M. Grabowski, UMN Extension
Photo 1: Brown blossoms and leaves infected with fire blight clinging to a crabapple tree
This June, many gardeners were surprised to see the blossoms on their crabapple trees turn brown and shrivel along with the leaves growing alongside them. In many cases blossoms and leaves became discolored all along several branches (Photo 1). In other cases individual clusters were affected. A month later, these brown blossoms and leaves remain attached to the tree and are likely to stay through the growing season. These unattractive brown clumps of foliage not only affect the beauty of the tree but have a significant impact on the progression of the disease.
These unusual symptoms are caused by the disease fire blight. Fire blight is caused by the bacteria Erwinia amylovora, and it affects trees and shrubs in the Rosaceae family. Fire blight can often be found on crabapple, apple and mountain-ash in Minnesota. Serviceberry, raspberry, cotoneaster, and hawthorn are susceptible but less commonly infected.
Although blossom blight is a rare occurrence in Minnesota, fire blight is not. The fire blight bacteria can infect young green shoots and leaves in addition to blossoms. In order to start an infection, the fire blight bacteria need warm wet weather (110 degree days above 65F to be exact) and a wound or natural opening. In a typical Minnesota spring, temperatures are too cool during crabapple blossom to allow the fire blight bacteria to multiply and start an infection. When weather warms up later in the season, fire blight often infects young shoots that have been injured by insect feeding, wind whipping or other factors. This year early warm wet conditions allowed the bacteria to blight blossoms in addition to shoots.
Regardless of how the bacteria first infect the tree, once an infection has begun, the fire blight bacteria can move through the infected blossoms or shoots and into the adjoining branches. With time the bacteria can move into the main trunk and even into the roots of the tree. Infection of the trunk or roots is lethal but infection of blossoms, shoots and branches can be pruned away if caught in time.
M. Grabowski, UMN Extension
Photo 2: Purple red discoloration of bark where the fire blight infected blossoms attach indicates progression of the disease into the branch. Healthy bark of this cultivar is yellow green
Gardeners that have observed symptoms of fire blight in their crabapple trees this spring should monitor trees throughout the growing season. The extent of the progression of the disease through the tree will vary depending on the resistance level of the tree. In highly resistant trees, the infection may not progress beyond the blossoms. In less resistant trees the infection will continue into the adjoining branch causing a canker. In highly susceptible trees, the infection can progress rapidly through the branch. Home gardeners can monitor the progression of the infection by looking for purplish brown discoloration of the bark (Photo 2) and continuing death of leaves along the branch.
Infections within branches should be noted for removal during the dormant season. February and March are ideal times to prune out branches infected with fire blight because the bacteria are not active at that time. Pruning cuts should not be made during the growing season (due to risk of the pruning wound becoming infected) unless the infection is close to infecting the main trunk. This may happen if suckers or water sprouts became infected or if the tree is highly susceptible. In that case, the branch should be cut 8-10 inches below the discolored bark on a cool dry day. Pruning tools will need to be sterilized after every cut with rubbing alcohol, a 10% solution of household bleach or Lysol®. Trees infected with fire blight should not be fertilized this summer as this will promote young succulent growth that is highly susceptible to infection. More information about fire blight can be found on the UMN Extension Yard and Garden page.