The source of a disease that has taken a massive toll on Israel and Southern California’s avocado industry has been identified by a plant pathologist at UC Riverside. Akif Eskalen recently discovered that the prevalence of the disease owed its success to a collaboration between beetles and a new species of fungi—the former serving as the vector of the disease and the latter being the cause of the disease and food for the beetle.
The beetle, known as the Tea Shot Hole Borer, is an exotic species that poses a threat to a variety of fruits and plants such as mango, silk oak, persimmon, citrus plants, macadamia nuts, lychee and guava. The beetle’s exceptionally small size (smaller than a sesame seed) makes it easy to evade capture and spread the Fusarium fungus.
The Fusarium fungus caused the disease known as “Fusarium dieback,” which occurs when parts of a plant begin to rot. “When a beetle burrows into the tree, it inoculates the host plant with the fungus it carries in its mouthparts,” stated Eskalen in an interview with the Highlander. “The fungus then attacks the vascular tissue of the tree, disturbing water and nutrient flow, and eventually causing branch dieback. The beetle larvae live in galleries within and the tree and feed on the fungus.”
Eskalen chanced upon the discovery of the beetle-fungus disease last February after being contacted by a homeowner in South Gate, Los Angeles. Recognizing dieback symptoms on a backyard avocado tree, the plant pathologist decided to embark on the research project.
Eskalen’s discovery has implications for the international community since the Tea Shot Holer Borer-Fusarium fungus combination is the same culprit behind the decline of avocados in Israel.
“The symptoms were unlike anything I have ever seen on avocado,” stated Eskalen. “I brought the samples into my lab, and found the beetle in the plant samples, associated with necrotic tissue. We were able to culture a fungus out of the symptomatic tissue and, using molecular techniques, we identified a new Fusarium species that the beetle was associated with.”
Eskalen requests that gardeners keep a watch for any indication of the beetle or fungus in their trees. Symptoms may include white chalky exudate around a small opening on the bark of the branches or the trunk, which could be the beetle exit hole. The wood may appear wet or discolored due to the exudate from Fusarium dieback.
“The first step is to identify its current distribution and host range, which is what my lab is working on,” added Eskalen. “Every day we are identifying a new host and we now know that this beetle/fungus can be very aggressive. The next step we are working on with (Richard) Stouthammer’s lab is identifying how the disease is transmitted, and what stops the beetle from moving the disease around.”
Eskalen’s lab has teamed up with scientists from the plant pathology, microbiology and entomology departments to uncover control measures for the fungus and beetle. Eskalen has collaborated with Alex Gonzalez, a field specialist, in an attempt to determine the current distribution and host range of the specimens. Meanwhile, Professor of Entomology Richard Stouthamer and Associate Specialist in Entomology Paul Rugman-Jones are studying the beetle’s biology and genetics.