Fire Blight
 | Author:
Allison Kosto, MSU Broadwater County Extension Agent
MSU Broadwater County Extension Agent |
Fire Blight
Allison Kosto
MSU Broadwater County Extension Agent
It’s interesting to see how things cycle in Mother Nature, and much of it is dependent on weather. For the past two years, we’ve had prime conditions for fire blight which can be destructive on trees. Fire blight is caused by a bacteria that is most damaging in apple, pear and crabapple trees, but can also be an issue, although less common, on hawthorn, lilac, serviceberries, blackberries, raspberries and mountain ash. It can destroy a season’s fruit crop, damage the structure of the tree, and diminish tree health. Highly susceptible trees can be killed in a single season, although this is rare.
The frequency of the disease varies from year to year. The weather is often a major factor in determining if it will be an issue but cultivar, tree age and tree health can also influence susceptibility. Fire blight tends to be most serious when spring temperatures during pre-bloom and bloom are warmer than average. If it is warm and rainy then the disease can spread rapidly. It can be spread by insects, rain, wind or birds. As the weather heats up and the humidity drops, the disease usually stops spreading.
Even though symptoms can be seen as soon as active growth begins in the spring, they are typically not noticed until they are well-developed. Symptoms include the characteristic black “shepherd’s crook” twigs, discolored bark and light brown to blackened leaves. During wet conditions, the fruit may have a creamy bacterial ooze. Infected fruit stays on the tree. It will gradually dry and stay attached to the branch.
Unfortunately, there is no cure for this disease and treatment options are limited for homeowners. If you have reoccurring issues with fire blight, then you may want to choose a cultivar that is more resistant. Annual pruning is important and avoid over-fertilizing. If you have fire blight, the first step is removing all the affected twigs and branches. Remove twigs and branches at least 12 inches below the edge of the visual infection. Sterilize your pruning tools between every cut to avoid spreading the bacteria. To sterilize, you can use household bleach, ethyl alcohol or household spray disinfectants. Completely remove and destroy any of the branches.
Pesticides are generally not recommended unless you have a history of fire flight. Copper products are the primary chemical option available to homeowners. They are only a preventative solution and often still don’t provide adequate control even with multiple applications. These products should be applied to open blossoms and the number of applications depends on the length of the bloom period. For pear trees, this might mean 5-12 applications per season. Copper can also cause scarring on the fruit and needs to be applied at least 50 days prior to harvest. There are some bacteria strains that are resistant to copper.
For assistance with diagnosing disease or pest issues on trees, contact the MSU Extension Office in Broadwater County at 406-266-9242 or allison.kosto@montana.edu with questions or bring a sample to 416 Broadway in Townsend.
