Fire blight is a bacterial disease that affects certain species in the rose family (Rosaceae). It is especially destructive to apples, pears, and crabapples. The disease also can occur on serviceberries, flowering quinces, cotoneasters, hawthorns, quinces, pyracanthas, blackberries, raspberries, and mountain ashes.
Leaves darken and wilt but remain attached to the tree (see above), giving it a scorched or burned look- hence the name. As symptoms progress, fruits remain attached to the tree as shriveled “mummies”. Cankers, which are sunken areas darker in color than the surrounding bark, form as the disease advances. If present on the main trunk, cankers often are fatal, as they eventually will girdle the tree.
Tree varieties with some degree of fire blight resistance*
|Golden Delicious||Seckel||Red Splendor|
* Resistance does not mean immunity.
To determine the full susceptibility of your fruit varietal, see this chart from CSU Extension.
The Erwinia amylovora bacteria overwinter in blighted branches and at the edge of cankers (areas of bark killed by bacteria). In spring, when temperatures frequently reach 65 F, the bacteria multiply rapidly.
Masses of bacteria are forced through cracks and bark pores to the bark surface, where they form a sweet, gummy exudate called bacterial ooze. Insects such as aphids, ants, bees, beetles, and flies, are attracted to this ooze, pick up the bacteria on their bodies, and inadvertently carry the bacteria to opening blossoms. Bacterial ooze splashed by rain can also spread the pathogen.
Once in the blossom, bacteria multiply rapidly in the nectar and eventually enter the flower tissue. From the flower, the bacteria move into the branch. When the bacteria invade and kill the cambial tissue of the branch, all flowers, leaves and fruit above the girdled area die.
Infection also can take place through natural openings in leaves (stomata), branches (lenticels), pruning wounds, insect feeding and ovipositing, and hail. Droplets of bacterial ooze can form on twigs within three days after infection.