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TREES, PLEASE: Tree Terror: Are Your Trees on a Hit List?

A greater and more diverse tree canopy is necessary for disease resistance in San Diego's vegetation. Image courtesy of Robin Rivet.

By Robin Rivet.

Today we face a force to rival Darth Vadar, and it ain’t pretty. The arrival and potential conquest by the Kuroshio and Polyphagus shot hole borers signals a new epoch for unintended global trade. It’s sad news, but especially scary if you value San Diego’s native vegetation. Many host plants for the destructive pathogenic fungus spread by these beetles are already identified: native and non- native species of sycamore; cottonwood;willow; alder; oak; boxelder; maple; buckeye; locust; mesquite; mulefat; and palo verde (Click here to see an interactive map depicting locations where wood tissue samples have tested positive for Kuroshio and Polyphagus shot hole borer specific Fusarium fungus.) Few native trees seem resistant. It is estimated that in the Tijuana Estuary alone, over 144,000 willow trees have been infected and are dying—with few beneficial predators in sight. Unless thwarted, imagine Mission Trails Regional Park in a few years.

UCANR Handout: Invasive Shot-Hole Borers + Fusarium Dieback

Adding to this horrific threat are attacks to many common ornamentals: Liquidambar; Mimosa; Xylosma; Brachychiton; acacia; coral; carrotwood; camellia; red-flowering gum; wisteria; holly; dombeya; plus kentia and king palms. All have evidenced dieback caused by Fusarium spread by these shot-hole boring beetles. It is small comfort that a few reviled species (including tamarisks, castor bean, and Ailanthus) are also tasty to these borers, since this lengthy host list also includes revered edible crops—our avocados, persimmons, and figs.

Couple this despair with an urban landscape already ravaged from years of drought, fire, and other destructive insect pests—in a region that has few native tree species to begin with—and we’re in big trouble. Dr. Greg McPherson, a scientist with UC Davis/USFS Pacific Southwest Research Station, projects a loss of 38% of all urban trees covering four counties throughout Southern California. This is catastrophic, and does not even take into account the extensive and ongoing removal of healthy trees incurred by ongoing urban sprawl; feckless and antiquated municipal policies; and a public that seems largely disinterested in improving Southern California’s paltry, urban tree canopy.

You likely know that the gold-spotted oak borer has already killed 100,000 oaks in San Diego’s back country, and that citrus greening disease, huanglongbing (HLB) spread by the Asian citrus psyllid (ACP), might soon destroy all our commercial and backyard citrus. At this moment, California appears to be the last place on the planet where massive amounts of pesticide aren’t needed to avoid totally ravaged citrus groves. Maybe you have seen the imported glassy-winged sharpshooter spreading various forms of Xylella fastidiosa? That bacterium also interrupts water uptake, causing leaf scorch symptoms and ultimate death to additional tree species like olives, almonds, mulberries, liquidambar, oleander, and grapes. Plus, we’ve just quarantined for the Mexican fruit fly. It’s all out there, and it’s frightening.

Is there hope? Nothing is certain, but vastly increasing tree variety and quantity seems essential. Think pecans, ginkgos, peppers, magnolias, melaleucas, and Torreys. And there are surely many more yet unnamed Mediterranean adapted species looking for new homes. We have the potential to grow much greater and more diverse tree canopy than we do, and we must act now.

Member Robin Rivet is an ISA Certified Arborist, UCCE Master Gardener and City of La Mesa Environmental/Sustainability Commissioner.

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