By Rick Thomason
University of Tennessee Johnson County Extension Director
Fire blight is a serious disease of apple and pear. The name “fire blight” describes the most characteristic symptoms of this disease: a brown-to-black scorched appearance of twigs, flowers and foliage.
The blossom blight phase is usually the first symptom observed in the spring. Blossoms and fruit spurs appear water-soaked, wilted, shriveled and finally turn brown to black. Shoot blight occurs when infections begin at the shoot tips and move rapidly down the shoots.
Leaves turn brown or black. Frequently, the tip of the blighted shoot bends over and resembles a shepherd’s crook.
Limb and trunk blight occur when the infection moves downward from infected shoots or fruit spurs into larger branches or the trunk. Dark, slightly sunken cankers (dead areas) are formed. On susceptible varieties, these cankers can girdle and kill the entire branch or tree.
During humid or rainy weather, blighted tissues often exude a milky-white, sticky ooze that later turns an amber color. This ooze, which is very apparent on blighted fruit, contains millions of bacteria that can cause new infections.
The bacteria overwinter in the trunk and branch cankers.
In the spring, the bacteria resume multiplication when temperatures are above 65 F. Their growth is favored by heavy dews or high humidity. The bacteria are carried by wind, rain and insects to blossoms or young, succulent shoots. Bacteria can enter the flowers and leaves through natural openings. However, wounds and injuries made by insects, hail, wind, humans and possibly frost are important means of entrance. New infections and disease development usually continue until rapid spring growth stops.
Cankers and blighted shoots should be removed, as they can serve as a source of bacterial spread. Delay the removal of infected shoots until the dormant season to avoid spreading infection to healthy shoots.
Dormant pruning also prevents the formation of new, susceptible shoots as would occur during the growing season, while fire blight is active. Examine the larger branches and trunks carefully for cankers, since these are the most likely overwintering sites for the bacterium.
If only a few trees are lightly infected, spring pruning to remove newly infected twigs and shoots can reduce spread of the disease if done carefully. The trees must be inspected daily and new infections must be promptly removed as soon as symptoms are observed. Cuts should be made 10 to 12 inches beyond the last evidence of disease. To avoid spreading the bacterium, the pruning shears should be sterilized after each cut. Household bleach, diluted one to four with water, has traditionally been used for this purpose. However, recent research has indicated that Lysol(r), at the same dilution, is just as effective as bleach and causes less corrosion of tools than bleach. Best results were obtained by soaking the tools for at least one minute, as opposed to dipping them.
Excessive pruning and high nitrogen fertilization will promote vigorous growth, which is susceptible to fire blight infection. Fertilization and pruning practices on susceptible varieties should be adjusted to promote moderate growth only.
The antibiotic streptomycin is the most effective spray material for controlling fire blight. However, streptomycin is useful only for prevention. It should not be used if symptoms are present. The critical time for fire blight prevention is during the bloom period. Apply streptomycin at 1 teaspoon/gal beginning when 20 percent of the blooms are open and continuing until petal fall. Streptomycin bloom sprays should be applied every five days under normal weather conditions, and every three or four days if the weather is unusually warm.
Streptomycin is most effective when applied alone, as a dilute spray, under slow drying conditions.
Source: UT Extension Publication SP277-R, “Fire Blight”