Fire blight, caused by the bacterium Erwinia amylovora, affects over 130 plant species in the rose family. In Minnesota, fire blight is most often seen on apple, crabapple, pear, mountain ash, and cotoneaster.
Photo: U of MN Plant Disease Clinic
Bacteria survive winter near the edge of cankers and become active in the spring during warm, humid weather. Bacterial ooze, a sweet and sticky substance containing bacteria and plant sap, flows out of infected trees through natural openings and cracks in the bark. Bacterial ooze can be spread to young succulent growth on nearby plants by wind, rain, or birds. Most often, pollinating insects attracted to the sweet smelling ooze are responsible for transmitting the bacteria to flowers of susceptible hosts.
Infected flowers first appear water-soaked, then shrivel, turning brown or black. As the infection progresses, leaves on the same spur turn dark brown or black as though scorched by fire (Fig. 1). The dark, shriveled leaves hang downward and usually cling to blighted twigs. Infected shoots, twigs, and suckers turn brown to black and often bend in a characteristic shepherd's-crook (Fig. 2). Infected immature fruit turns dark, shrivels, mummifies, and rots. Mummified fruit may cling to the tree for several months.
A canker is formed when an infection progresses into larger branches. The host may produce callus tissue that walls off the canker, or a host may continue to be infected causing death of the branch. Weather conditions are an important variable influencing the severity of fire blight. New infections may occur throughout the growing season during warm, humid weather, and are especially common after a summer hailstorm when bacteria are washed into wounds created by hail.
Cultural practices, such as proper pruning, fertilizing, and site selection, can help prevent or minimize fire blight. Young, succulent growth is very susceptible to fire blight, so avoid heavy pruning which stimulates excessive new growth. Prune young trees annually during dormancy (late winter) to eliminate the danger of large cuts which may promote the growth of suckers. Use a balanced fertilizer early in the spring to encourage tree growth during the first part of the season when temperatures are cooler and less conducive to the spread of fire blight. Excess nitrogen stimulates new growth, which is highly susceptible to fire blight, so fertilize only as needed. Fertilizer applied to the lawn is often adequate for the needs of nearby trees. Trees with dark green, well-developed leaves and adequate growth would not benefit significantly from an application of fertilizer. Plant new trees on the proper site to minimize stress.
Photo: U of MN Plant Disease Clinic
Diseased twigs, branches, and trees may be removed in late winter. Cuts should be made at least six inches below the diseased area into healthy wood and at a proper pruning site. Summer pruning can be hazardous, however highly susceptible trees may be killed if the disease is not pruned out as soon as possible. Infected twigs and sprouts which are pruned in summer should be cut at least twelve inches below the point of visible infection and at a proper pruning site. If fire blight is seriously damaging a cotoneaster hedge, cut the hedge about six inches above the ground in late winter. If only a few stems are blighted, they can be removed as described for trees. Pruning shears should be dipped for five seconds in a freshly made 10% bleach solution (one part bleach to nine parts water, equivalent to 1½ cups bleach in 1 gallon of water) between each cut.
If trees are severely infected or if fire blight continues to be a yearly problem, a copper-based fungicide like Bordeaux or a fire blight spray containing streptomycin sulfate can be applied. Follow directions on the label for application procedures. Read the label carefully and apply only as directed.
The following list includes varieties of apples and crabapples and their resistance or susceptibility to fire blight. Susceptibility to other diseases, especially apple scab, should be considered when purchasing new trees. Note that the resistance of the following varieties should be used only as a ranking for comparison between varieties. Resistant varieties can still become infected with fire blight!
|Most Resistant||Somewhat Resistant||Less Resistant||Least Resistant|
|Apples/*Crabapples grown for eating||Chestnut, Crab, Dolgo*, Haralson, Honeycrisp, Liberty, Red Baron, Red Delicious||Zestar!||Centennial*, Cortland, Keepsake, McIntosh, Minjon, Regent, State Fair, Sweet Sixteen, Whitney*, Zesta||Beacon, Fireside, Honey Gold, Wealthy|
|Ornamental Crabapples||Adams, Adirondack, Camelot, Lancelot||Candymint, Pink Princess, Red Splendor, Silverdrift||Golden Raindrops, Purple Prince, Red Jade, Spring Snow, SugarTyme||Madonna, Sinai Fire, Snowdrift|
Representative trade names may be included along with generic names. This information is supplied with the understanding that no discrimination is intended and no endorsement is implied.
Agrios, George N. 1978. Fire Blight of Pear and Apple, pp. 457-462 In: Plant Pathology, Second Edition. Academic Press, Inc., Orlando, FL. 703 pp.
Beer, S.V. 1990. Fire Blight, pp. 61-63 In: Compendium of Apple and Pear Diseases. The American Phytopathological Society, St. Paul, MN. 100 pp.
Van Der Zwet, Tom and Keil, Harry L. 1979. Fire Blight, A Bacterial Disease of Rosaceous Plants. USDA Agriculture Handbook #510, Washington, D.C. 200 pp.
NOTE: Resistance information on apples was updated by Dave Bedford at the U of MN Horticultural Research Center, and Harold Pellett and Nancy Rose at the U of MN Landscape Arboretum. University researchers will be continually screening for fire blight susceptible varieties. It is possible that additional data may change current resistant-susceptible categories of some varieties. Readers may wish to check in the future for updated information.
Chad Behrendt, Crystal Floyd