When apple or pear branch tips look scorched, with brown or black leaves hanging on them, a likely explanation is the disease “fire blight”, caused by the bacterium Erwinia amylovora. This aptly named disease seriously damages plants in the Rose family, especially apples and pears, seldom stone fruits, and many ornamentals (Figure 1). In commercial orchards, economic losses can be severe. In backyards, limbs die back and fruit is lost. Small trees may die.
Flowers and succulent shoots are usually affected first. Symptoms are a water- soaked appearance then a sudden wilting of succulent tips, followed by the shriveling of infested leaves, shoots, blossoms, and eventually fruit. Infected twigs typically form a “shepherd’s crook”.
The infection spreads from the flower to the fruiting spur, then to leaves, and ultimately to the woody tissue around the spur. Tips of limbs are infected and show symptoms first. The leaf stem (petiole) and midrib characteristically blacken and yellow bacterial ooze may occur on the leaf. Diseased apple leaves generally turn brown, while infected pear leaves turn black.
Cankers, can form on twigs and branches, eventually spreading into the trunk. Cankers are at first slightly sunken small brown to black areas. These may crack during the dormant season.
During the growing season the active edges of the cankers may appear raised or blistered and become more defined. The internal wood under and around the cankers becomes discolored with reddish brown streaks. These cankers may girdle the branch, killing it.
A tan-yellow bacterial slime is forced out of infected areas. The bacteria are then easily dispersed by insects and splashing water. This sugary bacterial ooze clogs the water carrying vessels of plants causing wilting.
Immature fruit can become infected with bacteria through natural openings in the skin, wounds, or infected fruiting spurs. Infected fruit first appears gray green, and water-soaked, then turns black. Sometimes a whitish to light tan fluid seeps out of the fruit. Eventually, fruit dries and shrivels on the tree.
Compared to its rate in flowers and fruit, the disease progresses more slowly in woody tissue, but once in the trunk, Erwinia amylovora can kill a tree.
Bacteria over-winter in the tissue under the bark at the margins of cankers. As the weather warms in spring, bacteria become active and form a sticky bacterial flow.
Bacteria may be spread in many ways. Birds, flies, pollinators and other insects crawl through or ingest this material and infect flowers, wounds, and natural leaf and twig openings. Insect vectors of the disease include ants, aphids, bees, houseflies, pear psylla, leafhoppers, and shothole borers among others. The host range includes over 200 species in nearly 40 genera.
Splashing water, either from rain or overhead irrigation, is another common way the disease is spread. Wind also carries bacteria.
Humans are often at fault for disseminating the inoculum by unsanitized pruning tools.
Blight symptoms appear within one to three weeks of infection, depending on temperature and moisture.
Warm weather, 65 °F or higher in a 24-hour period and humidity of 65% or higher during bloom greatly favor disease development, although fire blight can grow over a much wider range of 39-90 °F. Epidemics often occur following rain or hail storms where twigs and branches have suffered injury. Precipitation promotes development and dissemination of the disease. Sprinkler irrigation, high nitrogen fertilizers, severe pruning, and other factors that favor succulent new growth can all stimulate the disease.
Buy only fire blight resistant varieties and avoid susceptible plants. This is the first priority in preventing or controlling fire blight. Plant trees and shrubs in soil with good drainage to avoid stress that makes plants more susceptible to the disease.
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Skelly, J., and O'Callaghan, A., 2001, Fire Blight, Extension, University of Nevada Reno, FS-01-56
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