Trees, Shrubs, and Vines: Fireblight

Problem
Blossoms and leaves of some twigs suddenly wilt and turn black as if scorched by fire. Leaves curl and hang downward. The bark at the base of the blighted twig becomes water soaked, then dark, sunken, and dry; cracks may develop at the edge of the sunken area. In warm, moist spring weather, drops of brown ooze appear on the sunken bark.

Analysis
This disease is caused by a bacterium (Erwinia amylovora) that is very destructive to many trees and shrubs. (For a list of susceptible plants, see Plants Susceptible to Fire Blight.) The bacteria spend the winter in the sunken areas (cankers) on the branches. In the spring, the bacteria ooze out of the cankers and are carried by insects to the plant blossoms. Flies and other insects are attracted to the sweet, sticky ooze and become smeared with it. When the insects visit a flower for nectar, they infect it with the bacteria. The bacteria spread rapidly through the plant tissue in warm (65°F or higher), humid weather. Bees visiting these infected blossoms carry bacteria-laden nectar to healthy blossoms. Rain, wind, and tools may also spread the bacteria.

Solution
During spring and summer, prune out infected branches about 12 inches beyond any visible discoloration and destroy. Sterilize the pruning tools with rubbing alcohol after each cut. A protective spray of a pesticide containing basic copper sulfate or streptomycin applied before bud break in the spring helps prevent infection. Repeat at intervals of 5 to 7 days until the end of bloom. In summer or fall, after the disease stops spreading, prune out infected branches.