Why Are Our Trees Dying?

By Emile Fiesler, environmental consultant

Southern California has a new challenge: a new species of fungus. The fungus was unknown to science until 2013, when it was described and given the scientific name Fusarium euwallacea and common name fusarium dieback. Fusarium is a large and complex genus of fungi, most are found in soil and associated with plants. The majority of Fusarium species are harmless, but fusarium dieback is one of the exceptions; it can kill trees.

Fusarium dieback is not particularly choosy and is spreading in close to 60 tree species. Its spreading is assisted by its accomplice, a tiny beetle; also new to science. The tiny beetle is an invasive ambrosia beetle in genus Euwallacea, accidentally introduced from Asia, likely in lumber.

Ambrosia beetles are weevils that have a symbiotic relationship with their only food source: ambrosia fungi. The beetles tunnel into trees and other woody plants and release fungal spores to start their fungal gardens. The ambrosia fungus enters the plant’s xylem, where the plant’s sap transport takes place. The fungus digests the xylem and concentrates the obtained nutrients, on which the ambrosia beetles feed.

Most ambrosia beetles tunnel into weak or dying trees. Not so for the Polyphagous Shot Hole Borer (PSHB), as our new ambrosia beetle has been dubbed; it attacks healthy trees. Polyphagous means that it has a broad appetite. This name was given due to it boring in more than 50 local tree species. This is a misnomer, as PSHB’s appetite appears highly restricted to the Fusarium dieback fungus. This doesn’t change the devastating bottom line: that we could loose more than one third of all our local trees.

Some affected native trees are Western Sycamore, Coast Live Oak, Fremont Cottonwood, and Red Willow. PSHB also invades Avocado trees, many of our ornamental street trees, and even the highly invasive Castor Bean.

On June 18, 2016 I found a tiny (~ 2.5 mm / 0.1 inch long) beetle on the Palos Verdes Peninsula; see photo. It is apparently the first PSHB record for the Peninsula.

Keep a keen eye out for tiny holes in trees, less than a millimeter (0.04 inch) in diameter, often accompanied by sap stains. Once identified, it is best to sacrifice the tree and grind it into tiny pieces. The pieces can be covered with a tarp to create a solar heating effect that kills the fungus and its ally. If you notice an outbreak, contact the Los Angeles Agricultural Commissioner’s office at 1-626-575-5471.

When planting trees, choose drought-resistant trees and select locations with plenty of space to grow. The healthier the tree the less likely it will be affected.



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