Fire blight is more severe during times of increased temperatures -- those persisting between 65 and 75 F -- intermixed with rain events during bloom. Because of the cooler temperatures experienced in Montana earlier this spring, many susceptible trees and shrubs flowered later when temperatures were warmer.
"This year is particularly bad," said Barry Jacobsen, Extension plant pathology specialist at Montana State University. "It doesn't take long driving around town to see trees infected with fire blight."
Through models developed by Washington State University, the increase of fire blight severity can be determined by calculating the amount of degree hours four days prior to a rain event when the tree or shrub is in bloom. Degree hours are those in which the temperature rises above 60 F.
The symptoms of fire blight include blossoms that wilt and turn brown, succulent spring growth tip wilt, stems and leaves that brown or have a scorched appearance, a "shepherd's crook" on tender growth and, in severe infections, cankers in twigs, limbs and even trunks of trees.
The only way to control fire blight this time of year is to prune out the infection. Monitor trees for symptoms and remove any infection at least 8 to 12 inches below the infection. Ideally, infected limbs should be removed at their point of attachment, without damaging the branch collar. Information about proper pruning of fruit trees can be found online through MSU Extension publications at http://www.msuextension.org/publications.asp.
"You should make sure that you prune out the water sprouts too," said Jacobsen. "If fire blight is present, the sprouts can easily infect the trunk of the tree and the whole tree will be affected." Water sprouts, sometimes called suckers, are branches that grow vertically from lateral branches and along or at the base of the trunk.
When pruning, always disinfect tools between each cut with alcohol or a 10 percent bleach solution to stop the spread of the bacteria to healthy parts of the tree. Following pruning, remove all pruned material from the property.
In some cases, entire trees may need to be removed. If trees are replanted in the same area, avoid fire blight by planting resistant varieties. To find resistant varieties of apple and other trees that may grow in your area, contact your county Extension agent or go online at http://msuextension.org/publications/AgandNaturalResources/MT200812AG.pdf to find a MontGuide, a resource from Montana State University Extension on disease resistant apple varieties for Montana.
Contact: Toby Day is the Montana State University Extension Horticulture Specialist. E-mail him at firstname.lastname@example.org.