Fire blight is caused by a bacterium, Erwinia amylovora, and it only affects members of the rose family. This includes more than 75 different kinds of trees and shrubs, including apple and crabapple (Malus), cotoneaster (Cotoneaster), hawthorn (Crataegus), mountainash (Sorbus), pear (Pyrus), pyracantha (Pyracantha), quince (Chaenomeles), rose (Rosa), and spirea (Spiraea). This disease is indigenous to North America, but has been reported in New Zealand and Great Britain.
In early spring, about 14 days after the flowers have opened, the petals become water-soaked, turn brown, and then black. This condition, which affects single flowers or entire clusters, is called “blossom blight.” New leaf growth can also be affected; the leaves wilt suddenly and turn black or brown, giving the plant an appearance of having been scorched by fire. Often the new blackened tips curl over, forming a “shepherd’s crook.” Bark lesions, called cankers, can develop on the bark of larger branches that have become infected through flowers or twigs. Cankers are localized dead areas in the bark of twigs, branches, or trunks. They appear as unusually shrunken, depressed, or discolored areas of dying tissue, which may split open and expose the wood beneath. These depressed and discolored canker areas produce an opaque grayish or brown liquid, which is the source of infection.
The annual cycle of fire blight is not complicated. Bacteria overwinter at the margins of cankers. In spring, warm, wet weather, above 65 degree F, initiates bacterial activity, resulting in a canker “ooze.” This ooze is transmitted to flowers and twigs by water, birds, bees, and humans. One to three weeks later, fire blight symptoms appear.
- Remove all infected branches in spring as symptoms appear. Prune at least 6 inches below the visibly infected area. Pruning tools should be sterilized before and after each cut. Common household bleach (10%) or rubbing alcohol is a good disinfectant.
- Avoid an over-stimulation of plant growth with high rates of nitrogen fertilizers.
- In areas where fire blight is common, avoid planting susceptible plants. Some plant groups, such as crabapples, have resistant cultivars.
Contact the Plant Clinic (630-719-2424 or firstname.lastname@example.org) for current recommendations. Use pesticides safely and wisely; read and follow label directions