On Tuesday, December 8th, I attended an information session at the Audubon Center at Debs Park on a new disease affecting walnut trees, including the two California native black walnuts, Juglans californica (native to southern California), and J. hindsii (the northern native). Steve Seybold, chemical ecologist and forest entomologist of the Davis-based Pacific Southwest Research Station, USDA Forest Service, and an affiliate of the Department of Entomology, University of California, Davis, spoke to landscape professionals about this newly recognized disease.
Seybold was invited to speak to the group by consulting arborist, Angela Liu (angelaliuLA@gmail.com). Liu first saw evidence of the disease while preparing a report for the Audubon Center about the native walnuts at their facility. While examining the walnuts in Debs Park she noticed some disease problems. At the same time Liu learned of a new disease, Thousand Cankers Disease (TCD), affecting walnuts. Noticing what appeared to be evidence of the disease, Liu contacted UC Davis for positive identification (the sample tested positive for TCD). She also invited Seybold to give a presentation on what is known about the new and alarming disease.
Thousand Cankers Disease (TCD) has only very recently been observed. It appears to be caused by a tiny beetle, Pityophthorus juglandis that burrows into trees, carrying a fungal disease, Geosmithia sp. It is believed at this time that the beetle is native and has been around and widely distributed for a long time. It was first collected in Los Angeles County in 1959. Examination of the beetles shows evidence of a complex community of natural enemies further supporting the theory that it is native. The fungal disease, though, was only recently reported, in 2008. It may be non-native, or a new organism, though there is still much more research needed to determine where it came from and how to control it.
Evidence of the tiny walnut twig beetle appears in the form of pinholes in the branch bark of affected trees. The beetle is a phloem feeder that acts as a vector for the fungus. The fungus colonizes and kills the phloem of branches and stems, causing the development of stained bark cankers, hence the name, Thousand Cankers Disease. The disease is confirmed through testing for the fungus. Flagging, branch dieback, pinhole size holes in the bark, and staining are signs of the disease. Evidence of TCD has been widespread.
During the information session, Seybold shared displays of infected branches and samples of the beetle. He showed pictures of the male and female adult insects and larvae, and video clips of the beetles in action. Out in the field, Seybold pointed out the tiny pinhole-sized entrance and exit holes caused by the beetles in walnut trees at Debs Park. He also showed staining and cankers probably caused by the disease.
In addition to affecting native California black walnuts, the eastern black walnut (J. nigra) is highly susceptible. At this time the nut producing English walnut (J. regia) appears to be fairly resistant, though there is evidence of the disease in the Paradox rootstock, used in the walnut industry.
There are many questions about the course of the disease, the extent of mortality and resistance, and control. Researchers are concerned and are actively studying this new fungal disease for answers.
Arborist, Angela Liu, suggests that you contact a certified arborist if you think you are seeing evidence of TCD. If you have more questions you can reach Angela Liu at angelaliuLA@gmail.com.
For more information on Thousand Cankers Disease check out the following articles:
Beetle and Fungus One-Two Punch Threatens Black Walnut Trees, Scientists Warn. July 2, 2009. University of California, Davis, Department of Entomology.
Pest Alert Walnut Twig Beetle and Thousand Cankers Disease of Black Walnut. Colorado State University.
Fungal disease attacks black walnut trees in eight Western states by Li Lou in Sacramento Bee, December 13, 2009.