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° 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 mortal attacks.
Identification
“Shepard’s Crook” is the infamous sign of fire blight with a blackened or burned twig tip and appearing like a fish hook. 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 sign of infection.
Disease Cycle
The bacteria enter the plant through blossoms, leaves or wounds. Initially the bacteria emerge from the plants 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.
Management
Cultural:
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.
Pruning:
Often the bacteria will move down a twig killing the cambium all the way around and causing all growth above it to appear water soaked and dead. Other times only part of the twig will be girdled with bacteria 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 prune out entire limb if more than 50% has been girdled by the disease. If less than 50% then it can be cut out with sharp tools. Although not ideal, sometimes it is necessary to prune during the growing season when the bacteria is extremely active.
Reasons:
Current year growth infected
Safety
Clearance (e.g. house, walk, etc.)
If pruning during warmer months use a disinfectant on tools between each cut. Lysol or an alcohol solution of 70% will suffice. Winter pruning (December, 1st thru March, 1st) is ideal to avoid the spread of the disease. It just isn’t active during this time which means it is much less likely to spread from one cut to another.
Chemical:
Several sprays are available to be applied at time of blossom. The difficulty is that the flowers do not all open at the same time and we can’t 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 I commercial applicator is being used. We recommend two sprays once the first few flowers have opened and again 7-10 days later.