Horticulture : News
DATE: June 08, 2010
HARDIN COUNTY COOPERATIVE EXTENSION SERVICE
201 Peterson Drive
Elizabethtown, Kentucky 42701-9370
BY: Amy Aldenderfer
County Extension Agent for Horticulture
Fire Blight: What's a Homeowner to do?
What is it?
Fire blight, caused by the bacterium Erwinia amylovora, concerns orchardists as a serious disease of apple and pear. However, it also occurs on many landscape trees and shrubs in the rose family. Besides attacking apple and pear trees, including flowering crabapple (Malus spp.) and callery pear (Pyrus callaryana), fire blight appears commonly on several species of cotoneaster (Cotoneaster spp.), hawthorn (Crataegus spp.), and mountain ash (Sorbus spp.). It also occurs on firethorn (Pyracantha coccinea), and less commonly on serviceberry (Amelanchier canadensis), flowering quince (Chaenomeles japonica), cultivated quince (Cydonia vulgaris), flowering plum (Prunus triloba var. plena), spirea (Spirea vanhouttei), rose (Rosa spp.), Stransvaesia davidiana, and on Rubus species such as red and black raspberry and thornless blackberry.
The earliest symptoms in the flowers can be seen only by careful examination of infected spurs, which reveals dark and wilted individual flower bases or pedicels. As the infected bloom collapses, the infection spreads rapidly into the other flowers in the spur, causing the entire spur to wilt suddenly and die. Thus, growers often first notice that blossoms and leaves of infected terminals and spurs wilt suddenly and then turn dark brown to black as if scorched by fire. Diseased tissue usually remains firmly attached to the tree. From there, infections frequently spread to the supporting spurs and branches to form cankers. These cankers continue to increase in size as long as conditions remain favorable for disease development. The cankers have a shrunken surface and appear dark brown to purple in color. If a canker girdles a stem, the whole stem can die.
In the “shoot blight” phase of the disease, infected, rapidly-growing shoots wilt from the tip and develop a crook or bend at the growing point. At first the tissues appear water-soaked and dark green; soon, however, they become brown to black in color. Twig infections can spread back into their supporting limbs and cause cankers. Infections of basal sprouts (suckers) can rapidly kill a tree when the infection moves into the root collar area and restricts the vital flow of nutrients and water into the crown. Symptoms can develop rapidly.
The bacteria overwinter in cankers formed during the previous years’ infections and as “resident” bacteria on plant surfaces. As numbers of bacteria grow, they can be identified as droplets of bacterial ooze produced in the margins of cankers. Bacterial growth is generally favored by an adequate food base supplied by the host plant, poor internal defense mechanisms of the host, high humidity, and temperatures between 65 and 70°F. When conditions become favorable for bacteria to grow, populations can build rapidly. At 70°F, bacterial numbers double every 20 minutes; one cell becomes billions overnight, each capable of causing an infection. Given the right conditions, thousands of infections can occur within minutes; thus fire blight epidemics are explosive.
Control measures
The adequate control of fire blight in a severe disease year is difficult, if not impossible, to attain. However, most years are not severe disease years and adequate disease control can be achieved with some effort. It is very important to prevent infection of the flowers because once they become infected, they serve as a source of inoculum for the rest of the tree. Control measures are outlined below:
- Take extra care during the dormant season to prune trees according to sound horticultural principles. Trees properly thinned and shaped are generally less susceptible to fire blight. In addition, while trees are dormant, blighted twigs should be pruned just below the infected areas and destroyed. This practice, which should be done every year, eliminates an important potential source of inoculum for subsequent epidemics. The pruning must be done carefully, so that all infected branches are removed. This care is especially important in young plantings where unchecked canker development could easily kill the tree. It is not necessary to sterilize pruning tools for dormant pruning. Remove badly infected trees and old, neglected pear trees that could be sources of inoculum.
- Avoid any cultural practices which stimulate rapid tree growth and excess branch proliferation. As noted earlier, these conditions (often brought on by excess nitrogen, abnormally low fruit load, and/or poor pruning techniques) increase a tree’s susceptibility to fire blight. Avoid tree stresses as well, because stressed trees, once infected, are less able to slow the spread of infection within the tree.
- Use fire blight resistant plants. Several cultivars of apple, pear, or the various blight-susceptible ornamental species and cultivars are immune to fire blight. Nonetheless, some cultivars of these plants are more resistant to or tolerant of the disease than others. Planting less susceptible species or cultivars might be beneficial where practical. Avoid the interplanting of susceptible and resistant cultivars of the same species or different species at a single location. A common mistake is to plant susceptible pear trees in the apple orchard. Interplanting often serves as a source of additional bacteria from which an epidemic could develop.
For more information about fireblight, visit http://www.ca.uky.edu/agc/pubs/ppa/ppa34/ppa34.htm.
You can contact the Hardin County Cooperative Extension Service at 765-4121 or www.hardinext.org or via email at Amy.Aldenderfer@uky.edu
Educational programs of the Kentucky Cooperative Extension Service serve all people regardless of race, color, age, sex, religion, disability or national origin.
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