Caused by bacteria, fire blight is a common and frequently destructive disease. It primarily affects pear, but apple and crabapple are also susceptible. Fire blight is less common on hawthorn, spiraea, ash, and other related plants. The disease can destroy limbs or entire shrubs or trees.
Branch and trunk canker symptoms can appear as soon as the tree begins actively growing. The first evidence is a watery, light-tan bacterial ooze that exudes from cankers on branches, twigs, or trunks. The ooze turns dark after exposure to air, leaving streaks on branches or trunks. However, most cankers are small and inconspicuous, therefore infections might not be noticed until later in the spring when flowers, shoots, and/or young fruit shrivel and blackens. Fire blight might be localized, affecting only a flower, twig, or branch/shoot, forming a crook at the end of each infected shoot.
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. This flecking represents new infections the bacteria cause as they invade healthy wood. As the canker expands, the infected wood dies; areas of dead tissue become sunken, and cracks often develop in the bark at the edges of the canker. The pathogen tends to move in trees from the infection site toward the roots.
Conditions Favoring Disease Development: Rainy or humid weather with daytime temperatures from 75° to 85°F with night temperatures above 55°F. Fire blight affect vigorously growing shoots most severely, therefore conditions such as high soil fertility and abundant soil moisture, which favor rapid shoot growth, increases the severity of damage. In general, young trees are more susceptible than older trees.
Avoid promoting vigorous growth (e.g excess nitrogen fertilization and heavy pruning). Remove and destroy fire blight infected areas in summer or winter. If the infection is rapidly advancing and pruning is necessary in the spring, dip shears in 10% bleach between cuts. Remove all discolored tissue plus 6 to 8 inches more beyond the infection. Remove and destroy the pruned branches, shoots, etc. Spraying is an option although often they don’t provide adequate control even with multiple treatments. Treatments should be made to open blossoms when the average temperature exceeds 60°F. Apply at four to five day intervals during periods of high humidity and until late bloom is over.