It is unusual for a new disease or pathogen to be discovered. It is even more rare when a new disease is found to be caused by a common pest. This is Fusarium dieback, and it can kill your trees.
Fusarium dieback is a fungal disease carried by invasive borers. As borers burrow into trees, they carry three different fungal pathogens with them. These fungi form colonies within a tree’s vascular system, blocking the flow of water and nutrients. Trees infected with Fusarium dieback must be destroyed and disposed of by professional arborists.
Symptoms of Fusarium dieback
Since this disease affects many different types of trees, and is caused by different fungi, it is no wonder that there are different 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 the presence of 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 you have to look very closely, usually just below areas showing symptoms of disease. Eventually, you will start seeing branches die. If you cut into an infected branch, 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 cutting, be sure to disinfect your tools with a household cleaner, such as Lysol, to avoid spreading the disease.
Once Fusarium dieback has infected a tree, the wounds and weakened condition of the tree make it susceptible to many other fungal infections and other diseases.
Originally found in Israel, Fusarium dieback was first seen in Southern California in 2003. The carrier was believed to be the tea shot hole borer, 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 SoCal’s box elder trees, palm trees, black locust, and our beloved avocado trees. In 2015, a second variety of carrier, the Kuroshio shot hole borer, joined the party and started infecting the popular California palms (Washingtonia filifera). It is estimated that this disease now threatens 25% of all the trees lining Southern California’s streets.
To the naked eye, the two species of borer responsible for Fusarium dieback look identical. 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 can be found feeding on and breeding in over 200 species of woody plants. That’s a lot of potential hosts. To date, the disease has been found in more than 130 different host species.
While this disease prefers palms and ornamentals, such as maple, birch, and tulip trees, it is becoming a serious threat to avocados and California live oaks. California bay laurel, carob, chestnut, elderberries, figs, olive, peaches, persimmons, pineapple guava, pistachios, and pomegranate 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, chambers are built for eggs. Female beetles have developed a symbiotic relationship with three different fungi, which they carry around in their mouths, much the way we carry around bacteria in our gut. [The fungal pathogens of Fusarium dieback are Fusarium euwallaceae, Graphium euwallaceae and Paracremonium pembeum, if you enjoy the Latin.]
The fungi that set up housekeeping within the tree end up being food for the newly hatched beetle larvae. By eating the fungi, the larvae then become carriers of the disease. These fungal colonies develop very rapidly, once they are inside a tree, and there is no known treatment at this time. Complicating matters even more, not all infected trees will show signs of infection. Some infected trees simply serve as breeding grounds, without showing any signs of disease, and we don’t yet know why.
Healthy trees are far better able to protect themselves against borers. This means selecting plants appropriate to your microclimate, irrigating and fertilizing them properly, and providing healthy soil. Also, monitor your trees regularly.
Currently, Fusarium dieback is limited to Israel and Southern California. You can see a map of the disease, as it spreads through California, here. [It’s a big file, so it make take some time loading.] While Fusarium dieback has only come as far north as San Luis Obispo, to date, that can change overnight. Research is underway, to try and identify an effective lure that can be used to trap the borers.
If you suspect Fusarium dieback on a tree, please contact your local County Extension Office. Together, we may be able to slow or stop the spread of this disease.
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