Fire blight is a highly destructive and invasive disease among fruit trees. Entire orchards can be effectively destroyed if fire blight is not properly controlled or eliminated. To help control the outbreak of fire blight on pear trees, ongoing maintenance and control programs are used in conjunction with the timing of the growing season. Understanding the causes and signs of fire blight helps in the treatment and prevention of the disease.
Fire blight is a fungal disease that affects fruit trees, most notably pear and apple trees. This disease can become serious and kill the tree affected. It is a sporadic disease, and can survive cold winters as dormant spores in dead plant material around the base of the tree. Bosc pears are especially susceptible. Fire blight often attacks younger trees that are more prone to infection.
The cause of fire blight in pear trees is bacteria labeled Erwinia amylovora. This bacteria can be spread by insects, splashing rainwater or improperly washed pruning tools. Insects which can carry the bacteria include flies, bees, aphids and leaf hoppers. Bacteria enter healthy blossoms through the open stigma or anthers, traveling up twigs to larger branches into the trunk, and eventually reaching the root system. Temperatures of 65 degrees or higher combined with a trace of humidity or rain over a 24-hour period is enough to trigger the spread of the bacteria.
Signs of fire blight in pear trees are most evident when blossoms and spurs are developing. Infected blossoms turn brown and wilt. Twigs and branches infected with fire blight darken and develop a water-soaked appearance. The infected twigs or branches will turn black; their ends will curl into the "shepherd's crook," which is the most common sign of fire blight in pear trees. A yellowish tan ooze appears at the base of affected areas; this is often the first noticeable sign of infection. Trunks and limbs develop cankers; these appear as water-soaked areas at first, turning brownish red and then very dark brown or black. The bark will appear sunken in the area and eventually separate from the infected area. Cankers will encircle a limb and kill the limb. Dead leaves, fruit and blossoms will remain on the tree. The entire tree will take on a burnt look, giving the infection the name fire blight.
Fire blight can kill pear trees. Pear crops from infected trees can be destroyed. Young trees are the most susceptible and can be completely overrun by the disease in one season. Trees that survive the attack can be severely disfigured and stunted. Scars will develop in areas where cankers have been removed or infected limbs have been cut away.
Proper management of orchards, fertilizers and irrigation can help prevent fire blight on pear trees. Too much nitrogen fertilizer, excessive pruning and over-irrigation provide the best breeding ground for fire blight. Use of resistant varieties also helps slow down the spread of the disease. Keeping ground cover cleared of dead plant material removes the breeding ground for the initial bacterial infection. Applying antibiotic and copper sprays helps in preventing and slowing fire blight; blossom sprays of streptomycin or terramycin timed at the first stages of blooming help greatly diminish the infection. Diligent observation and removal of infected branches or twigs help keep fire blight from spreading. Destroying infected branches away from the trees reduces the chance of fire blight reinfecting the orchard.