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Fire Blight

In spring, disease symptoms can appear as soon as trees begin active growth. The first sign is a watery, light tan bacterial ooze that exudes from branch, twig, or trunk cankers (small to large areas of bark killed by the pathogen during previous seasons). The ooze turns dark after exposure to air, leaving dark streaks on branches or trunks. However, cankers may be inconspicuous and infections may not be noticed until later in spring when flowers, shoots, and/or young fruit shrivel and blacken.

Flowers are usually infected first. Infected flowers and flower stems wilt and turn black on pear trees, whereas on apple trees they turn brown. Blight infections often move into twigs and branches from infected blossom clusters, causing small shoots to wilt, forming a crook at the end of each infected shoot. Eventually the infected portion of the shoot turns black. Dead, blackened leaves and fruit cling to branches throughout the season, giving the tree a scorched appearance, hence the name "fire blight."

When the pathogen spreads from blossoms into wood, the newly infected wood underneath the bark has pink to orange-red streaks. Fire blight may also spread into wood surrounding an overwintering canker. 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 caused by bacteria as they move out to infect healthy wood. 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. The pathogen tends to move in trees from the infection site toward the roots. In fall, leaves on infected pear shoots often turn red and then black.

Blossoms and leaves wilt, turn black, shrivel and die.

Cause(s):

Probably caused by fire blight, caused by a bacterium; especially susceptible are apple, flowering crabapple, pear, pyracantha, and quince. Fire blight, caused by the bacterium Erwinia amylovora, is a common and frequently destructive bacterial disease of pome fruit trees and other related plants. Pear and quince trees are extremely susceptible. Apple, crabapple, and Pyracantha species are also frequently damaged. Fire blight occasionally attacks hawthorn (Crataegus species), Spiraea, Cotoneaster, toyon (Photinia species), juneberry or serviceberry (Amelanchier species), loquat, mountain ash (Sorbus species), and other related plants. Fire blight infections can destroy limbs and even entire shrubs or trees.

Control(s):

Fire blight development is influenced primarily by seasonal weather. When temperatures in the range of 75° to 85°F are accompanied by intermittent rain and hail, conditions are ideal for disease development. The succulent tissue of rapidly growing trees is especially vulnerable. Thus, excess nitrogen fertilization and heavy pruning, which promote such growth, should be avoided. Trees should not be irrigated during bloom. Monitor trees regularly, and promptly remove and destroy fire blight infections. If fire blight has been a problem in the past, apply blossom sprays. Sprays prevent new infections but will not eliminate wood infections; these must be pruned out. In years when weather conditions are very conducive to fire blight development, it may be difficult if not impossible to control the disease.

During late summer all infected twigs and branches should be pruned out and burned or buried, and pruning tools sterilized with alcohol or 10% bleach.

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