Fire blight is a serious, contagious bacterial disease caused by Erwinia amylovora. It poses a major threat to fruit orchards and ornamental plants, rapidly destroying susceptible trees in a single growing season. The pathogen primarily affects trees and shrubs in the Rosaceae family. Since the bacteria survive the winter in infected plant tissue, fire blight can spread quickly once the growing season begins.
Recognizing Fire Blight and Susceptible Trees
Look for visual symptoms that suggest the plant parts have been scorched by fire, which gives the disease its name. Infected blossoms and leaves wilt quickly, turning dark brown or black, often remaining attached. A distinctive sign is the “shepherd’s crook,” where the tip of an infected young shoot wilts and bends into a hook shape.
As the infection moves into older wood, it forms cankers—areas of dead, sunken bark on branches or the trunk. A clear, milky, or amber-colored sticky liquid, known as bacterial ooze, may exude from these cankers or infected shoots, particularly in warm, humid weather.
The most susceptible trees belong to the pome fruit group, including apples, pears, and quince. Related ornamental plants are also vulnerable, such as hawthorn, cotoneaster, pyracantha, and serviceberry.
How Fire Blight Bacteria Moves Between Trees
The bacteria can be transported from an infected tree to a healthy one through several mechanisms. Spread typically begins in the spring when overwintering cankers exude bacterial ooze containing millions of pathogens. This ooze attracts insects, especially pollinators like bees, which carry the bacteria on their bodies from diseased flowers to new, open blossoms on healthy trees.
Rain and wind splash also move the bacterial ooze from cankers or infected tissue onto other parts of the same plant or to nearby trees. The bacteria enter new plants through natural openings like flowers and stomata, or through wounds caused by hail, wind, or insects.
Human activity provides a direct pathway for transmission, primarily through unsanitized tools. Pruning shears, saws, or grafting equipment used on infected tissue can easily transfer the bacteria to a healthy branch or tree if they are not sterilized between cuts.
Stopping the Spread Through Sanitation and Management
Effective control relies on prompt action, primarily the physical removal of infected wood. When pruning, make the cut significantly below the visible symptoms to ensure all systemic bacteria are removed. A best practice is to cut into healthy wood 12 to 18 inches below the lowest sign of infection.
Proper sanitation of tools prevents human-assisted spread. Pruning tools should be sterilized between each cut by dipping them in a solution of 10% bleach (one part bleach to nine parts water) or rubbing alcohol. All removed infected material must be destroyed immediately, either by burning or deep burial, and should never be left on the ground or composted where the bacteria can persist.
Chemical controls are mainly preventative, not curative for established infections. Copper-based products and antibiotics, such as streptomycin, are applied during the bloom period to prevent bacteria from entering the open flowers, which are the most common entry points. These treatments are most effective when timed according to weather conditions that favor the disease, such as warm temperatures and high humidity. Reducing tree vigor through moderate nitrogen fertilization also helps, as succulent new growth is highly susceptible to infection.