Christian County Extension Office
Apples, pears, crabapples, and even some ornamentals are infected by fire blight, a destructive bacterial disease. Fire blight damage is noticeable when infected leaves suddenly turn brown as if scorched by fire. This disease can result in reduced fruit, loss of limbs, or even tree death. Quince, mountain ash, spirea, hawthorn, pyracantha, and cotoneaster are also susceptible to fire blight. Infections occur in different parts of the tree and enter the plant by different means. The most common infection sites include:
- Twig Blight: This is the most obvious symptom of fire blight. In the spring, infected leaves, generally the youngest on the shoot, quickly wilt and turn dark brown or black but remain attached to the twig. The twig often will form a cane-like crook at the tip. The stem may also ooze a watery exudate.
- Blossom Blight: Diseased blossoms become water-soaked and turn brown. Blossom blight can reduce the current and following year’s fruit crop. Droplets of milky tan-colored bacterial ooze may be visible on diseased tissue.
- Canker Blight: As infections move downward from twigs and blossoms, localized cankers form on the trunk and limbs. Branch cankers become sunken and darkened and eventually cracked and creviced.
- Trauma Blight: Wounds to foliage or shoots from hailstorms, high winds, or driving rain can also serve as an entry point for the bacteria.
Fire blight bacteria becomes active in the spring when temperatures are above 65 degrees and humidity is 60%. Droplets of bacterial ooze will appear on the canker surfaces of previously infected trees. The bacteria are spread to blooms, foliage, and twigs by insects (i.e., honeybees, ants, beetles). Rainfall promotes widespread dispersion of the bacteria through splashing. The bacteria enter the tree through natural openings or wounds in the bark and actively re-infest other parts of the tree, until vegetative growth ceases and the terminal bud is formed (about a month after flowering).
Control of fire blight requires a total program. The use of several practices in an integrated manner should minimize fire blight damage.
- Plant resistant varieties of apple, pear, and crabapple. Resistant apple varieties include Jonafree, Prima, Red Delicious, and Red Free. Resistant pears include Kieffer, Magness, Moonglow, and Old Home. Many new crabapple varieties are also resistant to fire blight.
- Prune and discard infected twigs and branches with cankers. You can either prune as soon as infection is noticed to help remove inoculum or if there are a lot of infected areas you may wait for dormant pruning this winter. Cuts should be made several inches below the last visible evidence of disease on the shoot. If removing an entire shoot, it is better to leave a 2 to 3 inch stub than make a smooth cut near the trunk. This way if re-infection occurs it will likely appear on the tip of the stub which can be removed later. Dispose of all infected prunings. Clean your pruning equipment by dipping them in a solution of one part Clorox or Lysol to nine parts water after every cut. Oil the blades after pruning to prevent corrosion.
- Fertilize trees correctly. Excessive nitrogen promotes vigorous growth which is more susceptible to fire blight. Avoid improper fertilization by soil testing and making light applications in early spring or late fall after growth has ceased.
- Control insects. Aphids and other insects can carry bacteria to shoots and create new entry points when feeding. Follow a pest management program to reduce the number of insect pests in your orchard.
- Chemical control. During bud swell (late dormancy), an application of copper fungicide (e.g. Kocide or other fixed copper) should be applied, especially if fire blight was severe last year. This copper application should reduce amounts of bacterium present on branches and spurs, reducing risk for disease development. Do not apply copper after ¼ inch green leaf stage.
- Disease Risk Assessment & Weather Models. Plant disease prediction models utilize weather data to analyze disease risk. The University of Kentucky maintains weather stations and incorporates this data into disease risk predictions models. You can find models at weather.uky.edu
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UNIVERSITY OF KENTUCKY, KENTUCKY STATE UNIVERSITY, U.S. DEPARTMENT OF AGRICULTURE, AND KENTUCKY COUNTIES, COOPERATING