Splashing rain or insects transmit the bacteria to nearby blossoms or succulent growing shoots. In spring when the weather is sufficiently warm and moist and trees resume growth, a small percentage of the cankers become active as bacteria multiply and ooze from branch or twig surfaces in a light tan liquid. LIFE CYCLEįire blight bacteria overwinter in cankers on twigs, branches, or trunks of host trees.
In fall, leaves on infected pear shoots often turn red and then black. The pathogen tends to move in trees from the infection site toward the roots. As the canker expands, the infected wood dies, turns brown, and dries out areas of dead tissue become sunken, and cracks often develop in the bark at the edges of the canker. This flecking represents new infections the bacteria cause as they invade healthy wood. If the bark is cut away from the edge of an active canker, reddish flecking can be seen in the wood adjacent to the canker margin. The bacteria also spread into the wood surrounding overwintered cankers that have become active in spring. When the pathogen spreads from blossoms into wood, the newly infected wood underneath the bark has pink to orange-red streaks (Figure 5). Once infected, the plant will harbor the pathogen indefinitely. Less susceptible varieties might be severely disfigured. Dead, blackened leaves and fruit cling to branches throughout the season, giving the tree a scorched appearance, hence the name “fire blight.” Infections can extend into scaffold limbs, trunks, or root systems and can kill highly susceptible hosts. Succulent tissues of shoots and water sprouts (root suckers) also are subject to infection. Fire blight infections might be localized, affecting only the flowers or flower clusters, or they might extend into the twigs and branches, causing small shoots to wilt (Figure 4) and form a crook at the end of each infected shoot. Infected flowers and flower stems wilt and turn black on pear trees and brown on apple trees. Open flowers are the most common infection sites (Figure 3) and remain susceptible until petal fall. The amount of fruit loss depends upon the extent and severity of the disease. However, most cankers are small and inconspicuous thus infections might not be noticed until later in spring when flowers, shoots, and/or young fruit shrivel and blacken. The ooze turns dark after exposure to air, leaving streaks on branches or trunks. The first sign is a watery, light tan bacterial ooze that exudes from cankers (small to large areas of dead bark that the pathogen killed during previous seasons) on branches, twigs, or trunks. In spring, branch and trunk canker symptoms can appear as soon as trees begin active growth. The disease can destroy limbs and even entire shrubs or trees.
Fire blight is less common on hawthorn ( Crataegus species), Spiraea, Cotoneaster, toyon ( Photinia species), juneberry or serviceberry ( Amelanchier species), loquat ( Eriobotria), mountain ash ( Sorbus species), and other related plants. Apple, crabapple ( Malus species), and firethorns ( Pyracantha species) also are frequently damaged. Pear ( Pyrus species) and quince ( Cydonia) are extremely susceptible. Red-streaked wood underneath the bark in a fire blight canker.įire blight, caused by the bacterium Erwinia amylovora, is a common and frequently destructive disease of pome fruit trees and related plants.
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