Symptoms and Signs
Symptoms of branch canker and dieback include exuding reddish sap that dries to a brown and white powder. Bark may be cracked, darkly discolored, or slightly sunken. With older cankers, the bark may be friable (crumbly) and easily removed from the damaged area. Under the canker, the inner bark and wood is reddish brown to brown instead of the normal pale color. When the branch is cut transversely, a characteristic wedge-shaped canker extending deep into the xylem may be visible. If much of the xylem becomes infected, limbs may collapse and leaves quickly turn brown, but remain attached.
Symptoms observed on avocado trees with branch canker and dieback include
- shoot blight and dieback
- leaf scorch
- branch cankers
- stem end rot of fruit
Branch cankers closely resemble Phytophthora trunk canker. Branch cankers usually occur higher above the ground, beginning around the first main branch crotch or higher. Branch canker can affect twigs and smaller branches, as well as the upper trunk and large limbs. Branch cankers sometimes extend deep into wood, whereas Phytophthora cankers only discolor a shallow layer of outer wood. Except when trees are young, branch canker is usually not as serious as diseases caused by Phytophthora spp.
Branch canker and dieback can be a serious problem in new plantings; stock sometimes arrives from the nursery with latent infections at the graft union. When the infection kills the graft union, the dead scion retains a dry brown canopy, and shoots and green leaves sprout from the rootstock. The graft union may be unusually swollen and rough before the young tree dies. Inside the graft union, dark, discolored wood extends through the entire width of the small trunk.
This disease is less important on established, older trees. Some of the smaller branches and sometimes large limbs can die back. Usually, not the entire tree is affected and the tree remains productive. In severe cases, the main trunk may be girdled, killing the tree.
Comments on the Disease
Members of the fungal family Botryosphaeriaceae are known to cause branch cankers on a variety of woody hosts, including avocado. The disease was previously known as Dothiorella canker because the pathogen most often isolated at the time was known as Dothiorella gregaria (teleomorph B. ribis).
Botryosphaeriaceae spores enter and initiate infection primarily through pruning wounds on the trunk or branches. More frequent pruning, such as would occur in a high-density grove, can increase dissemination of this pathogen among trees, leading to an increase in canker development and a possible decrease in yield as branches with cankers are pruned out.
Heavy rainfall causes increased spore production and infection. Spores spread by air and rain or irrigation splash that are hitting infected tissues. Trees that are stressed are much more susceptible to this disease. Common stresses include poor irrigation, low-quality irrigation water, nutritional deficiencies, or severe insect and mite feeding. Drought stress especially promotes symptom development and triggers latent infections to develop into disease.
Look for diseases and disease-promoting conditions regularly throughout the grove using recommended monitoring methods. 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, where pycnidia (spore-forming structures) and spores of the pathogen persist.
- Dispose of dead wood and old fruit well away from avocado trees.
- Prune and harvest only during dry conditions.
- Correct environmental and nutritional stresses, and minimize other pest problems.
- Appropriate amount and frequency of irrigation is especially important. Leach soil periodically and use low salinity water if salt toxicity is a problem.
Nurseries should use stringent sanitation measures, disinfest propagation material, and consider treating graft unions with an efficacious fungicide (e.g., FRAC Group 1 - MBC).