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.
There is no single management practice that completely controls fire blight. A combination of cultural practices to reduce tree susceptibility and disease spread, and chemical control to protect against infection can reduce the disease. 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.
Trimtect does not have direct antibiotic activity against the fire blight pathogen (Erwinia amylovora), but can reduce the host susceptibility. A foliar spray treatment in the spring temporarily suppresses rapid shoot elongation during the peak infection period by the fire blight pathogen potentially reducing the susceptibility of the host to shoot tip infections. Cambistat, a tree growth regulator, can also be used to minimize shoot growth, which will minimize the shoot blight stage of a fire blight infection..
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.
Copper hydroxide will effectively reduce shoot and blossom infections, and will leave a visible white residue. Bacastat will reduce only the twig stage, not the blossom stage of infection. Optimal results with Bacastat occur when applied prior to the onset of visual symptoms.
Signs of Damage
Fire blight creates different types of symptoms, depending on the plant part that is attacked and when the plant is attacked.
- 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.
- 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 tree.
Photo: Patrick Anderson, Rainbow Ecoscience
Trees At Risk
Plants in the rosaceous family including
- Ornamental pear
- Mountain ash
- Bacteria over winters at the edges of the cankers, in the wood.
- New infections usually begin in the flowers each spring.
- When conditions are wet and warm in the spring, the bacteria multiplies and may appear as creamy ooze on plant tissue.
- The bacteria is spread to natural openings or wounds by wind, water, insects, people, or pruning tools.
- Hot summer weather generally slows or stops the disease.
University of Georgia Extension
Washington State University
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