BP-30-W
Ornamental Diseases

Cooperative Extension Service
Purdue University
West Lafayette, IN

Fire Blight

Paul C. Pecknold, Extension Plant Pathologist

Fire blight is a bacterial disease that is particularly destructive on many varieties of apple and pear. It may also damage certain ornamental plants, such as flowering crabapple, hawthorn, mountain ash, cotoneaster, pyracantha, and spirea. If not controlled, fire blight can destroy the blossoms and fruit and may damage or kill the plant by stem infection.

Symptoms

Fire blight usually first appears during bloom. The blossom clusters wilt and turn dark brown or black. This is followed by twig blight infection of the current season's growth. The most obvious symptom of twig blight is a scorched appearance of affected stems in which the leaves wilt, turn brown, and cling to the stem. It is this stage that gives the disease the name "Fire Blight."

Often the tips of blighted twigs have a crooked appearance resembling a fish hook. Fire blight may continue to spread downward from the blighted twigs into main scaffolding limbs and trunk. The outer bark of infected branches becomes shriveled, while the inner bark appears water-soaked and reddish-brown. There is usually a distinct separation of the infected (cankered) and healthy tissue. The cankered areas are often slightly sunken and have a darker appearance than that of adjacent healthy bark tissue.

Cause

Fire blight is caused by the bacterium, Erwinia amylovora. The bacteria overwinter in cankered limbs, and in spring, droplets of sticky, amber-colored ooze form from these cankers. These droplets contain large numbers of bacteria. Insects and spattering rain carry the bacteria from the droplets to blossoms and twigs. More fire blight bacteria ooze from these new infections, and insects and rain again carry them to new areas of the tree and orchard. Fire blight is most damaging in years when spring temperatures are above normal with frequent rains. During cool springs the blossoms blight phase is usually not significant.

Figure 1: Blighted twig showing typical crooking of the apical

Control

No single practice can insure complete control of fire blight. However, you can reduce the disease if you employ a combination of both cultural and chemical control measures as outlined below.

Sanitation. Fire blight-infected limbs and branches should be pruned during late winter when there is much less chance of spreading fire blight bacteria on cutting tools. However, often it is necessary to make immediate cuts to prevent the disease from going into the main framework of the tree. This is especially critical on young trees with diseased branches attached to the main trunk. Use great caution when pruning infected limbs during spring or summer. Cut 8 to 12 inches below the diseased tissue and, most important, sterilize cutting tools between each cut. A 70 per cent denatured alcohol solution, made by mixing 3 volumes of denatured alcohol with 1 volume of water, is recommended for sterilizing cutting tools. A 10 per cent solution of liquid laundry bleach (sodium hypochlorite) can also be used, but this preparation is corrosive to most pruning tools. If it is used, the tools should be thoroughly rinsed and oiled after cutting.
Chemical control. The most effective chemical control of fire blight is achieved by the application of streptomycin (Agri-Strep) during bloom. Because blossoms open over a period of several days, 3 to 4 applications during bloom are necessary. Apply the first bloom spray shortly after the first blossoms open. A second spray is applied when about half of the blossoms that were not open during the first spray do open. A third spray should be applied when the remaining blossoms open (full bloom). Additional sprays may be needed if the bloom period is unusually long. Do not use streptomycin after bloom, except within 24 hours after a hail storm. Excessive use of streptomycin may result in the development of resistant strains of the fire blight bacterium. Follow all label instructions regarding amounts of pesticide to use, method of application, and safety warnings.
Cultural practices. There is added danger of severe fire blight infection when an excessive amount of new growth occurs. Rapidly growing, succulent twigs which have been stimulated by excessive fertility or heavy pruning are extremely susceptible to the fire blight bacteria. Therefore, it is best to use a balanced fertilizer with fairly low nitrogen content for moderate growth.
Remove suckers which develop on the trunk or main scaffolding limbs, since they are also very susceptible to fire blight infection.
Resistance. Use fire blight resistant plants, if available. Though few varieties of apple or pear are immune to fire blight, many do have limited resistance to the disease. For information on varietal resistance levels, contact your county Extension office.

Reference to products in this publication is not intended to be an endorsement to the exclusion of others which may be similar. Persons using such products assume responsibility for their use in accordance with current label directions of the manufacturer.

REVISED 5/01

It is the policy of the Purdue University Cooperative Extension Service, David C. Petritz, Director, that all persons shall have equal opportunity and access to its programs and facilities without regard to race, color, sex, religion, national origin, age, marital status, parental status, sexual orientation, or disability.

Purdue University is an Affirmative Action employer.

This material may be available in alternative formats.

1-888-EXT-INFO

http://www. extension.purdue.edu/extmedia