Fire blight is a common disease of plants in the rose family caused by the bacterium Erwinia amylovora. Apple, crabapple, pear, fire thorn, hawthorn, and quince are some of the most susceptible species. Symptoms first appear in the spring as new shoots turn black and bend downward forming a "shepherd's crook". Leaves droop and turn a dark color. Individual branches may look like they were scorched by fire.
The bacteria can survive the winter in cankers (sunken diseased areas) and diseased shoots. If moisture levels are high and the temperature is between 65 and 85 degrees, white bacterial ooze can start to drip from the diseased areas. The sticky, sweet ooze is attractive to insects that may pick up bacterial cells and carry them to healthy parts of the plant, such as flowers and leaves.
Sanitation is the most important management recommendation for controlling fire blight. The best time to prune is when the plants and the bacteria are dormant. Also, pruning while plants are dry helps prevent spreading the bacteria because they can be carried in water. Pruning cuts should be made about 12 inches below the diseased areas. Whenever possible, plant fire blight resistant cultivars. Bactericides (antibiotics or copper sulfate) are available, but they are typically only used by commercial growers. Without good sanitation bactericides will be ineffective.
This article originally appeared in the May 24, 2002 issue, p. 69.