You may have noticed dead sprigs at the tips of tree branches in your park’s fruit trees. This burnt-looking die-back is the namesake symptom of fire blight, a common disease that affects plants in the rose family including apples, pear, peach, quince, spirea, and pyracantha.
Fire blight is caused by the bacterium Erwinia amylovora. Native to North America, the disease was first documented in apples, pears, and quinces in the 1780s in the Hudson Valley. Before the introduction of apples and pears, it likely occurred in wild crab apples, and young trees are more impacted than older, more established trees. Although extensive research has been done on treatments for the disease in commercial orchards, according to The Organic Center, “There is not cure for fire blight, and there is no single ‘silver bullet’ that will prevent fire blight infection.” That being said, watching for the first signs of infection and acting early can help reduce the disease’s impact.
Symptoms of Fire Blight
Dormant in cold weather, fire blight is transmitted in spring during blossoming periods and is most active between 75-95˚ Fahrenheit. Insects, wind and rain spread the bacteria from infected tissue into plant blossoms, where the infection multiplies. After petal fall, symptoms are first visible when the base of the flower (receptacle, ovary, and peduncle) becomes water-soaked and dull. As the fruiting body develops, it becomes brown/black and omits bacterial ooze. When the infection spreads, leaves wilt and turn dark brown, and the tip of the shoot bends into a “shepherd’s crook.” The dead shoot clings to the plant long after it has died. Eventually, dark cankers develop on the plant where the infection overwinters. The disease can also enter the plant through damaged leaf surfaces (from hail or insects). See Dr. Dave Rosenberger’s blog for good photographs of early symptoms.
Systemic infection of fire bight in a pear tree in the Fort Ross Orchard. The entire tree was subsequently removed (NPS Photo).
Treatment of Fire Blight
Fire blight should not be removed during periods of high humidity and diseased material should not be composted. During the winter, trees should be monitored several times, in case cankers were missed in previous removal efforts. Avoid over-fertilizing trees and limit winter pruning to prevent excessive shoot growth, as older wood is less susceptible to the disease.
When pruning infected trees, sterilize tools between cuts and between work on different trees using a 10% bleach solution or a small torch (to prevent rusting). If the disease is systemic and cankers have recurred after several attempts at removal, consider removing the entire tree. When replacing fruit trees, explore the use of fire blight resistant rootstock and cultivars.
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