Fire blight is a bacterial disease of trees and shrubs in the rose family caused by Erwinia amylovora, which is found in much of the world. It can be a catastrophic plant disease and can cause the destruction of whole orchards of apples and pears. Of pear tree diseases, fire blight is the most destructive. It also affects other plants, such as loquat and raspberry. The name comes from the appearance of the disease, in which the leaves and stems are wilted and darkened, looking like they have been burned.
The type of bacterial damage it inflicts causes fire blight to be classified as a vascular wilt. The bacteria and its by-products build up in the part of the plant that transmits water and nutrients, causing this system to become plugged. The above-ground parts of the plant then wilt and die.
The first appearance of fire blight is usually during flowering in the spring. The flowers wilt and turn black. The infection spreads to the twigs, which wilt from the tip downward. The infected leaves cling to the stem, and both the leaves and the stem become brown. The bacteria can spread downward into the main limbs and trunk, and can kill the whole plant if it reaches the root. Plants that survive can over-winter with the disease and then infect surrounding plants in the spring.
Once plants are infected, fire blight can form lesions that exude a liquid containing the bacteria. This exudate can spread to parts of the same plant or infect other plants. It is easily splashed by rain drops, or carried by insects or birds. Tissue that is already injured is at great risk of becoming infected. Also, bees carry the bacteria to other flowers. Fire blight is a highly contagious plant disease.
For plants that are already infected, one should cut 8-12 in (20-30 cm) below the infected branches and dispose of them. There is a clear delineation between the parts of the limb that are infected and those that are healthy. It is very important to disinfect all gardening or cutting tools after each use, so the infection is not spread to other limbs or plants. The standard recommendation is to use a solution of 70% denatured alcohol, since bleach can damage the tools. Partially resistant apple and pear trees are available and should be used whenever possible.
In some areas, it is possible to use chemical control. This can be difficult, however, since many strains of the fire blight bacteria have evolved resistance to antibiotics. One can try using streptomycin soon after the first flowers open. It is necessary to make three to four applications during the flowering period. Unless there is a hailstorm, avoid using streptomycin after the flowers are finished blooming in order to minimize the chances of developing resistant bacteria.
Cultural practices can help to minimize the susceptibility of the plants to the pathogen. Over fertilization or overly aggressive pruning can cause the rapid growth of susceptible twigs. Both practices should be avoided.
Fire blight is one of the bacterial diseases that is more of a problem during wet, hot weather. The bacteria multiply much more quickly at temperatures over 63° F (17° C) than they do at those under 56° F (13° C). Initial infection of the blossoms is not usually a problem during cool springs. Commercial growers often use forecasting models, based on the temperature and amount of bacteria present, to predict whether antibiotic application during bloom will be necessary. Several European countries have tried to eradicate this pathogen, but it continues to spread.