Fusarium Dieback

New diseases or pathogens are rare these days. But scientists found one carried by a common pest. It is called Fusarium dieback, and it can kill your trees. [See a map of its presence in SoCal here.]

Fusarium dieback is a fungal disease carried by invasive borers. As borers burrow into trees, they spread three different fungal pathogens. These fungi form colonies within a tree’s vascular system, blocking the flow of water and nutrients. Trees infected with Fusarium dieback must be removed and disposed of by professional arborists.

Symptoms of Fusarium dieback

Since this disease affects many different types of trees and has multiple pathogens, it is no wonder that there are a variety of symptoms. Infected avocado trees, for example, will exhibit sawdust-like frass (bug poop), gumming, and sugar volcanoes. Sugar volcanoes are white discharges of sugary sap. On other host trees, you may see dark, greasy-looking areas on the bark, withered leaves and stem tips, and white mycelium under the bark. Mycelia are the vegetative growths of fungi.

As the infestation progresses, perfectly round, tiny borer entry and exit holes may become visible. These holes are only 0.03 inches in diameter, so look very closely, usually just below areas showing disease symptoms. Eventually, branches will start to die. If you cut into an infected limb, you will see that the wood is discolored, brown, or black. If you scrape the bark away from entry or exit holes, you will also see discoloration. After each cut, disinfect your tools with a household cleaner to avoid spreading the disease.

Disease background

Fusarium dieback originated in Israel and spread to Southern California by 2003. Botanists are not sure how far the disease will spread from there. At first, scientists thought the tea shot hole borer was the carrier, a common pest of tea plants in Sri Lanka. DNA testing, however, showed that this was an entirely new species, now named the polyphagous shot hole borer. 

By 2010, this borer, and the disease it carries, had become a serious threat to Southern California’s box elder trees, palm trees, black locust, and our beloved avocado trees. In 2015, a second carrier, the Kuroshio shot hole borer, joined the party and started infecting the popular California palms (Washingtonia filifera). This disease currently threatens 25% of all the trees lining Southern California’s streets.

Beetle description

The two borer species responsible for Fusarium dieback look identical at first. Female beetles are tiny and black, only 0.07 to 0.1 inches long. Males are brown and even smaller, at only 0.06 inches long. Female beetles can fly and will leave their birthplace to find other host trees to use as nurseries, carrying the disease with them when they go. Males do not fly and generally stay in the tree of their birth. These beetles are most active during summer and fall.

Fusarium dieback hosts

Shot hole borers feed on and breed in over 200 species of woody plants. That’s a lot of potential hosts. While this disease prefers gums, palms, pines, and privets, it is also a threat to apples, bay laurel, carob, chestnut, elderberries, figs, grapes, olive, persimmons, pineapple guava, pistachios, pomegranate,  stone fruit, and stone pine are also vulnerable to Fusarium dieback.

How infection occurs

This disease starts when female beetles bore into tree trunks and branches, creating galleries of tunnels. Within the tunnels, they build egg chambers. Female beetles have developed a symbiotic relationship with three different fungi, which they carry around in their mouths, much the way we have bacteria in our gut. [The fungal pathogens of Fusarium dieback are Fusarium euwallacea, Graphium euwallaceae, and Paracremonium pembeum if you enjoy the Latin.]

Those fungi set up housekeeping within the tree and become food for the newly hatched beetle larvae. By eating the fungi, the larvae then become carriers of the disease. These fungal colonies develop rapidly once inside a tree, and there is no known treatment at the time of publication. Not all infected trees will show signs of infection. Some infected trees serve as breeding grounds without showing any signs of disease. We don’t yet know why.

Preventing disease

Healthy trees are far better able to protect themselves against borers. So select plants appropriate to your microclimate, irrigate and fertilize them properly, and provide healthy soil. Also, monitor your trees regularly.

If you suspect Fusarium dieback on a tree, contact your local County Extension Office. Together, we may be able to slow or stop the spread of this disease.