Fire blight, caused by the bacterium Erwinia amylovora, is a serious disease of many ornamental species in the Rosaceae family. Certain varieties of apple, flowering crabapple, pear, pyracantha, and quince are highly susceptible. Other plants, such as hawthorn, Japanese quince, mountain ash, rose, cotoneaster, spirea, loquat, and amelanchier, may occasionally also be damaged.
Blossoms, young fruits, small twigs, leaves and water sprouts are susceptible to infection. Blossoms and leaves wilt suddenly, turn dark brown to black, shrivel, and die, but usually remain attached to the plant. Leaves are often infected through the petiole and, on some plants, blackening of the petiole and adjacent tissue can be seen before the whole leaf dies (Fig. 1). Infected fruit becomes leathery in appearance. Cankers on limbs are characteristically shrunken and dark brown to purplish in color. Dark streaking in the wood often extends several inches beyond any surface discoloration. Secondary infections start in the small twigs, progress down the stem, and may eventually involve whole branches (Fig. 2). Severely blighted plants appear to have been scorched by fire (Fig. 3). A characteristic symptom of fire blight is the bending of the blighted terminal, which resembles a shepherd's crook (Fig. 4).
During the late summer when bacterial growth slows, all infected twigs and branches should be pruned out and burned or buried. Because the bacteria are usually found somewhat in advance of the obviously diseased tissue, the cut should be made through healthy wood 6 to 8 inches below the point of visible infection. Cankered areas on large branches and trunks should be pruned out during the dormant season. Pruning tools should be sterilized frequently with rubbing alcohol or dilute household bleach (1 part bleach to 9 parts water) during this operation to prevent introducing the bacteria to new wood.
Fire blight is worse on succulent tissues. Avoid excessive use of nitrogenous fertilizers and excessive pruning, both of which promote succulent growth. Remove water sprouts that form on susceptible tree species as they appear. Water sprouts are very susceptible to infection and can provide direct access of the bacteria to the trunk.
Varying levels of resistance to fire blight have been found among cultivars of some common landscape ornamentals (Tables 1-2). Resistant cultivars should be considered for new plantings.
Crabapple cultivars with resistance to fire blight and other major crabapple diseases (scab and powdery mildew)
*Adams has poor resistance to Japanese beetles.
Tolerant or resistant ornamental pear cultivars
|P. betulaefolia 'Dancer'|
P. calleryana 'Alfred'
P. calleryana 'Bradford'
|P. calleryana 'Capitol'|
P. calleryana 'Cleveland Select'
P. calleryana 'Fauriei'
Adapted from previous publication by R. C. Lambe
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Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; M. Ray McKinnie, Interim Administrator, 1890 Extension Program, Virginia State University, Petersburg.
May 1, 2009