By Dr. Thomas T. Yamashita
Ceratocystis canker (commonly referred to as mallet wound canker) is a disfiguring but treatable disease of almond trees inflicted by the fungus Ceratocystis fimbriata. C. fimbriata is an extremely prevalent and adaptable organism that can survive in host species as diverse as sweet potato, cacao, cherry, coffee, mango, sycamore, taro, apricot, and of course, almond.
The most obvious symptoms of Ceratocystis canker are the namesake cankers which appear on the limbs and trunks of infected almond trees.
Ceratocystis canker is a type of fungal gummosis, the first evidence of which manifests in the form of long, wet patches of bark, with orange or amber-colored gum exuded at the edges. This moist area of bark eventually browns and becomes sunken. Cankers can continue to appear moist or even soaked, but may also take on a dry appearance.
However, the affects of Ceratocystis canker are not limited to the surfaces of infected trees. Shaving the bark away will reveal a whitish mycelial (fungal) growth, and beneath that brown-stained wood. Infected tissue may penetrate all the way into the heartwood of the tree, and extend as much as 2 feet beyond the margins of the overlying canker.
Cankers can be found on the trunks, limbs, and branches of affected trees, and often initially have a patchy or striped look. Over time, cankers will spread, and can encircle or ‘girdle’ affected limbs. Limbs with cankers will experience wilting, with smaller girdled limbs dying quickly once fully encircled, while larger limbs may survive for years.
Ceratocystis canker is spread by a variety of opportunistic sap-feeding beetles and fruit flies.
When an almond tree is injured, the wound will exude sap in order to protect the exposed wood. This draws the attention of a variety of species of fruit flies, driedfruit beetles, sap beetles, and oribatid mites. These insects often carry with them the spores of C. fimbriata, which they pick up—either by casual contact or through ingestion—while feeding on diseased trees.
While feeding on the sap secreted from a freshly wounded tree, the insects will deposit the spores. (Spores can also be passively transported to tree wounds by wind, rain, and sprinklers.) Once the spores come into contact with the tree, they germinate and invade the bark and xylem tissues. While the wood beneath often takes on a brown stain, these tissues rarely harbor the fungi, but instead are exposed to injury by the damage to the tissues above.
Almond trees are particularly susceptible to Ceratocystis canker because of the injuries commonly inflicted through careless agricultural practices.
The common name ‘mallet wound canker’ betrays the fact that the prevalence of this disease has been greatly aided by the practices of generations of California almond growers. Almond trees are commonly damaged by tractors and trimming equipment, as well as by shakers during harvesting.
While infection often occurs through damage to tree trunks and limbs, even damage to small branches and twigs is sufficient for infection. This means that even careful growers often make their trees prey to Ceratocystis canker during trimming. A trimmed branch may get stuck, and in the process of pulling it free, break off innumerable small twigs and create dozens of potential infection points.
Controlling Ceratocystis canker requires a combination of extreme care, prompt treatment of damaged bark, and removal of diseased tissue.
The best means of controlling this disease is avoiding causing damage to trees. Thus, growers are advised to take special care in their orchards, and to train laborers in the proper use of equipment and to show care when trimming trees or harvesting. It is also recommended that irrigation be reduced or eliminated entirely for the 2 to 3 weeks prior to harvesting, as the presence of water increases the likelihood of tissue bruising. For the same reason, pruning should be avoided prior to wet weather.
If the bark of a tree has been damaged, it is vulnerable to infection by C. fimbriata for up to 14 days. Ne Plus Ultra, Nonpareil, and Mission cultivars are particularly prone to infection. For prevention, damaged bark should be cut out, and the wound shaved and then sprayed (not painted) with the following mixture:
- Interior white latex paint – 3 quarts
- Water – 1 quart
- Daconil fungicide – 1 ounce
- Silicone surfactant – ½ ounce
This treatment should be repeated once per month for 4 months, or until a callus has fully formed over the wound.
You may also opt to use a commercial insecticide paint product as an alternative to the above regimen.
There is no cure for infection. No product currently on the market is capable of clearing C. fimbriata from affected tissues. However, cankers can be surgically removed. Extreme care must be shown in doing so, as any infected tissue left behind will result in the reoccurrence of canker. Multiple surgeries may be necessary to fully remove infected tissue.
The best time to perform surgery is between the months of December and February, when temperatures are lowest, and insects less active. However, conditions should also be dry, which may prove challenging if your region is especially prone to wet weather during this period.
Surgery consists of removing infected bark and the surrounding 1 inch of bark, then cutting out ¼” to ½” of the infected wood beneath. If surgery is performed during cold, dry weather, treating the area is unnecessary. However, if you must perform surgery during periods when insects are active, treat the wound with the mixture described above.
Until a callus has formed over the wound, avoid disturbing the tree as much as possible. Shakers should not be clamped on the regions surrounding the surgery area, as this may damage the callus and reopen the wound. Continue to keep a close eye on the tree and repeat the surgery if the canker reappears.