When was the last time you took a close look at the apple tree in your back yard? If the answer is “Erm….” then you may want to jot that down on the “to do soon” list. Why? Because of Erwinia!
Spring has been particularly hot and humid this year in Southern Illinois and Missouri (2014), creating ideal conditions for a disease known as Fire Blight to develop rapidly throughout the region. Fire blight is a bacterial disease caused by Erwinia amylovora which attacks Apple, Pear, and several other tree species during warm, rainy spring weather. The pathogen causes a range of symptoms – from destruction of blossoms, fruit, and twigs, to loss of the entire tree, depending upon the cultivar and the severity of infection. According to both the Purdue and Cornell Extension offices, it’s one of the most destructive diseases of apple and pear trees.
So how do I know if my trees have Fire blight?
Trees typically become infected as insects move from blossom to blossom in the spring time, unwittingly carrying bacteria from infected trees with them. Rain then washes the bacteria into natural openings at the flower base, allowing the pathogen to move in. Once inside Erwinia amylovora can move systemically (throughout the inside of the plant), but also produces bacterial ooze on infected areas of the tree’s surface (cankers, wilt, etc) where insects are sure to pick it up and carry it off to the next victim.
Within 2 weeks of the initial infection, a variety of symptoms begin to appear around the infection site:
- Blossoms wilt, gray, and die
- Flower/fruit clusters around the blossom die back
- ‘Shepherds Crook’ – Branch tips/young growth browns and curls down on itself. Considered the characteristic symptom of Fire blight.
- Spotted fruit – spots are typically reddish brown or black in color, and spread across the fruit’s surface.
- Cankers – lesions of necrotic (dead) tissue on the branches, fruit, or trunk of the tree. Often, these areas are sunken. Cankers typically appear later in the year as tree growth slows, and serve as overwintering sites for the bacteria.
“That doesn’t sound so bad.”
Although losing a few apples or having unattractive dead branch tips throughout the canopy can be a nuisance, these symptoms don’t really seem that bad. But Fire blight isn’t considered one of the “most destructive” diseases for nothing – Apple and Pear trees afflicted with Fire blight tend to die within a matter of years.
Once a plant is infected with Erwinia amylovora, it cannot be cured. At best, symptoms are noticed early and diseased tissue is quickly pruned off, slowing the pathogen and reducing innoculum. If Fire blight is not caught early and contained, the bacteria spreads to the main trunk, causing the tree to die. Young trees in particular die quickly.
What can I do about Fire blight?
The best way to deal with Fire blight is not to deal with it at all – purposely choose apple cultivars and root stocks that are resistant to Fire blight. Your local university extension office should be able to make a good recommendation on what types are resistant and work well in your area. Also consider talking to your local tree care expert about using a bactericidal spray during blooming season when the disease is at its worst to reduce the chance of infection.
If you think you might see symptoms or know that your tree has Fire blight already, then it’s time to make some potentially tough decisions. Once infected, it’s impossible to “cure” a tree, and the disease is rarely caught often enough to eradicate it. Once the pathogen reaches the main stem trees die, with young trees dying within a matter of weeks while mature trees may take 2-3 years. Your options are to attempt to slow disease progression with careful watching and pruning, or remove the tree entirely once the presence of the pathogen has been confirmed. For best results, consult with your local university extension office or trusted tree care provider to determine severity of infection and treatment (or removal) options available to you.
Watching and Pruning
You may be able to slow disease advancement be removing blighted twigs and other parts early in the summer time. Before choosing this option, consider carefully – while you may be able to prolong the tree’s life span, you’ll also be providing a home for the pathogen in your yard, significantly increasing the likelihood of the infection spreading to other trees and plants in your neighborhood.
If you ultimately choose to prune and watch, it is important to use sharp, clean cutting tools to do the work, and to make all cuts a minimum of 10 inches away from visible symptoms to increase the likelihood of “getting it all.” Clean cutting tools between each cut by dipping them in a 10% bleach solution to prevent accidental spread of the pathogen into healthy tissue.
Removing Fire blight
Be prepared to remove the tree. More often than not, trees infected with Fire blight end up dead within a matter of years, and in the meantime, serve as a source of innoculum (infectious particles) for the surrounding trees. Contact your local tree care provider for tips, advice, or hire them to do the work for you.
Regardless of weather you watch or remove, make a point of removing any leaf or twig debris from around the base of the tree, especially if you know the tree is already infected. This reduces the amount of bacteria around to infect, or re-infect, the tree, and is a good general habit to get into regardless. Just make sure you aren’t throwing your bacteria infected waste onto the compost pile! The last thing you want is to discover you’ve just put the Erwinia amylovora into another part of the garden!