Branch canker and dieback of avocado, formerly Dothiorella canker
Branch cankers and limb dieback of avocado are caused by a complex of fungal species in the families Botryosphaeriaceae and Diaporthaceae, such as Phomopsis (=Diaporthe) species. These fungi can also infect and decay avocado fruit and leaves.
Identification
Symptoms of branch canker and dieback include diseased or dead tissue in bark and wood (cankers), leaf scorch (leaf death), shoot death (dieback), and stem end rot of fruit. Cankered bark exudes reddish sap that dries to a brown to white powder. The canker commonly is an area of bark that is cracked, darkly discolored, slightly sunken, or all of these. With older cankers, bark may shed from the damaged area or be crumbly and easily removed. When outer bark over cankers is removed, the inner bark and wood is brown, orangish, or red instead of a healthy pale color.
When much of the xylem under a canker becomes killed by the fungi, the limb dies outward and upward from the canker. Brown, dead leaves remain attached to the dead limb.
Symptoms of branch canker and dieback closely resemble those of Phytophthora trunk canker and crown rot (Phytophthora mengei =P. citricola, formerly called citricola canker). When bark is removed from a canker, unhealthy brown to orange or reddish tissue is apparent with both diseases. However, branch canker and dieback generally occurs higher above the ground, beginning around the first main branch crotch or higher. Phytophthora cankers generally occur on the lower trunk and root crown. Branch canker and dieback infects much smaller limbs, such as twigs and small branches, as well as the upper trunk and large limbs.
When cut into with a knife, branch canker and dieback is sometimes found to extend deep into wood. Phytophthora cankers discolor only a shallow layer of outer wood. Except when trees are young, branch canker and dieback generally is not as damaging as Phytophthora canker.
Life cycle
The fungi that cause branch canker and dieback can infect only through wounds. Heavy rainfall increases spore production and infection potential, and the spores are spread by splashing water and wind. Stress makes trees much more susceptible to this disease. Common causes of stress are low-quality irrigation water, nutritional deficiencies, and severe insect or mite feeding. Poor irrigation practices especially make trees more susceptible to branch canker and dieback.
Damage
Branch canker and dieback can be a serious problem in new plantings. Young trees sometimes arrive from the nursery with latent (not yet apparent or developed) infections at the graft union. After planting when the infection kills the graft union, the dead scion retains a dry, brown canopy and green shoots and leaves sprout up from the rootstock. The infected graft union may be unusually swollen and rough prior to the young tree’s death. Cutting inside at the graft union reveals dark, discolored wood that can extend through the entire width of the small trunk.
Branch canker and dieback is usually of minor importance in established, older trees. Scattered small branches and sometimes large limbs can die back. Usually, branch canker and dieback will not affect an entire, older tree, and the tree as a whole will remain productive. In unusually severe cases, the main trunk of older trees becomes girdled, killing the tree.
Solutions
Avocados on Mexican rootstocks are generally more resistant to this disease than are Guatemalan cultivars. Consider planting rootstock cultivars that are resistant to this disease including Barr Duke, Borchard, Duke 7, Duke 9, Dusa (Merensky 2), Latas (Merensky 1), Topa Topa, and Toro Canyon. Buy young trees from a reputable, commercial supplier that sells disease-free plants.
Where branch canker and dieback is a problem, rely primarily on sanitation and good cultural practices to control it. Prune out dead limbs and twigs. Dispose of dead wood and old fruit well away from avocado trees. Prune and harvest only when the weather and leaf, wood, and soil surfaces are dry. Correct any environmental or nutritional stresses and keep all pest problems to a minimum. Maintaining an appropriate amount and frequency of irrigation is especially important. Leach the soil periodically and use low-salinity water if salt toxicity is a problem.
Adapted from Integrated Pest Management for Avocados and Pest Management Guidelines: Avocados, University of California Statewide Integrated Pest Management Program (UC IPM). |
Limb killed by branch canker and dieback.
Bark removed revealing brown, orange, and reddish diseased vascular tissue and wood.
Fungi that cause branch canker and dieback also infect leaves (shown) and fruit.
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