Fire blight is a bacterial disease that attacks raspberries, apple, crabapple, mountain ash, cotone aster, saskatoon berry, and spirea. The blossoms of these trees and shrubs suddenly turn brown and wilt. Later, the twigs may appear as if scorched by fire, and may turn brown or black, curling into a characteristic "shepherd's crook". The leaves on infected branches often remain on the trees all summer and well into the winter. The disease also produces cankers, which are discolored areas that are slightly sunken and tend to crack around the edges. These cankers may produce a slimy, bacterial ooze that contains millions of bacterial cells. Flies, bees, ants, and beetles can spread these disease cells from blossom to blossom and from tree to tree. The disease can also be spread by rain, wind and pruning shears. Fire blight can develop during early summer when there are alternating periods of rain and sunshine. Severe infections can ultimately be fatal to some trees or shrubs.
Once fire blight has infected a tree or shrub, eradication is the only way to control it. During the dormant period, prune out the infected plant material, cutting at least nine inches below the diseased wood. Disinfect pruners between each cut by dipping them in a solution of one part household bleach to nine parts water for ten seconds. Burn the diseased prunings.
Prevent problems by planting resistant varieties, or spray existing plants with an antibiotic such as Agristrep, every five to ten days during bloom. Excessive fertilization, especially with nitrogen, can make plants susceptible to fire blight. Fertilize only when soil tests show a nutrient deficiency. Rapid growth can make the tree or shrub susceptible to other problems as well; steady growth is best.
References:
Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997