Michelle Grabowski, University of Minnesota Extension Educator
This past summer, many Minnesotans noticed dead brown wilted leaves on apple, crabapple and mountain ash trees caused by the bacterial disease known as fire blight (Read Midsummer Trouble for Trees & Shrubs) Although symptoms of fire blight are most apparent in spring and summer months, one of the best times to manage this disease is right now.
Fire blight is caused by the bacteria Erwinia amylovera. This pathogen can infect all members of the Rosaceae family, but most commonly causes problems on apples, crabapples, and mountain ash trees in Minnesota. In the spring and summer months, the fire blight bacteria infects leaves and blossoms, turning them black to brown. The infection often starts at the tip of the branch and moves systemically downward, resulting in entire branches that look blackened or scorched by fire.
Photo 2: Discolored sapwood due to fireblight. Michelle Grabowski
It is within these infected branches that the fire blight bacteria survive Minnesota’s harsh winters. Branch infections are known as cankers and can be recognized by gardeners as an area of the branch with cracked, discolored and sometimes sunken bark. If the bark is peeled back with a sharp knife, reddish brown streaks can be seen in the light colored sapwood of the tree. Healthy sapwood should be off white to light green in color.
Photo 3: Fireblight canker on mountain ash. Michelle Grabowski
In addition it is common for the leaves and blossoms killed by a fire blight infection to remain attached to the infected branch. These are an indicator to gardeners to look more closely at the branch to determine where the branch canker is. Trees that suffered from fire blight in the previous summer should be carefully inspected for cankers regardless if leaves remain attached or not since wind can knock old leaves to the ground.
Any cankers that are found within the tree canopy should be pruned out and destroyed in February and March. During these months, it is too cold for the fire blight bacteria to be actively spreading within and between trees. In spring, as the weather warms up, however, the bacteria will multiply and ooze out of the canker in a sweet sticky liquid. This bacterial ooze is carried to new trees on wind driven rain, by insects, or on pruning tools and the hands of gardeners. It is therefore critical to remove branch cankers before the start of the new growing season to prevent new infections.
Gardeners should make the pruning cut to remove fire blight cankers at least 8 inches below visible signs of the infection. After each pruning cut, tools should be cleaned with a 10% bleach solution, rubbing alcohol, or a household anti-bacterial cleaner like Lysol® or Listerine®. All infected branches should be burned, buried or disposed of in the trash.