Fire blight is a highly destructive disease of apple and pear that can occur in commercial orchards and home plantings. Many landscape trees and shrubs in the rose family are also susceptible to this disease. Fire blight can cause severe damage in a very short period of time. Because precise conditions are needed for infection, disease appearance is erratic from year to year.

Fire Blight, Nicole Ward Gauthier

The earliest disease symptoms are observed on infected spurs when the bases of individual flowers or pedicels (flower stems) wilt and darken. As blooms collapse, infection spreads rapidly into other flowers in the cluster, causing the entire spur to wilt suddenly and die. Diseased tissues usually remain attached to the tree.

by Nicole Ward Gauthier & Cheryl A. KaiserInfections frequently spread from blossoms to supporting spurs and branches, resulting in stem lesions or cankers. Fire blight cankers appear shrunken with a dark brown to purple color. As cankers increase in size, they can girdle stems or branches; as a result, tissues above these infection sites die.

Bacterial cells can build up during the blossom and spur blight phases of fire blight and infect rapidly-growing shoots. Blighted shoots wilt from the tip and develop a crook or bend at the growing point, commonly referred to as a ‘shepherd’s crook.’ This phase occurs after bloom.

Trunk and rootstock infections can occur from the internal movement of the fire blight bacterium through water conducting-tissue or from infected water sprouts.
The fire blight organism, Erwinia amylovora, survives from one year to the next at the margins of previously formed branch and trunk cankers. In most years, fire blight begins during the bloom period and, as long as the environment is favorable, it will continue through petal fall and/or until shoot elongation stops.

Fire blight is generally favored by:

• High relative humidity or rainy conditions.
• Temperatures between 65°F and 70°F.

Under favorable conditions, bacterial populations can build-up rapidly. At 70°F, numbers of bacterial cells double every 20 minutes; one cell can become one billion cells overnight, each capable of causing a new infection.

The key to fire blight management is preventing the infection of flowers. Once flowers become infected, they serve as a source of inoculum for the rest of the tree. Management of fire blight requires an integrated approach that relies primarily on cultural practices and is supported by the judicious use of bactericides.

While few cultivars of apple, pear, and the various ornamental host species are immune to fire blight, some cultivars are more resistant or tolerant than others. Whenever possible, plant resistant cultivars and resistant cultivar/rootstock combinations. For information on fire blight tolerant apple and pear cultivars, consult the Midwest Tree Fruit Pest Management Handbook, ID-93.

Implementing the following cultural practices is important in managing this disease:

• Avoid any cultural practice which stimulates rapid tree growth; young succulent tissue is susceptible to infection.
• Fertilization, especially nitrogen application, should be adequate for tree health without promoting rapid growth and prolonged succulence.
• Prune trees to improve air circulation and to promote rapid drying of foliage. Aggressive pruning will stimulate growth, so selective pruning is recommended.

Pruning can play an important role in a comprehensive fire blight management program, and when done properly, should reduce inoculum and tree damage. However, while removal of sources of the pathogen is desirable, pruning when the disease is active can further spread the pathogen. Thus, pruning out fire blight strikes during the growing season is a controversial issue. Currently UK recommends that pruning blighted twigs and cankered branches be delayed until winter.

Timely chemical sprays may be used as preventative measures to control fire blight during the spring when the pathogen is at the surface of cankers and on flowers. After the bacterium has invaded tissues, bactericides are not effective. Fungicides will not control fire blight. Refer to the Midwest Tree Fruit Spray Guide (ID-92) for application rates and other details

Submitted by Nicole Ward Gauthier & Cheryl Kaiser, University of Kentucky, Department of Plant Pathology