It's about 150 years old, 50 feet in height, and measures about five feet in diameter. And it's dying.
What's killing it is thousand cankers disease (TCD), an emerging insect-fungus complex.
It's killing a lot of black walnut trees.
In fact, TCD has caused profound damage to black walnut trees not only in urban areas of California and other western states, but in Pennsylvania, Tennessee and Virginia, according to a newly published review by UC Davis-affiliated scientists and their colleagues.
The article, “Status and Impact of Walnut Twig Beetle in Urban Forest, Orchard and Native Forest Ecosystems,” published in the Journal of Forestry, updates the spread of the disease, and chronicles the role of the bark beetle, Pityophthorus juglandis, and the canker-producing fungus, Geosmithia morbida, in killing walnut trees, especially black walnuts.
Native to southwestern United States and northern Mexico, the bark beetle, about half the size of a grain of rice, “has invaded urban, orchard and native forest habitats throughout the United States, as well as Italy,” said lead author and forest entomologist Steven Seybold of the Pacific Southwest Research Station, USDA Forest Service, Davis, and a lecturer and researcher with the UC Davis Department of Entomology and Nematology.
Walnut twig beetles (WTB) tunnel into branches and trunks of walnut (Juglans) where they create galleries for mating and reproduction. They carry spores of the fungus into their galleries, and the resulting fungal infection causes formation of cankers, which coalesce and girdle branches and stems.
“The walnut twig beetle is a significant pest of very large trees because it sequentially attacks the small branches--though ironically not the twigs--all the way down the trunk to the soil line,” said Seybold, a pioneering scientist of TCD, who first found TCD evidence in Davis in 2008. “Most bark beetle species are not this thorough in using all of the phloem tissue in their plant hosts. In Davis right now, in the courtyard next to Sophia's Thai Restaurant, 129 E St, the tiny beetle is gradually killing the largest northern California black walnut tree in the city. It has taken nearly a decade, but the crown of the massive tree is nearly completed killed.”
Seybold estimated that the E Street tree is about 150 years old, "maybe older." It measures almost 65 inches or just over five feet.
Seybold noted that the disease is “unique because of its multifaceted negative impact on walnut trees involved in landscaping, food production, and forestry. Walnut trees are valuable ecologically and for food and timber, so the walnut twig beetle is a good model in which to study the impact of a bark beetle on forest and agro-ecosystem services.”
The five co-authors of the synthesis article include Stacy Hishinuma and Andrew Graves, two USDA forest entomologists with UC Davis connections. Hishinuma, who works in the Pacific Southwest Region, San Bernardino, and holds a doctorate in entomology from UC Davis, studied in the Seybold and Mary Lou Flint labs, UC Davis Department of Entomology and Nematology. Graves, who works in the Southwestern Region, Albuquerque, N.M., is a former postdoctoral fellow in the UC Davis Department of Plant Pathology.
Other co-authors are Professor William Klingeman III of the Department of Plant Sciences, University of Tennessee, and forest entomologist Tom Coleman with the USDA Forest Service's Southern Region, Asheville, N.C..
UC Davis doctoral student Jackson Audley of the Seybold lab and Richard Bostock lab (Plant Pathology), contributed photos of dead and dying walnut trees in the Davis area, along with UC Davis doctoral student Corwin Parker and Hishinuma. Audley, who is researching an ensemble of behavioral chemicals that repel the walnut twig beetle from landing on English walnut trees, conducts his research in a commercial orchard near Winters.
“WTB is one of a few invasive bark beetles in North America where expanding distribution and impact have been pronounced enough to affect other species, communities, and ecosystems to the extent that services provided by urban forests, agroecosystems, and wildland areas have been altered,” the co-authors concluded in their paper. “We envision that ecological impacts of WTB will continue to unfold across a wider geographic area to affect various types of key services, i.e., provisioning (e.g., timber and nontimber products); regulating (e.g., air and water quality/quantity, climate regulation); and cultural (e.g., recreation, aesthetics, shade) services.”
Scientists first collected the beetle in North America in 1896 in New Mexico, 1907 in Arizona, 1959 in California, and 1960 in Mexico, but never considered it a major pest of walnut trees until black walnuts began deteriorating and dying in New Mexico in the early 2000s. Walnut tree mortality that occurred in the early 1990s in the Wasatch Mountains of Utah and in the Willamette Valley of Oregon is now attributed to TCD.
