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.
Fig 1 – Blossom blight phase of fire blight in which bacteria infect blossoms during bloom.
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.
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.
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).pdf
Disease and Insect Control Programs for Homegrown Fruit in Kentucky http://www.ca.uky.edu/agc/pubs/id/id21/id21.pdf