Identifying Fusarium dieback disease

A new beetle that vectors a Fusarium dieback on avocado trees has been discovered in and near Los Angeles County. The newly discovered Euwallacea sp. Beetle (Polyphagous Shot Hole Borer) is indistinguishable from the tea shot hole borer beetle (Euwallacea fornicatus) and is believed to be a new species of beetle. 

The beetle and Fusarium fungus have a symbiotic relationship. The beetle burrows into the avocado tree and inoculates the tree with the fungus carried in its mouthparts. The tree eventually exhibits symptoms of branch dieback because the fungus affects the vascular tissues that distribute water and nutrients from the roots to the rest of the tree.

Symptoms of Fusarium dieback disease

• Look for a single entry/exit hole with surrounding white powdery exudate that is either dry, or surrounded by wet discoloration of the outer avocado tree bark.
• During the early stages of colonization, there may not be any visible injury to the bark. Scrape off bark around an infected area or entry/exit hole to look for brown discolored necrosis caused by the fungus.
• The entry hole on an avocado tree is about 0.033 inches wide.
• The beetle larvae live in galleries within the tree and feed on the fungus. Examine the area around the gallery for the discolored necrosis caused by the fungus.

Additional host plants

Certain plant species appear to be a preferred host for the beetle. These include box elder, castor bean, English oak, sycamore, liquidambar, goldenrain tree and coast live oak. Other host plants include: Box elder, Big leaf maple, Mimosa, American sweet gum, Silk tree, Coral tree, Titoki, Blue palo verde, Palo verde, Tortuosa, Weeping willow, Red Willow, Trident maple, Japanese maple, Evergreen Maple, Chinese holly, Brea, Black bean, and Camelia.

If these plants are close to your avocado groves, inspect them for evidence of the beetles. The majority of beetle exit/entry holes are found at the base of the plant.

If no beetle holes are present, it may be a good idea to remove the plants. If beetle holes are found, there is no proven manner in which the plants can be destroyed without allowing the beetles to disperse.

If you suspect you have found this beetle or see symptoms of Fusarium dieback in your avocado groves, contact your local farm advisor, pest control advisor, county Ag Commissioner office, or the CAC office at 949-341-1955. Dr. Akif Eskalen, from the University of California, Riverside, can assist in confirming the existence of the pathogen. He can be reached at 951-827-3499, or email at [email protected].