Fireblight Disease on Pears
Fire blight, unlike most fruit tree diseases, is caused by a bacteria that can be spread from tree to tree by bees, aphids, pear psylla, and other insects. The bacteria mainly attack twigs and young shoots.
Affected branches wither and turn black or brownish black, as if scorched. Most branch tips, once infected, wilt rapidly, taking on a shepherd's crook shape. The bacteria enter the tree through the blossoms or through lush, tender new growth. Once inside, they multiply rapidly and begin to work toward the roots. An orange-brown liquid will ooze from pustules on the tree, particularly on warm days. This liquid contains a great number of bacteria, which may be spread by rain or insects. The bacteria form a canker under the bark and survive there through the winter, infecting more trees the following year.
Pruning to Control Fireblight
Fire blight is a very serious disease in most parts of the country; it can wipe out all susceptible trees in an orchard in one season. It can be controlled, however. Fire blight is most damaging during bloom, when the blossoms become entry points for the bacteria. If you have a mini orchard -20 to 30 pear trees - it would pay to use an antibiotic spray or dust during bloom and shortly after to prevent the blight bacteria from reproducing. Fire blight spreads rapidly in periods of warm, humid weather, so check the trees carefully at such times for signs of wilting. Remove all suckers and cut off any infected branches before the bacteria attack the tree further. Using a sharp set of pruning shears, cut off the wilting branch at least 8 inches below the point of last visible wilt. After each cut, dip the shears in a solution of 1 part household bleach to 9 parts water to avoid transmitting bacteria from one branch to another. (When you are finished, wash and oil your tools to prevent damage from the bleach.) Discourage lush growth because it is very susceptible to fire blight damage. If possible, remove other plants that may serve as hosts for the disease, including wild apples, hawthorns, mountain ash, and cotoneaster hedges. Prune out fire blight cankers in the winter when trees are dormant. Leaves remaining on blighted branches in autumn can indicate trouble spots. If you prune before the sap starts to flow in the spring, you don't have to sterilize your tools after each cut.
Control Pests Too
Control aphids and pear psylla to prevent them from spreading the disease. The chief pest of pear trees in eastern and western (but not central) United States is the pear psylla. This reddish- brown insect rapidly develops resistance to chemical controls; it causes significant damage by spreading pear decline and fire blight and sucking out the plant sap. The insects emit a sticky substance called honeydew on which a black fungus grows. Yellow jackets may congregate around the black fungus, indicating pest activity. Control adults with dormant oil spray in the fall when they are most susceptible. A fall spraying is better than a spring spray because it will not affect many of the beneficial insects that are present in the trees in very early spring. If necessary use a dormant oil spray in the spring to inhibit egg-laying and to kill any active adults present. The adults begin to lay eggs when the temperature gets up to 70° F, sometimes at slightly lower temperatures on a sunny day with no wind. You will have to use a 10-power magnifying glass to see the little yellow eggs at the base of the bud scales. As soon as you find any eggs, the first oil spray should go on. Spray again 7 days later. The insects don't seem to like laying eggs on this oily surface. Be sure to cover the tree thoroughly - until it's dripping. During the growing season use an insecticidal soap spray to keep pear psylla activity down.
This article is a part of our Fruit Gardening Guide for Pears.