July 20, 2019
If you see brown or blackened leaves, a tan oozing substance or streaks on the branches of certain ornamental or fruit trees or if it looks like your fruit tree has been scorched by fire, your tree may be suffering from a fire blight infection.
Fruit trees such as pear and quince are extremely susceptible. Apple and crabapple are also frequently damaged. Fire blight on ornamentals is less common, but those that are susceptible include firethorn (also known as pyracantha), hawthorn, spiraea, cotoneaster, toyon, juneberry or serviceberry, loquat and mountain ash.
The fire blight bacteria overwinters in long, narrow cankers on the tree and will present as wet-looking, brown, irregularly shaped, elongated lesions that develop in the bark and outer sapwood of spurs, branches and the tree trunk.
Before you grab your saw or pruning shears, carefully study and photograph the clues. There are specific management techniques that you can follow to save your tree, but you must act quickly and prune correctly, because fire blight is a fast-spreading, destructive disease.
Typically, in the spring, branch and trunk canker symptoms can appear as soon as the tree begins active growth. Open flowers are the most common infection sites, and the flower and flower stems will wilt and turn either black or brown depending on the type of fruit tree.
Successful removal of a fire blight infection is done in summer or winter when the bacteria are no longer spreading through the tree. At these times infections have ceased enlarging and canker margins are clearly visible.
Using the right sharpened tools for the job results in clean cuts and makes the job easier. Use hand pruners to snip twigs and branch tips less than 1/2 inch in diameter. For limbs up to 1½ inch in diameter, use long-handled pruners, which have better leverage. Pruning saws are best for branches more than 1½ inch in diameter.
When removing infected branches, the location of the cut is critical. The cut should be 8 to 12 inches beyond the visible damage to make sure you have removed all of the diseased tissue.
A telltale dark ring in the branch will indicate if the cut is not deep enough. To locate the correct cutting site, follow the infected branch to its point of attachment and cut at the next branch juncture down. This will remove the infected branch and the branch to which it is attached.
If the infection has traveled into a trunk or major limb, the wood often can be saved, but needs careful management by scraping off the bark down to both the outer and inner bark.
If you remove both the outer and inner layer of bark at the infection site, you will find the tissue closest to the infection site is brown. Further from the site it becomes red or orange streaks, and then red flecking. When scraping, look for long, narrow infections that can extend beyond the margin of the canker or infection site. If any are detected, remove all discolored tissue plus 6 to 8 inches more beyond the infection. This procedure is best done in winter when trees are dormant and bacteria aren’t active in the tree. Don’t apply any dressing to the wound. If the limb has been girdled, scraping won’t work, and the whole limb must be removed.
The disease spreads on contaminated tools, so it is important to disinfect the tools after each cut. A one- to three-minute soak in a mixture of one part unscented chlorine bleach and four parts water is an effective method of disinfecting tools. Chlorine bleach has a corrosive effect on tools if left on the metal too long. After pruning, wash the tools with soap and water, rinse them thoroughly and pat them dry before putting them away.
For more information on fire blight, including how to manage infected trees, check out the University of California integrated pest management program at ipm.ucanr.edu/PMG/PESTNOTES/pn7414.html.