Symptoms and Signs
Fusarium dieback is caused by a complex of fungal species colonizing galleries made by the polyphagous shot hole borer or the Kuroshio shot hole borer.
External Symptoms: A host tree's visible response to disease varies among host species. On avocado, sugary exudate (also called a sugar volcano) and frass may be noticeable before the tiny beetles are found. The beetle's entry and exit holes, which are about 0.03 inch (0.85 mm) in diameter, can be located beneath or near the symptoms. Advanced fungal infections will eventually lead to branch dieback.
Internal Symptoms: The fungi interrupt the transport of water and nutrients in branches of affected trees. Infected wood is discolored brown to black. Scrape away bark around beetle entry and exit holes to easily see discolored wood. Cross-sections of cut branches show the extent of infection.
Comments on the Disease
Fusarium dieback is a recent, invasive, beetle-vectored disease that causes damage on avocado and more than 39 other tree species. The disease has spread in urban forests and wild lands in the Los Angeles basin since early 2012, and in Orange and San Diego counties since early 2013 and Ventura County in 2015.
Rapid spread of the beetle and fungi throughout various land-use areas is attributed to the diverse range and quantity of suitable hosts in Southern California
Currently there are no control measures for this disease. Early detection of infestations and removal of infested branches will help reduce vector beetle numbers and the extend of disease spread.
- Chip infested wood onsite to a size of one inch (2.5 cm) or smaller. If the branches are too large to chip, solarize them under a clear tarp for several months.
- Sterilize tools with either 5% bleach, Lysol cleaning solution, or 70% ethyl alcohol to prevent the spread of the disease through pruning tools.
- Avoid movement of infested firewood and chipping material out of infested area.
- For more information visit the UC Davis Eskalen Lab website.