Fire blight is a bacterial disease most often seen on apples, crabapples and pears. Occasionally it is seen on mountain ashes and lilacs. Other plants that are susceptible to this disease are cherries, choke cherries, June berries, apricots, blackberries, raspberries, strawberries, roses, and spireas.
Symptoms of fire blight include blossoms and leaves on terminal branches turning brown or black as though scorched by fire, shoot tips bending into the shape of a shepherd's crook, and fruit developing blackened areas, surface blisters with red halos, and browning or soft rotting.
Cankers are dead and diseased areas that develop on large or small branches, and their bark is slightly depressed, with a definite line delimiting diseased from healthy bark. The bacteria overwinters in live tissue at the margins of live cankers and becomes the source of new infections the next spring as a sweet, bacteria-rich ooze that pushes out through openings in the bark. Droplets of this ooze can also develop on infected shoots. Insects contaminated with this ooze spread the bacteria to flowers and other plant parts. Rain, hail, and birds also spread the bacteria.
Prune away infected branches and destroy them. Diseased branches can be removed in late winter. Cuts should be made at least 4 inches below the diseased area. Infected branches which are pruned in the summer should be cut l0 to l2 inches below the point of visible infection. Since fire blight is so very contagious, the pruning shears needs to be cleaned between cuts. Pruning shears should be dipped for l0 seconds in a l0% bleach solution between cuts. Annual pruning of healthy branches should be limited to dormant times (late winter or early spring). Streptomycin sulfate is an antibiotic for fire blight that is sold as Agrimycin, Agristrep, or Fire Blight Spray. Bordeaux mixture is a less effective chemical control.
Good to intermediate resistant varieties of apples include Haralred, Sweet l6, Red Baron, and Dolgo. Excellent to good resistant ornamental crabapples include Adams, Centurion, Radiant, Red Splendor and Sargent.