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Fire blight is a bacterial disease that is especially destructive on apple, pear, quince and crabapple.  The disease also can occur on hawthorne, mountain ash, serviceberry, pyracantha, cottoneaster and other species in the rose family.

Disease Cycle

Fire blight is caused by the bacterium Erwinia amylovora.  The bacteria overwinter in blighted branches and at the edge of cankers (areas of bark killed by bacteria).  In spring, when temperatures reach 65 degrees F and rainfall occurs, bacteria resume activity and multiply rapidly.  They are forced through cracks and bark pores to the exudate called bacterial ooze.  Insects such as bees, ants, flies, aphids, and beetles are attracted to this ooze, pick up the bacteria on their bodies, and inadvertently carry the bacteria to opening to opening blossoms.  Bacterial ooze splashed by rain also may spread the pathogen.

Once in the blossom, bacteria multiply rapidly in the nectar and eventually enter the flower tissue.  From the flower, the bacteria move into the branch.  All flowers, leaves, and fruit above the point of entry die.

Young branch tips can be infected through stomata (air openings on the leaves), lenticels (air opening on branch), or more commonly through wounds created by pruning, insects or hail storms.  Droplets of ooze can form on these infected twigs within three days.  Fruit may be infected through insect wounds.  Cankers eventually develop from branch or blossom infections.

Diagnosis

Symptoms of blossom blight are first seen about the time of petal fall.  Infected blossoms appear water-soaked and wilt rapidly before turning dark brown.  Leaves wilt, darken and remain attach to the tree giving the tree a fire-scorched appearance, thus the name fire blight.  Infected twigs exude a white creamy ooze containing bacteria in droplets or fine, hair-like strands.  Tips of twigs turn black and curl over when dry, causing the "shepherd's crook" appearance.  Infected fruits also exude a white ooze and eventually dry and remain attached to the branch.

Cankers appear as dark, slightly sunken places on branches or the main stem in July or August.  The inner bark turns from green to brown, but the appearance may vary with certain varieties.  Droplets of bacterial ooze may appear on the canker.

Varietal Susceptibility

Different varieties of pear, apple, and crabapple have different degrees of susceptibility.  No variety is immune when conditions are favorable and the pathogen is abundant.  The amount of disease found in a variety from year to year also is affected by local weather conditions.

Control

There is no cure for this disease so prevention is the best solution for al growers.  Fire blight control methods include use of resistant varieties, cultural practices, pruning, spraying, and sanitation measures.

Variety Selection

Care should be taken to select cultivars that are less susceptible to fire blight.  Varieties also should be selected for adaptability to the growing area to prevent stresses that may predispose the tree to other disease causing agents.

Cultural Practices

The structure and mineral content of your soil are important in the management of fire blight.  Trees planted in poorly drained soil are more susceptible.  Since fire blight development is favored by young succulent tissue, heavy application of nitrogen fertilizers should be avoided.  Only add enough nitrogen to achieve 12 to 18 inches (30 to 46 cm) of branch growth each year.

Pruning

Remove newly infected young twigs as soon as possible in the spring.  This will reduce infection on the tree and the amount of bacteria available to infect healthy blossoms and shoots.  In young twigs, cuts should be made at least 12 inches (30.5 cm) below the visible edge of infection.

On dry days during the winter, all blighted twigs, branches, and cankers should be removed.  Limbs should be pruned about 4 to 6 inches (10 cm) below the visible infection.

To remove a canker on large stems, first make a cut through the bark down to the wood 4 inches (8 cm) outside the canker margin.  The cut should not have any sharp angles.  Next, cut and scrape away all infected bark down to the wood.  Exposed wounds should be treated with a disinfectant such as household bleach (1 part bleach to 4 parts water) or a 70 percent alcohol solution.

CAUTION!!! All tools used in pruning must be surface sterilized after each cut.  Dipping tools in household bleach or ethyl alcohol, or using household spray disinfectants should be sufficient.

Prevention and Sanitation

During pruning procedures, care should be taken to avoid unnecessary wounds to the tree.  Soft-sole shoes should be worn to prevent bark injuries when climbing trees.  All infected branches should be promptly removed from the site and destroyed to decrease the chance of new infections.

Spraying

Sprays should be used as preventative treatments since they have little effect on existing infection.  Blossom infections may be expected if temperatures remain between 65 degrees F and 86 degrees F for a day or more during flower bloom and there is at least a trace of rainfall or the relative humidity remains above 65 percent for 24 hours.  If these weather conditions occur, or are anticipated, and/or occur with abundant shoot growth or there are fruit with injuries, streptomycin or copper sprays should be applied.  Streptomycin sprays are available under the brand names of Agri-Strep and Agri-Mycin 17.  Several forms of copper (copper sulfate with lime, copper hydroxide) suitable for fire blight control are available.  Copper sprays are best used during dormancy and bud break since they may cause damage to leaves and fruit.  Spraying should be done in the cooler hours of the day to allow for better absorption and less evaporation.  Sprays should not be applied within 50 days of apple harvest or within 30 days of pear harvest.  Always consult your local Cooperative Extension agent or district forester for updated spray rates and related information.

Orchard Managers

Sprays should be applied at 50 percent bloom, full bloom, three days after full bloom, and at 50 percent petal fall.  During full bloom, streptomycin sprays are most effective at a concentration of 75 to 100 parts per million (ppm).

After petal fall and during continued periods of high humidity, sprays of streptomycin should be made at seven-day intervals to prevent fire blight from occurring on new shoots and fruit.

Concentrated spraying can be effective if at least 24 ounces (680 g) of streptomycin are used per acre (4,047 square meters).  When severe blight conditions exist, irrigation water should be limited to stop terminal growth and promote hardening.

Homeowners

For a homeowner with only a few trees, a simpler spraying approach can be used.  Streptomycin should be applied starting at 50 percent bloom.  This procedure should be repeated  twice at four- to five-day intervals.  If the bloom period is long and high humidity persists, additional applications may be necessary.  Always follow label directions for any pesticide.

Courtesy of the Colorado State University Cooperative Extension (8/88)

    by J.E. Boyd and W.R. Jacobi