Fire blight is a disease caused by the bacterium Erwinia amylovora, posing a threat to many species within the Rosaceae family. This bacterial infection primarily attacks pome fruits, such as apples, pears, and quinces, along with ornamental trees and shrubs like hawthorns and cotoneasters. The pathogen spreads rapidly through blossoms, rain splash, insects, and contaminated tools, making it one of the most destructive diseases in orchards and home gardens. Once established, fire blight moves quickly from tender new growth into older, woody tissue, potentially killing an entire tree. The bacteria survive the winter within sunken cankers on branches, which serve as an inoculum source for the following spring.
Identifying Fire Blight Symptoms
Confirming fire blight requires looking for symptoms, particularly during or shortly after the spring bloom period, when warm, humid weather favors the infection. The most recognizable symptom is the “shepherd’s crook,” where the tips of young, infected shoots wilt, turn brown or black, and curl over in a characteristic hook shape. These blighted leaves and shoots shrivel and turn dark, giving the plant a scorched appearance, which gives the disease its common name.
A direct sign of the bacteria is the presence of bacterial ooze, which appears as small, watery droplets ranging in color from milky white to amber. This sticky exudate forms on infected limbs, shoots, or fruit, especially under humid conditions. The ooze contains millions of bacteria, which are then spread by rain, wind, and pollinators, initiating new infections. Unlike many fungal blights, the dead leaves often remain attached to the branch throughout the season.
Immediate Removal Through Pruning and Sanitation
The primary action for controlling an active fire blight infection is the physical removal of diseased plant material to prevent pathogen spread. Pruning must be aggressive and precise, making the cut into healthy wood well below any visible signs of infection. Experts recommend cutting 8 to 12 inches below the lowest point of discoloration or canker formation on the branch. This is necessary because the bacteria can be present in the vascular tissue far ahead of the visible symptoms on the bark surface.
Sterilizing pruning tools between every single cut is required to avoid transferring the bacteria to healthy tissue. Effective disinfectants include a solution of 70% rubbing alcohol or a 10% solution of household bleach; quaternary ammonium disinfectants are also available and less corrosive. The tool blades must be thoroughly wiped or dipped in the solution for at least 30 seconds to ensure the bacteria are killed. All removed material, including the infected wood, should be collected and destroyed immediately, preferably by burning or sealing it in plastic bags for disposal, and never added to a compost pile.
Applying Chemical and Biological Control Methods
Chemical treatments for fire blight are preventative, focusing on protecting vulnerable blossoms from infection rather than curing established cankers. Application is precisely timed to the bloom period, when flowers are open and temperatures favor bacterial growth. Copper-based products, such as Bordeaux mixture or fixed copper formulations, are used during the dormant season and again in early spring to reduce the number of bacteria overwintering on the bark surface.
For active control during bloom, specific antibiotics like streptomycin are used, though their use is often restricted due to concerns about antibiotic resistance. Commercial growers manage this risk by limiting applications and rotating them with alternative products. Newer biological control options utilize beneficial microorganisms, such as certain Bacillus species, which competitively colonize the flower surface. These bacteria physically occupy the space and consume the resources that Erwinia amylovora needs to establish an infection, offering an alternative for disease suppression.
Managing Environment and Host Resistance
Selecting resistant varieties of apples, pears, and ornamentals lowers the likelihood of future outbreaks. While no variety is completely immune, resistant cultivars have mechanisms that limit the spread of the bacteria within the plant tissue.
Controlling vegetative growth is a useful strategy because the bacteria easily infect soft, succulent new shoots. Gardeners should avoid excessive nitrogen fertilization, which encourages the production of this susceptible new growth, especially in the spring. Managing irrigation to avoid wetting the foliage and ensuring good air circulation through proper pruning helps reduce the warm, humid conditions that allow the Erwinia amylovora bacteria to multiply and spread.