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How to protect your trees from fire blight disease

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By Dennis Morgeson

 

If shoots on your apple and pear trees are turning brown, “hooking” over, and dying, they probably have fire blight disease. This bacterial disease is showing up throughout the state and is very difficult to control. Fire blight can infect apple, pear, hawthorn, mountain ash, serviceberry, flowering quince, rose, as well as raspberry and blackberry trees and bushes.

Fire blight usually enters apple and pear trees through their blooms. We had warm weather in April when these tress flowered, as well as intermittent rain showers and heavy dews that favored fire blight development. The bacteria enters flowers through their nectaries and begin to multiply. As they multiply they move down the stem and into the limb. Once at the limb, the bacteria form a colony and cause a canker on the stem. If the canker spreads the circumference of the stem, the stem will die. Cankers serve as a source of innoculum for the rest of the tree and from season to season.

Control of fire blight is very difficult. As with bacterial infections in humans and animals, bacteria infections in plants spread rapidly and are extremely difficult to control, once a major outbreak has occurred.

There are few control methods that are effective for homeowner use. In early spring, just before bud break, spray trees and shrubs that are susceptible to fire blight with fixed copper. This will help reduce the number of over wintering bacteria. Avoid spraying copper after one-half inch of growth has occurred. It can damage the fruit.

Commercial growers spray Streptomycin, an antibiotic several times during bloom time. Streptomycin can and will kill fire blight bacteria when it comes into contact with it.  Homeowner use of Streptomycin is not recommended because increased use of this type of chemical can cause resistance and cross over pathogens to humans, although it is unlikely. Also, Streptomycin isn’t usually sold in small enough quantities for it to be cost effective for homeowners.

The best control method is pruning. At the first sign of fire blight damage or a canker, prune it out. Prune just below the damaged point into healthy wood. After every cut, dip your pruners into a 10 percent bleach solution to kill any bacteria that may be present. If you don’t sanitize your pruners, you run the risk of re-inoculating at every cut point. Be sure to burn or discard the pruned limbs far from your garden. They can still be sources of innoculum.

Be careful when mowing or weed eating around your trees and shrubs. Every wound is a source of entry for fire blight bacteria. If the bacteria gets into the main trunk of the tree, it can kill it. Often after hail storms there is a fire blight outbreak.

Apply insecticides as needed to susceptible plants. Insects can spread diseases, such as fire blight as well. Leafhoppers, plant bugs, and psylla can vector fire blight, causing an infection at each feeding point. Also, remember that a healthy plant, such as one that is adequately fed and watered, will be more likely to overcome an infection than a stressed one. However, don’t over fertilize! Extremely fast succulent growth is very susceptible to a fire blight infection.
Finally, the best defense against fire blight is to plant resistant varieties of the plants you want to grow.  Resistant plants will get fire blight but it will be less severe and easier to control than on non-resistant cultivars. For information about disease resistant plants and recommended varieties for Kentucky, call me at the Washington County Extension Office at 1-859-336-7741.