Fire Blight

Fire blight, caused by the bacterium Erwinia amylovora, it is a serious disease that affects new leaves, fruit, flowers, and stems of over 75 species of trees and shrubs in the rose family including: apple, crabapple, hawthorn, pear, pyracantha, cotoneaster, spirea, flowering quince, and mountain-ash. Temperature, humidity, insect vectors, and wounding can all affect the severity of infection. If not controlled, it can cause flower and fruit blight, twig and branch dieback, or even kill the plant.

In spring, branch and trunk canker symptoms can appear as soon as trees begin active growth. 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. The ooze turns dark after exposure to air, leaving streaks on branches or trunks. 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 amount of fruit loss depends upon the extent and severity of the disease.

Injuries on tender young leaves and shoots, caused by wind, hail, or insect punctures, are easily invaded by the fire blight bacteria. Such infections lead to shoot blight. Ideal conditions for infection, disease development, and spread of the pathogen are rainy or humid weather with daytime temperatures from 75° to 85°F, especially when night temperatures stay above 55°F.

Fire blight bacteria generally don’t move uniformly through the bark but invade healthy wood by moving in narrow paths up to 1 1⁄2 inches wide in the outer bark ahead of the main infection. These long, narrow infections can extend 2 to 3 feet beyond the edge of the main infection or canker. If you expose bark from an infected woody area, you will see that the diseased tissue closest to the main canker is brown. Farther out, the infection turns red and then appears as flecking. Just beyond the visible infection the tissue will look healthy.

Fire blight development is influenced primarily by seasonal weather. When temperatures of 75° to 85°F are accompanied by intermittent rain or 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 shouldn’t be irrigated during bloom. Monitor trees regularly, and remove and destroy fire blight infections. (See Removing Diseased Wood.) If fire blight has been a problem in the past, apply blossom sprays. Sprays prevent new infections but won’t eliminate wood infections; these must be pruned out. In years when weather conditions are very conducive to fire blight development, it can be difficult if not impossible to control the disease.

Streptomycin is an antibiotic that is acceptable for use to protect trees but may be difficult to obtain. Do not use streptomycin after symptom development since it may lead to antibiotic resistance in the bacterial population.

Aluminum tris is a bactericide used prior to and during bloom.

Copper sprays are toxic to many species of bacteria. Copper sprays are best used during dormancy and prior to bud break because they may damage leaves and young fruit. Do not apply sprays within 50 days of apple harvest or within 30 days of pear harvest. Do not mix with oils or phytotoxicity issues can occur. Copper is available in several forms and sold under various trade names, including Bordeaux mixture.

Prohexadione-calcium is a plant growth regulator that reduces longitudinal shoot growth by inhibiting gibberellin biosynthesis. Prohexadione-calcium does not possess antibacterial activity but alters host biochemistry and tissues in ways that are not favorable for infection by E. amylovora. The length of time that shoot growth is inhibited depends on the application rate and tree vigor. Prohexadione-calcium is ineffective for control of the blossom blight phase of fire blight.