Fire blight is a bacterial disease which can severely damage apples and pears. Blossoms, fruits, fruit spurs, twigs, and branches are affected and sometimes the entire tree may be killed.
Blossom blight is usually the first symptom observed in the spring. Blossoms appear water-soaked, wilted, shriveled and finally turn brown to black.
Twig and leaf blight symptoms are similar to those of blossom blight. Blighted twigs and watersprouts wilt at their tips giving the appearance of a shepherd's crook.
Fruit blight generally occurs in immature fruits. The fruit first appears oily or water-soaked; often droplets of milky-like ooze form on the surface. The fruit shrivels, becomes mummified and remains attached to the branch.
Limb and trunk blight occur when the bacteria move downward from infected blossoms, twigs and shots into larger branches or the trunk
During humid or rainy weather, blighted tissues usually exude a milky-like, sticky liquid or ooze containing the bacteria. The appearance of ooze on the surface of diseased blossoms, fruit, twigs or would is a very distinctive diagnostic characteristic of fire blight.
The bacteria overwinter in association with diseased and dead tissue (cankers) on the tree trunk. In the spring, bacteria are carried by wind, rain and insects to blossoms or growing, succulent shoots. Bacteria can enter through natural openings in the flower or through stomata in the leaves; however, wounds and injuries made by insects, hail, wind or by pruning are important means of entrance. From these primary infections, the bacteria may be carried to other blossoms, twigs or fruits resulting in secondary infections. These secondary infections may continue throughout the growing season and each infection may terminate in a canker.
Control is difficult, but many control practices help reduce the severity of fire blight. All of the control measures are not necessary or feasible in every planting.
1. Resistant Varieties -- Although varieties vary in resistance to fire blight, none are immune. In North Carolina, the more susceptible apple varieties are Jonathan, Golden Delicious, Lodi, Yellow Transparent, Stayman and Gala, with Red Delicious generally less affected. The pear varieties Bartlett, Bosc, and Clapp's Favorite are highly susceptible. Kieffer, Orient, Starks Delicious, and Dawn are less so and Magness and Moonglow are relatively resistant.
2. Removal of Infection Sources -- Removal of all cankers and blighted twigs before growth starts in the spring is very important. Cuts should be made 4-6 inches beyond any evidence of dead tissue. Pruning to remove newly infected twigs and shoots reduced spread of the disease if done carefully. The trees must be monitored daily and new infections must be promptly removed as soon as symptoms are seen. This method is more economical for the individual having only a few trees. Cuts should be made 10-12 inches beyond the last evidence of disease. Care must be taken to prevent spread of the bacteria by hands or cutting tools. If infections are cut out, use bleach (Clorox) dilute 1:10 with water to sterilize the cutting tools between each cut.
3. Insects -- Sucking insects, such as aphids, may carry the bacteria to terminals and shoots and create wounds which can be an entry point for the bacteria. Control of insects can reduce spread of the bacteria and the occurrence of infections. During bloom, bees can carry fire blight bacteria during pollination. Insecticides should not be used during bloom, instead this is the time that streptomycin sprays should be used.
Streptomycin sprays will not harm the bees, but will aid in preventing the bacteria from infecting the blossoms.
4. Cultural Practices -- Generally succulent, rapidly growing twigs and shoots are most susceptible. Management practices which reduce tree growth without an excessive loss of tree vigor are helpful. Such practices include not using excessive amount of nitrogen fertilizer. Watersprouts which develop near the base of the scaffold limbs, crotch, or trunk of the tree should be removed promptly.
5. Chemical -- Agricultural streptomycin (Agrimycin, Agri-Strep) in sprays at 100 parts per million (8 oz/100 gal or 1 teaspoon/gal) applied at 5-day intervals beginning when 5-10% of the blossoms are open and continuing until petal fall with 2-3 sprays after petal fall at 7-10 day intervals will reduce blight. Streptomycin should not be applied closer than 50 days to harvest on apples and 30 days on pears.
Remember, fire blight
develops more rapidly when temperatures are 65-90 F combined with humid or
rainy weather, and that once blight is established it is very difficult to
Recommendations of specific chemicals are based upon information on the manufacturer's label and performance in a limited number of trials. Because environmental conditions and methods of application by growers may vary widely, performance of the chemical will not always conform to the safety and pest control standards indicated by experimental data. All recommendations for pesticide use were legal at the time of publication, but the status of registration and use patterns are subject to change by actions of state and federal regulatory agencies. Last printed 04/91
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Last update: 21 April 2002 by Tom Creswell