Fire blight is a bacterial disease of apple, pear, hawthorn, pyracantha, mountain ash, cotoneaster, flowering quince and many other trees and shrubs. Even though it is always present in the urban landscape, the severity of new infections varies greatly from year to year depending on weather. A cool (65 degrees F), wet spring will encourage bacterial growth and we typically see a rampant outbreak. Some plants will suffer minor tip dieback while others will suffer fatal attacks.
“Shepherd’s Crook” is the infamous sign of fire blight with a blackened or burned twig tip that appears like an upside down “J”. Fruit and flowers will also be shriveled and burnt, or dry looking. Flagging branches (dead limbs with dead leaves still attached) and cankers are the most obvious signs of infection.
The bacteria enter the plant through blossoms, leaves or wounds. Initially, the bacteria emerge from the plant’s interior forming an “ooze” on the bark that insects like. It sticks to their bodies and they then carry it to blossoms where it enters the vascular system.
It can also get in via splashing water droplets that find their way through wounds from hail, pruning, insect or animal damage as well as natural openings like the stomata in leaves.
Once inside the tree, it kills the cambium and moves down the stem until it is stopped by the plants natural defenses or other intervention. Sometimes plants can wall off an attack and other times they cannot stop the attack. This has to do with the resistance of a particular variety as well as the plants overall health.
Damage is typically cosmetic and rarely harmful to plant health. Severe infestations can cause twig dieback on some plants and can be fatal in some instances. Damage is always to soft plant parts like leaves, flowers and new growth.
Annual pruning to reduce the amount of supple, new growth is desirable as it is particularly vulnerable to attack. Over fertilizing with nitrogen, especially quick release nitrogen, should be avoided.
Often, the bacteria will move down a twig killing the cambium all the way around the trunk or branch, causing all growth above it to appear water soaked and dead. Other times, only part of the twig will be girdled and growth above the canker will appear healthy and untouched by the disease. This is a tricky situation when pruning. There is a chance the branch will survive should the infected portions be removed and there is a chance it will continue to spread if it is left. Consideration must be given to overall health and percentage of dieback from the attack.
The rule of thumb is to prune out the entire limb if more than 50% has been girdled by the disease. If less than 50%, it can be cut out with sharp tools.
The idea of pruning trees infested with fire blight in the growing season has historically been known as a bad practice. However, this adage is somewhat misleading as trees can be pruned without leading to future problems and may even be necessary to reduce the amount of blight in a tree that could have a greater negative impact on the health of the plant if it is left intact for the remainder of the growing season. The added implementation of disinfectant tools (e.g. 70% alcohol or Lysol) in between cuts will also diminish the possibility of re-contamination.
Dormant pruning is ideal to avoid the spread of the disease. Fire blight isn’t active during this time which means it is much less likely to spread from one cut to another. However, proper pruning techniques coupled with disinfectant tools limit the potential for spread when pruning in the growing season.
Several sprays are available to be applied at time of blossom. The difficulty is that the flowers do not open at the same time and it’s impossible to know when an insect will be by to pollinate each flower. The best solution is to spray the flowers every few days. This can be very expensive if a commercial applicator is being used. We recommend two sprays, once the first few flowers have opened and again 7-10 days later.
A trunk injection of an antibiotic is also available and acts more as a curative than a preventative control. It is systemic, which means it will act similarly as antibiotics do for animals when ingested or injected.
Limitations include: tree health, obstruction of chemical transport inside plant (e.g. Cankers, dieback), and bacterial resistance to Oxytetracycline.