Sorbus: Fireblight

Problem
The blossoms and leaves of infected twigs suddenly wilt and turn black as if scorched by fire. The 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. Young trees may die.

Analysis
This plant 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 cankers on the branches. In the spring, the bacteria ooze out of the cankers. Bees, flies, and other insects are attracted to the sticky ooze and become smeared with it. When the insects visit a mountain ash 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. Insects visiting these infected blossoms later carry bacteria-laden nectar to healthy blossoms. Rain, wind, and tools may also spread the bacteria. Tender or damaged leaves may be infected in midsummer.

Solution
Prune out infected branches 12 to 15 inches beyond any visible discoloration and destroy them. Sterilize pruning shears 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, will help prevent infection. Repeat at intervals of 5 to 7 days until the end of bloom.