As apple flowering-season
approaches, growers should begin thinking about management of fire blight. This bacterial disease can cause severe
damage on apples, pears, and related ornamental plants during warm, rainy
spring weather.
There is no single
method that will provide consistent and reliable control. Management of fire
blight requires an integrated approach that relies primarily on cultural practices
and is supported by the judicious use of bactericides.
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Fig 1 – Blossom blight phase of fire blight in which bacteria infect
blossoms during bloom.
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Disease Development: The fire blight bacterium overwinters primarily in cankered or diseased branches and trunks. During spring, bacteria-laden ooze is exuded from canker margins. Splashing rain and insects carry the pathogen to blossoms (Fig 1), and bees further spread the pathogen as they pollinate.
If weather is warm and
rainy, populations of the causal bacterium (Erwinia amylovora) double
every few hours, and more than a million bacterial cells can colonize a single
floral stigma. Rain or dew then washes the bacteria into openings at the base
of blossoms. Resulting symptoms are called blossom blight. Infections can spread from blossoms to
supporting spurs and branches, causing cankers that eventually kill entire
branches (Fig 2).
Even if there is no
blossom infection, shoot infections may occur. Bacterial cells infect
externally through shoot tips, as young, succulent tissue is susceptible during
periods of rapid growth. This phase of fire blight is called shoot blight or
shepherd’s crook.
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Fig 2 – Flower and shoot infections can spread to branches, causing
cankers that eventually kill entire limbs.
The fire blight bacterium overwinters in cankers and dead wood.
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Bactericides: During bud swell (late dormancy), an application of copper fungicide (e.g. Kocide or other fixed copper) should be applied, especially if fire blight was severe last year. This copper application should reduce amounts of bacterium present on the surfaces of branches and spurs, reducing risk for disease development. Do not apply copper after ¼ inch green leaf stage, as can be phytotoxic (cause foliar burn).
During bloom, beginning at the first sign of open blossoms, a bactericide
such as streptomycin (e.g. Agri-strep) should be applied at 4- to 5-day
intervals through petal fall. A minimum
of two applications is recommended. Another type of bactericide, oxytetracycline
(e.g. Mycoshield) may be substituted, but it is not as effective as
streptomycin. Oxytetracycline may be
mixed with streptomycin bactericides to help reduce the risk for resistance
development. Disease risk assessment
sites (see below) may be used to improve timing and efficacy of bactericide
applications. Note: Home orchards are usually not sprayed with
antibiotics, so the preventative copper spray is critical.
After bloom, certain weather conditions can increase risk for shoot
infections. This shoot blight phase can
be severe during rapid shoot development, especially under warm, rainy
conditions. The growth regulator
prohexadione calcium (e.g. Apogee) reduces terminal growth, reducing succulent
tissue that is most susceptible to infection.
Pruning: Growers should remove all damaged, dead, or
diseased wood from trees during dormancy, before bacteria become active this
spring. This will help eliminate large
amounts of infective inoculum.
Disease Risk Assessment &
Weather Models: Plant disease
prediction models utilize weather data to analyze disease risk. The University
of Kentucky maintains weather stations and incorporates this data into disease
risk predictions models. Models can be found at http://wwwagwx.ca.uky.edu/plant_disease.html
More information: See also our newest fact
sheet Fire Blight http://www.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/PPFS-FR-T-12.pdf
Midwest Tree Fruit Spray Guide http://www.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/MW_tree_fruit_spray_guide_ID-92_(2013)[1].pdf
Disease and Insect Control Programs for Homegrown Fruit in
Kentucky http://www.ca.uky.edu/agc/pubs/id/id21/id21.pdf
