Fire blight (Erwina amylovora) is a devastating bacterial disease that affects apple, pear and other members of the Rosaceae family. According to the USDA, fire blight costs apple growers in the United States over $100 million per year due to crop loss and the costs of disease control.
Damage caused by fire blight can be identified by the scorched appearance of affected branches, leaves and fruit. Areas affected by fire blight often appear blackened, shrunken and cracked with necrotic lesions that ooze a viscous discharge.
The disease enters the tree through the stomata of tender new shoots, open blossoms and new leaves. Damaged tissue also creates an entry point for the bacteria. Typical damaged-tissue entry points include punctures and tears caused by sucking insects, damage from hailstorms and areas of recent pruning.
Bacteria in the exudate are transmitted between trees by bees, birds, insects, rain and wind. As the disease is moved, it can also cause secondary infections in the same tree, increasing pressure on the infected tree. Due to its easy transmission, an entire orchard can be wiped out over the course of a single growing season.
Once a tree is infected, the bacteria move systemically through the tree, rapidly infecting the entire tree. When the bacteria reach the roots of the tree, death often occurs.
Fire blight is generally treated through early spring spray applications of antibiotics, including streptomycin and terramycin, prior to blossom. However, strep-resistant strains of bacteria have recently been detected in California and Washington. Late winter or early spring microinjection applications of oxytetracycline hydrochloride have also been shown to successfully prevent fire blight in nonbearing trees.
When a fire blight infection is detected, it is essential that the infected portion of the tree be pruned 8 to 12 inches below the infected area. Ensure that pruned limbs are completely removed from the area once pruned. To prevent further spread of the disease, pruning tools must be disinfected between each cut. Failure to do so will likely lead to the spread of the bacteria in subsequent cuts.
The discriminate use of a slow-release nitrogen fertilizer may also improve the health and vigor of affected trees, giving them a better chance of survival. Caution must be taken to not overfertilize trees, however, as the development of new soft tissue may create new entry points for the fire blight bacteria.
Trees are most susceptible to fire blight during the cool, wet season. During this time, inspect trees for new fire blight infections and prune affected trees as early as possible to prevent the spread of the disease.