Fire blight is a bacterial disease caused by the pathogen Erwinia amylovora that attacks many trees and shrubs (apple, pear, cotoneaster and mountain ash are four of the most common targets). The disease usually appears in late spring or early summer when it attacks the blossoms and very young growth at the tips of the branches. The ends of the stems take on a burnt brown appearance which gives the disease its name as the plants look like they have been burned.
The twigs, flowers and leaves turn brown and the terminal of each stem often takes on a shepherd’s crook appearance as it curls. If fruit has formed it turns brown and hard and does not mature but stays on the plant. Later in the season fire blight can attack whole stems of plants and in the case of cotoneaster, the leaves become bright yellow or orange and appear to be on fire — another origin of the disease’s common name. It is easily spread by birds, insects, rain, and infected garden tools.
There is no cure for fire blight and limited protective measures that can be taken. The best treatment is to remove all diseased parts as soon as possible, disinfecting the pruning tool after each cut with a bleach or alcohol solution. Regular pruning and removal of infected material will control all but the most severe attacks. Because the disease can be spread by rain, it is prudent to avoid overhead irrigation. Another tactic is to plant resistant varieties of plants or to avoid susceptible species, so some gardeners will not use cotoneaster for that very reason, but showy mountain ash and oak leaf mountain ash appear to be resistant to the disease.
Raspberries are also susceptible to fire blight, that turns the berries and the tips of the branches brown. It is the same pathogen but a different strain than the one that attacks apples, pears, mountain ash and cotoneaster. This strain of fire blight is relatively uncommon, but is becoming more prevalent in the United States, so we might see more of it in the future.
One raspberry variety that is most susceptible to fire blight is “Boyne,” but some cultivars that are quite resistant are “Nova,” “Ruby” and “Avon.” Coloured raspberries, such as the yellow variety “Honey Queen” also show a high level of resistance.
If you have a raspberry patch, next spring keep a close watch on it as it comes into leaf and begins to bloom. At the first sign of tip damage, have the pruners ready to combat fire blight before it gets a head start. Perhaps someday there will be a cure for this troublesome bacterial infection.