Fire blight, Erwinia Amylovora, is a very destructive bacterial disease of trees in the rosaceous family, such as apple and pear trees. Fire blight usually starts with the blossoms or flowers and moves into the twigs and branches causing infected twigs to bend over, creating a “shepherd’s crook” appearance. Warm rainy springs and open wounds allow rapid spread of the disease. Fire blight is most severe before and during bloom when spring temperatures are warmer than average. If not managed, fire blight can destroy the blossoms, fruit, and stems of the plant, and even kill the plant.
Fire blight had two types of symptoms depending on the plant part that is attacked.
- Blossom Blight
- This stage appears around the time of petal fall. Infected blossoms look like they have been soaked in water, wilt, and then turn dark brown.
- Shoot Blight
- This is the most obvious stage of symptom expression, which appears one to several weeks after petal fall.
- As the bacteria progresses, leaves wilt, turn brown and remain on the tree. This creates a fire-scorched look; hence the name “fire blight.”
- The branch tips on infected twigs may bend over to create a “shepherd’s crook”.
- Fire blight can also cause dark, sunken cankers that have a narrow callus ridge along the outside. This callus ridge differentiates cankers caused by fire blight from cankers caused by fungal pathogens.
- A creamy bacterial ooze may appear on cankers or fruit.
- Fruit may dry and remain on the tree.
- Primary hosts for fire blight are plants in the rosaceous family including cotoneaster, firethorn, crabapple, hawthorn, ornamental pear, and mountain ash
- The bacteria overwinters at the edges of the cankers, in the wood.
- The bacteria create a creamy ooze during warm and wet conditions which can spread to natural openings or wounds in the tree by wind, water, insects, people, or pruning tools.
Managing Fire Blight
The type of management for fire blight will be determined by which symptoms of the disease you are aiming to control. A combination of cultural and chemical practices will yield the best results.
Treat trees that expressed visible symptoms in previous growing seasons or high-value trees growing adjacent to symptomatic trees. If not managed, fire blight can destroy the blossoms, fruit, and stems of the plant, and even kill the plant.
Applications are typically made in the spring to suppress symptoms that would appear later in the growing season. Treatments can range from foliar sprays to protect the blossoms and branches, to trunk injections to reduce the severity of the shoot blight stage and leaf scorching. Growth regulators may be recommended to reduce shoot elongation which can reduce the severity of the shoot blight stage.
Other Treatment Practices
- During dormancy (dry weather) prune infected branches.
- During growing season prune at least 12 inches below the diseased area and disinfect pruners between cuts.
- Avoid the use of high nitrogen fertilization, which promotes increased shoot elongation.