present on the fruit surface.  Infected fruit gradually shrivel and can remain attached through the winter.

Cankers (slightly sunken areas of dead bark tissue)  develop when the infection progress into woody tissue.  The canker margins may crack as the bark dries out in late summer or fall.  Small droplets of amber ooze are especially evident in spring.  The infected tissue just under the bark will show streaks of reddish brown.  

Monitoring

Be vigilant when weather conditions favor blight development during bloom.  When temperatures rise above 65°F for several days, there is a greater chance of infection when moisture arrives.  The Cougarblight model can predict infections.

In late spring, scout orchards for infected/wilted blossoms.  Continue scouting for infections once per week until the weather turns hot. 

During pruning in winter, look for dead twigs that still have leaves attached.  These are old shoot infections.

Management

An effective management and control program for fire blight should include both cultural and chemical aspects. 

Cultural Management

• Reduce fire blight inoculum by removing other hosts such as pyracantha, hawthorn, cotoneaster, and wild crabapple growing near the orchard. 
• Select moderately resistant cultivars, such as Red Delicious or Early McIntosh, or resistant rootstocks, such as Geneva or M.7.  See page 15 for a list of varieties.
• Prune out limbs with blight during the dormant pruning season and regularly prune out strikes in spring and summer.  Cut 8-12 inches below the reddish color that can be found in the cambial layer beneath the bark.  If you can catch infections early (such as on blossoms or leaves), remove twice the length of visible dead tissue.
• Except during dormant pruning, tools must be disinfected between cuts, or blight may be carried to other branches or trees throughout the orchard.  Use household cleaning wipes that contain bleach.  

Chemical Control

• Apply a copper spray in early spring, just before buds swell.  This treatment will slow the bacterial growth on plant surfaces.  Do not use copper every year, as it can affect soil organisms or wash into groundwater.  Do not apply sprays containing copper to Anjou pears; russet may result.
• Cougarblight is a forecasting model that predicts infection risk.  It is available on the Utah TRAPs website or TRAPs mobile app.  For other states, check with your local county Extension office.  When infections are predicted, apply a suitable antibiotic.  Blossom sprays are effective for 3-5 days because new blossoms open and need protection if conditions continue to favor disease development. 
• Oxytetracycline or Kasumin should be used in areas where streptomycin resistance has been reported, e.g. Utah County in Utah.