“Currently, good cultural practices and sanitation of infested materials are the primary strategies for disease management within orchards and also for prevention of spread of the disease and vector to regions with low rates of infection,” according to the UC Statewide Integrated Pest Management Program (UC IPM)>
UC IPM recommends that trees with less than 50 percent live crown be removed to reduce the buildup of walnut twig beetles and inoculum in the trunk and larger scaffold branches. "Chemical control with either fungicides or insecticides is not recommended for management of thousand cankers disease," UC IPM says.
Which brings us back to the massive black walnut tree that is dying on E Street in Davis. If it dates back to 1868, that's the year the city of Davis was founded. Its namesake, Jerome C. Davis, owned a stock farm on the site.
Who was in the White House and who was in the California governor's mansion that year? Andrew Johnson and Henry Haight, respectively. That was also the year that trustees founded the University of California in Oakland; Clark Kerr became the first president. And 1868 was the year Memorial Day was first observed in the United States...and when author Louisa May Alcott published the first volume of her coming-of-age novel, Little Women.
The little tree in Davis became of age, too, growing into a giant tree offering shade, shelter and sustenance. Who would have thought that a tiny insect, half the size of a grain of rice, would play a major role in its demise?
UC Davis entomology doctoral student Stacy Hishinuma has received and accepted a position in the USDA Pathways Internship Program with the Region 5 field office of the USDA Forest Service, San Bernardino.
Hishinuma studies the walnut twig beetle, Pityophthorus juglandis, which in association with a newly described fungus, Geosmithia morbida, causes thousand cankers disease (TCD) of walnut and butternut trees.
The walnut twig beetle is believed to be native to Arizona, California, New Mexico and Mexico. In 2006, plant pathologist Ned Tisserat and entomologist Whitney Cranshaw of Colorado State University identified the pathogen in declining black walnut trees in central Colorado. The disease has now spread east of the Mississippi to states in the heart of the valuable black walnut timberlands. Most recently it was reported from Indiana. Latest collection records show that the beetle and pathogen are now known from nine states in the western United States and seven states in the eastern USA. In 2013 the disease was also reported in Italy marking the first time that it occurred in Europe.
Hishinuma works with major professor Mary Louise Flint and is co-advised by chemical ecologist and forest entomologist Steve Seybold of the Pacific Southwest Research Station, USDA Forest Service, Davis, an affiliate of the department. Flint is an Extension specialist emeritus with the UC Davis Department of Entomology and Nematology and a former associate director with the UC Integrated Pest Management Program.
“USDA will provide some financial support during the end stages of her Ph.D. thesis, in exchange for 320-640 hours of work and training over the next 14 months,” said Seybold. Then, in June 2016 her position will be converted to that of a permanent entomologist with the USDA Forest Service office in San Bernardino after she has completed her thesis. She will be responsible for forest insect survey, detection, and management on four national forests in southern California ranging from San Diego to Monterey counties.
“The highly competitive internship and guaranteed position are a credit to her and her achievements,” Seybold said.
Seybold and Flint assisted her in developing the internship, as did Richard “Rick” Bostock, UC Davis professor of plant pathology.
Hishinuma won the 2013 Western Forest Insect Work Conference Memorial Scholarship for her research on TCD and presented her work at group's 65th annual conference, held March 31-April 3, 2014 in Sacramento. She also received two scholarships from the California Garden Clubs, Inc. (CGCI) and a McBeth Memorial Scholarship to support her research on TCD.
Seybold's research group has led the effort to characterize the disease in California and to develop a nationwide detection program for the beetle. They recently published two papers in the journal PLOS ONE that characterize the genetic diversity and invasion patterns of both the pathogenic fungus and the beetle in the United States. Scientists believe that TCD occurs only on walnut, butternut, and wingnut, but it is most damaging to native black walnuts, Juglans californica, J. hindsii, and J. nigra although the disease has been recorded on at least 10 species of walnuts or their hybrids in California. Often the first symptoms of TCD are flagging and yellowing leaves and branch dieback, Seybold said. Affected branches show sap staining and pinhole-sized beetle holes. Beneath the surface are dark stains caused by the funguhe news is startling, but not totally unexpected.
Chemical ecologist and forest entomologist of Steve Seybold of the USDA Forest Service, Pacific Southwest Research Station, Davis, Calif., and a faculty affiliate of the UC Davis Department of Entomology, is leading a research group to characterize the disease in California.
