Fire blight is a significant disease of various fruit trees. It is one of the most common diseases to afflict fruit trees in spring and summer. Apples, crabapples, and pears are especially prone to infection. Fire blight is caused by the bacterium Erwinia amylovora. The bacterium can infect the branches, twigs, leaves, flowers, and fruit of susceptible plants. Plants that are severely infected generally experience widespread foliar dieback. Infections that progress into the main trunk often result in plant mortality. Under optimal conditions, fire blight can reduce blooms and fruit yield, or eliminate fruit altogether for a single growing season.
Distribution & Habitat
Fire blight occurs globally, wherever susceptible plants are present. Outbreaks are most prevalent in locations where conditions are favorable to infection.
Fire blight infects over 130 plant species of the rose family. It most frequently infects apple, crabapple, firethorn, mountain ash, and quince. Infections also occur on cotoneaster, hawthorn, juneberry, loquat, pyracantha, raspberry, and serviceberry.
The bacteria overwinter in cankers on the branches or trunks of infected trees. In spring, moist conditions encourage the bacteria to proliferate. As the bacterial population increases, a gelatinous ooze is excreted through natural openings in the wood, or cracks in the bark. Insects, particularly pollinators, are attracted to the ooze. As the insects make contact with the ooze, they become laden with the bacteria. The insects subsequently navigate to susceptible flowers, and other sections of the tree, unwittingly transporting the bacteria with them. The bacteria is then transferred from the insect to the tree, where it quickly becomes established. The bacteria multiply for several weeks, before infiltrating the host through natural openings and wounds on the branches, twigs, leaves, flowers, and fruit. Outbreaks may occur when insects carrying the bacteria visit a bevy of trees. The bacteria may also be disseminated to susceptible trees by air currents, or splashes of rain.
Once an infection has been initiated, the bacteria progress in narrow paths through the outer bark. As the infection moves through the bark, the live tissue is killed, and cankers form on the branch. The cankers often girdle the affected branch, causing it to die back. Eventually, the bacteria may advance into the tree’s main trunk, and root system. When this occurs, trees often succumb to infection. Warm, rainy weather in spring and summer stimulates the bacteria, causing repeated disease cycles, and increasing the severity of the disease.
Symptoms of Infections
Initial symptoms of infection appear in early spring, as trees break dormancy, and resume growth. The ooze produced by the bacteria is one of the most conspicuous symptoms. The watery, white to tan liquid can be observed exuding from infected branches, twigs, and trunks. The ooze turns black or brown once it has been exposed to the air.
Infected flowers and flower clusters will appear damp, before drooping and shriveling. As the infection deepens, the flowers become discolored, turning brown or black. Flowers that have succumbed to infection will often remain attached to the tree throughout the growing season. Infections may be localized, affecting only individual flowers or flower clusters. Infected fruit turns brown or black, and becomes desiccated. The fruit may cling to the branch for several months, before being shed.
Young leaves, twigs, and water sprouts wilt, turning grayish-green, then bending downwards to form a characteristic crook. The leaves and tips of infected twigs darken to brown or black. Trees with multiple infected shoots may appear scorched. Infected leaves will often linger on the tree for the remainder of the growing season.
Cankers can be identified by small to large areas of dead bark that have been killed by the bacteria. Most cankers are miniscule, making them difficult to detect. The bark on branch and trunk cankers will brown or blacken. As the wood decays, the bark sinks inwards. Eventually, the bark may crack or peel. If the bark is removed, brown staining of the sapwood may be observed.
- When planting, select cultivars that exhibit an increased resistance to fire blight. Resistant varieties of crabapple include ‘Adams’, ‘Adirondack’, ‘Camelot’, ‘Candymint’, ‘David’, ‘Dolgo’, ‘Lancelot’, ‘Pink Princess’, ‘Red Splendor’, ‘Silverdrift’, and ‘Tina’. Varieties of pear that are less susceptible to fire blight are ‘Bradford’, ‘Capitol’, and ‘Red Spire’. Most cultivars of apple are vulnerable to fire blight, though some are especially prone to infection. These include ‘Fuji’, ‘Gala’, ‘Golden Delicious’, ‘Granny Smith’, ‘Gravenstein’, ‘Jonathan’, ‘Mutsu’, ‘Pink Lady’, and ‘Yellow Newton’.
- Diseased twigs and branches should be promptly removed from infected trees. Prune infected twigs and branches in late winter, when the tree and bacteria are dormant. Dormant pruning will prevent the bacteria from being transmitted to other trees or plant parts. If pruning is required during the growing season, sterilize pruning equipment with a solution comprised of nine parts water and one part bleach. After pruning, safely dispose of the infected trimmings. Trimmings may be buried, or burned. Avoid composting diseased trimmings.
- If the main trunk has been infected, cull the tree and stump to eradicate the bacteria.
- Fungicidal applications may provide limited protection to blooms. Applications must be performed on open blossoms. The number of applications required is determined by the length of the bloom period. Administer applications at two to three week intervals until the blooming period has ceased.
- Vigorous plants are favored by the bacterium. As such, avoid excess nitrogen fertilization of plants during the growing season.
- Limit the irrigation of susceptible plants while they are in bloom. This will reduce the incidence of infection.
Photo courtesy of Michigan State University.