Fire blight is a bacterial disease that causes concern to growers with apples, pears, crabapples, and ornamental Callery pears. You might also see infection on other rosaceous hosts, such as cotoneaster, hawthorn, quince, firethorn, and mountain-ash. Fire blight is caused by a bacterium (Erwinia amylovora), and the bulk of the infection occurs during flowering when temperatures are warm (mean daily temperature of at least 60 degree F) and conditions are wet. The causal bacterium may spread by wind, water, equipment, and animals. Rain or insects may move the bacterium from cankers and bark to open blossoms, vigorous shoot tips, and leaves. As long as warm, wet conditions continue during bloom, the bacterium can continue to infect. Management practices focus on controlling the bacterium during flowering. The bacterium is also known to cause infection directly through wounds made during a hailstorm.
Look for water-soaked or wilted new growth that quickly turns brown to black and remains attached to the stem. Also, dark cankers may develop in the wood, especially on edible and ornamental (Callery) pears.
Because the bacterium originates on old cankers and bark infection, removal of branches killed by fire blight is extremely helpful in disease control. This should be done when the tree is dormant or after the infection, when weather is hot and dry. Disinfecting pruning equipment is an important step is reducing disease spread. Use 10% household bleach or similar disinfectant between cuts.
If you are thinking of planting new trees, do a bit of searching for resistant varieties. It will be well worth your search. Among the ornamental Callery pears, ‘Aristocrat’ and ‘Autumn Blaze’ have good resistance reports. Also refer to the University of Illinois Report on Plant Disease on fire blight at http://www.ag.uiuc.edu/~vista/horticul.htm.
Pesticides may be used for fire blight prevention. A dormant (early-season) application of copper may be of some benefit to kill bacteria as it oozes from cankers. Copper compounds are usually applied before green tip (when the tips of green leaves begin to emerge from buds). Copper products may include Camelot, Champion, Chipco Aliette, Junction, Kocide, Magellan, Nu-Cop, Phyton 27, and others. Antibiotics are used in commercial fruit-production areas to prevent fire blight. Sprays are made during bloom, specifically when no more than 10% of the blooms are open on the tree. There are few antibiotics available or recommended for ornamental plants. It is best to rely on nonchemical management practices to control fire blight on ornamental plants.
A few other helpful management tips include avoiding high-nitrogen fertilizers that promote lush susceptible growth, removing water sprouts when they are small, and removing nearby neglected pear and apple trees.