A new member of the team is UC Davis entomology graduate student Stacy Hishinuma (right), who just received a McBeth Memorial Scholarship to help fund her research.
TCD is caused by the walnut twig beetle, Pityopthorus juglandis, in association with a newly described fungus, Geosmithia morbida.
“The beetle is believed to be native to Arizona, California, New Mexico and Mexico,” Hishinuma said, “but was never associated with walnut tree mortality until just recently.”
The disease was first noticed in canker-riddled black walnuts in Utah and Oregon in the early 1990s, but scientists attributed that to environmental stress. In 2006, plant pathologist Ned Tisserat and entomologist Whitney Cranshaw of Colorado State University identified the pathogen in declining black walnut trees in central Colorado. TCD is now found in eight western states (Arizona, California, Colorado, Idaho, New Mexico, Oregon, Utah and Washington), plus Tennessee.
“My project focuses on understanding more about the biology of the beetle and fungus and documenting the frequency and progression of TCD in various walnut species throughout California,” Hishinuma said. “The McBeth scholarship is funding my travel to field sites across California.”
Hishinuma, who is seeking her master’s degree in entomology, works with major professor and integrated pest management (IPM) specialist Mary Lou Flint and is co-advised by Seybold. Flint, with the statewide UC IPM Program, is the associate director for urban and community IPM, and a Department of Entomology extension specialist.
The fungus enters the tree when the beetle tunnels into the bark to prepare egg galleries. “The fungus is probably carried as conidia on the beetle’s elytra or wing covers,” said Seybold, who has been studying the chemical ecology and behavior of bark beetles for more than 25 years.
Scientists believe that TCD occurs only on walnut, predominantly native black walnut, Juglans californica and J. hindsii, although the disease has been recorded on 10 species of walnuts or their hybrids in California. Often the first symptoms of TCD are flagging and yellowing leaves and branch dieback, Seybold said. Affected branches show sap staining and pinhole-sized beetle holes. Beneath the surface are dark stains caused by the fungus.
A tree can survive the stress produced by a few cankers, but when high populations of the beetle enter the tree and the numerous small cankers coalesce, the disease girdles twigs and branches. Eventually TCD attacks the main stem of the weakened tree down to the soil line.
Last summer, a USDA/UC Davis research team began tracking the pathogen and the beetle throughout the state, particularly in commercial orchards.
To prevent spread, infected trees should be removed and destroyed immediately by grinding or burning to ensure that beetles are destroyed, Seybold said. Infested walnut for chips, firewood, or woodworking should not be moved to new areas. Possible detections can be reported to the local agricultural commissioner’s office or to the local UC Cooperative Extension office.
Hishinuma, from Burbank, completed her undergraduate work at UC Davis in animal biology, with an entomology emphasis. She began her graduate studies this fall after working for a year in southern California assisting with research on the goldspotted oak borer, a relatively new pest in southern California that kills oak trees. Beetle-infested firewood contributes to the problem, as it does with thousand cankers disease.
The McBeth Memorial Scholarship was established in 1986 by Barbara McBeth Woodruff (1924-2007) in honor of her parents, Ira Guy McBeth and Rose McBeth, and her sister Frances McBeth Black. Ira Guy McBeth, an entomologist who received his doctorate from the University of California in 1915, made notable contributions to the citrus industry and served as an executive in agricultural, chemical and development companies.
Meanwhile, a thousand questions about thousand cankers disease...and maybe soon...solutions.
Steve Seybold discussed thousand cankers disease in a seminar presented May 19, 2010 to the UC Davis Department of Entomology. The seminar was webcast and is archived on the entomology website. (USDA Forest Service Pest Alert on Thousand Cankers Disease of Walnut (targeting eastern U.S. region). (May 2010)
UC IPM website (Authors: Andrew Graves, postdoctoral researcher, formerly with the UC Davis Department of Plant Pathology; Mary Louise Flint, UC IPM Program and Department of Entomology, UC Davis; Tom Coleman, USDA Forest Service, Forest Health Protection, San Bernardino; and Steven Seybold, USDA Forest Service, Pacific Southwest Research Station, Davis)
Detected in Tennessee (News release, Aug. 9, 2010, UC Davis Department of Entomology website)
Detected in California (News release, July 2, 2010, UC Davis Department of Entomology website)