Fire blight is a common springtime disease of apple, pear, and related species, including crabapple, hawthorn, cotoneaster, and quince.
Signs and symptoms
Leaves on affected branches wilt and turn black, appearing as if scorched by fire. The most characteristic symptom is the curling of affected shoots into curved "shepherd's crooks". Cankers (areas of sunken or discolored bark) may develop on limbs, and the blighted shoots may produce sticky ooze in wet weather.
Fire blight is caused by a bacterium. The bacterium survives the winter in cankers on infected branches. In the springtime, sticky bacterial ooze formed at the edges of the cankers is carried to healthy plants by insects, wind, and splashing rain. Healthy plants may become infected through blossoms or wounds.
Several management tactics can help reduce the impact of fire blight. Sanitation is most important, and infected branches should be pruned out of the tree. It is best to prune when the plant and bacterium are dormant, during the winter. Infected branches should be pruned at least 12 inches below the area that looks diseased, to remove all of the bacteria.
Resistant varieties should be planted whenever possible. Although they are not completely immune to fire blight, resistant varieties are less impacted by the disease than are other varieties. Pears tend to be more susceptible than apples.
Commercial growers can use bactericidal sprays, such as copper sulfate or streptomycin, during the bloom period. However, bactericides are ineffective without proper sanitation.
See this article for more information on fire blight.