Symptoms and Signs
Ceratocystis cankers first develop as elongated water-soaked areas with amber-colored gum at the canker margins. Infected tissue turns dark brown, and eventually the affected area becomes sunken. Dark stains may permeate the heartwood and extend longitudinally 20 inches or more past the margin of the canker on the bark. Cankers can girdle and kill infected branches, scaffold limbs, or entire trees. A limb 4 to 6 inches in diameter can be girdled in 3 to 4 years. However, some cankers become inactive and heal after two or three growing seasons. Trees consistently damaged by tree shakers year after year are most severely affected by this canker.
Cankers caused by Ceratocystis fimbriata may be confused with branch cankers caused by Phytophthora spp.
|Ceratocystis canker||Phytophthora cankers|
|Gumming: gum balls generally occur at the margins||Gumming: gum balls occur throughout the diseased area|
|Speed of canker growth: cankers grow slowly from year to year||Speed of canker growth: cankers advance rapidly but stop when the pathogen dies out in hot weather and do not resume.|
|Canker location: cankers are always associated with bark injuries.||Canker location: cankers can develop on uninjured portions of the tree or around pruning wounds,|
Comments on the Disease
Ceratocystis canker is a widespread disease of almond. Cankers often develop in areas of the trunk or branches that have been damaged by equipment, especially mechanical shakers during harvesting. Cultivars most susceptible to the disease are Mission, Ne Plus Ultra, and Nonpareil.
Obvious wounds, such as those caused by harvesting equipment or pruning, are the most common entry sites for Ceratocystis, but inconspicuous wounds on small twigs or branches are also susceptible. When pruned branches are pulled from the tree, small twigs are broken. If these or minor bark abrasions are infected by Ceratocystis, small cankers form. Once infection has occurred, there is no cure. Cankers will continue to expand each year until scaffolds are girdled and die.
Ceratocystis is spread by several species of sap-feeding beetles and a fruit fly. These insects feed on the fungi in diseased trees, including other stone fruit trees, and either ingest fungal spores and later excrete them or come into contact with the spores and transport them on their bodies to new locations. Insects that have fed on spores as larvae can retain the fungus through pupation and emerge with it as adults. The fungus can also contaminate adult insects in winter months if the adults inhabit old bark wounds.
Following a bark injury, most almond trees are susceptible to Ceratocystis infections for 8 to 14 days. Once it infects cambium, the fungus can invade healthy bark tissue and young xylem tissues. Dark stains permeate the sapwood and heartwood, but the fungus seldom penetrates further than the xylem of the previous year's growth. The fungus grows more rapidly in smaller branches, and these branches are killed sooner.
The most effective way to prevent Ceratocystis canker is to avoid shaker injury to trunks and scaffolds. If the bark is injured, shave the rough portions to promote callus formation.
To control established cankers, you can perform tree surgery. Cankers are not easy to remove, however, and if you miss some of the infected tissue, the fungus can continue to grow and the canker will return. More often than not, tree surgery must be done year after year until you finally remove all infected tissue, and it may not be economically feasible.
The best time to carry out tree surgery is from December to February; both the pathogen and the insect vectors are less active at this time. However, surgery at this time of year may provide an opening for infection by aerial Phytophthora species and wood-decay fungi. Also, surgery must be done when dry weather conditions are predicted, as rain may allow other pathogens of almond wood to produce infectious inoculum.
- Remove infected bark and 0.25 to 0.5 inch of the woody tissue underneath the bark. Extend the cut at least 1 inch beyond the visible canker margin.
- If surgery is done in the winter months when few insects are active, leave the wound undressed. The value of putting dressings on tree wounds following surgery has not been established, and it slows the healing process.
- Recheck the area the following year. If the canker has returned, repeat the process.
Avoid clamping the shaker head on areas of the tree that have had surgery, as this can reopen wounds on a healing callus.