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Extension 4-H Youth Building
Ames, Iowa 50011-3630
(515) 294-9915

6/28/04

Contacts:
Krystal Tentinger, Plant Pathology, keeko@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033, jmcguire@iastate.edu

Garden Column for the week of July 2, 2004

Don't Get Burned by this Common Tree Disease

By Krystal Tentinger
Plant Pathology Student
Iowa State University

You may look out your window and be shocked to see that one of your trees looks like it has been torched by fire. It could be a crabapple or Bradford pear. Many homeowners have one of these trees in their front yard or landscape because of their beautiful spring blooms. What most homeowners are not aware of is that these trees could be at risk for a disease called fire blight. Fire blight is a bacterial disease that affects a wide range of plants, but is most destructive to apples and pears. This disease is found on many species of the rose family and is seen on cherry, hawthorn, quince, cotoneaster, mountain ash, service berry, spirea, rose and raspberry to name a few.

Diseases often have common names that relate to what they look like on the plant. Fire blight is no exception, and it refers to infected branches that look as though they had been scorched by fire or heat. After infection, blossoms and fruit can wilt and darken. The infected leaves darken, turn a dark brown or black color, and bow down. New shoots that are infected quickly wilt and droop, making what most call a 'shepard's crook.' The branch tip will be curved in a very sharp angle and looks like a fishing hook. These infected branches and fruit remain on the tree.

Fire blight develops rapidly in warm, humid weather. It can be transferred very easily from tree to tree and is spread by splashing rain, wind, and insects such as flies, ants, aphids, and beetles. This disease needs to have an opening in the plant for infection to occur. This can be natural openings, such as stomata (holes on the plant where water and air can be transferred), or forced openings, such as insect feeding wounds or areas affected by hail damage. Infected trees harbor the bacteria and can be a source of inoculum (source of disease) for other trees in following years.

Fire blight also can progress into the limbs and trunk of the tree in the form of a canker. Cankers are sunken, darkened areas with a distinct ridge between healthy and infected tissue. This canker is a perfect place to keep for the bacteria to survive over winter, and can be the initial points of infection in the spring. When cankers overtake the trunk, or young trees are affected, this can mean death for the tree.

Fire blight is managed by combining several control strategies:

Choose a proper site. When planting trees and ornamentals, make sure that the site has proper drainage and soil pH for the plant selected. Stress caused by improper soil conditions may make a tree more vulnerable to disease.

Choose a cultivar with resistance to fire blight. A number of cultivars of pear, apple, crabapple, and other woody ornamentals have shown a level of resistance to fire blight. Homeowners should check that level of resistance before buying.

Prune infected branches during late fall. Branches that show infection or cankers should be pruned during late fall at least 10-12 inches below the lower edge of the canker.

Control insects. Insects are a main factor in the occurrence of this disease. Insect feeding will make wounds where the bacteria can easily enter. Also, insects carry the bacteria from plant to plant. An insect control program may need to be implemented, especially in a commercial fruit production setting, in order to minimize damage caused by fire blight.

Destroy infected trees. Surrounding trees that are infected could be harboring the bacteria and thus should be destroyed. Also, heavily infected trees should be destroyed if pruning is not beneficial.

Spray trees with a bactericide. Chemical sprays can be beneficial to prevention or stopping the transfer of fire blight. An antibiotic such as Streptomycin can be used during the bloom period, and make sure to follow guidelines for spraying.

Do not over fertilize. Additions of nitrogen can cause rapid growth, which puts the tree at risk for infection.

Using a variety of these methods is the only way to properly stop the advance of this disease. With good use of control options, homeowners and others will ensure that their trees will not get "burnt" by fire blight. For images of the disease and further information, please refer to "Tree Fruits - Insect and Disease Management for Backyard Fruit Growers in the Midwest" IDEA 3, a publication from Iowa State University Extension.

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Editors: A color photo, suitable for publication, is available at right. Click on the thumbnail photo to go to the fullsized photo. The fullsize photo is 252K.

Caption: Fire blight


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