A newly discovered disease caused by a previously
undescribed fungus hitchhiking on a tiny native bark beetle, is
infecting and killing hundreds of black walnut trees in California and
seven other Western states.
The havoc wreaked
by the combined pests, coined "Thousand Cankers Disease," represents a
serious threat to black walnut trees, says chemical ecologist and
forest entomologist Steve Seybold of the Davis-based Pacific Southwest
Research Station, USDA Forest Service, and an affiliate of the
Department of Entomology, University of California, Davis.
"The
black walnut trees could go the way of the American chestnut or
American elm," warns entomologist Lynn Kimsey, chair of the UC Davis
Department of Entomology and director of the Bohart Museum of
Entomology, which houses one of the largest insect collections in North
America.
"By itself the very tiny walnut twig beetle, does
relatively little damage," Seybold said. But combined with the
aggressive fungus, it can kill a walnut tree in one to three years.
Despite the "twig" in its common name, the walnut twig beetle also
bores holes in large branches and even in the trunk of walnut trees.
The
beetle, Pityophthorus juglandis, native to Arizona, California, New
Mexico, and Mexico is widely distributed in California, from San Diego
to Shasta counties. Known since 1959 as just another specimen in the
drawers of California insect museums, it has emerged on the radar
screens of entomologists and plant scientists because it has been found
in abundance on dying walnut trees statewide. The disease has also
been found in Colorado, Arizona, New Mexico, Idaho, Utah, Washington,
and Oregon.
"It's a hard time for hardwoods," said Seybold,
who organized and chaired a symposium at the Entomological Society of
America's 65th annual meeting, held last fall in Reno. "This is
behaving like an invasive pathogen that has run amuck."
Scientists
are concerned that the disease may also impact English walnut and
California walnut production. "There are hints that the fungus may have
infected English walnuts in Utah," Seybold said, "and there are several
symptomatic English walnut trees at the USDA National Germplasm
collection located in nearby Winters but beyond that we do not know the
extent of the threat to the industry."
The fungus, with its
barrel-shaped spores, appears to be an undescribed and perhaps exotic
species within the genus Geosmithia, said postdoctoral researcher
Andrew Graves of the UC Davis Department of Plant Pathology. Graves,
part of a Davis-based team working on the project since June 2008, has
noted that there are seven named species of Geosmithia.
Colorado
State University plant pathologist Ned Tisserat, who placed the fungus
in the genus, Geosmithia and named the disease, "Thousand Cankers,"
told the ESA symposium: "It is really, really a scary disease; it's
as bad as butternut (walnut) canker." Butternut (Juglans cinerea) is
also known as white walnut.
Graves, who also holds a
doctorate in entomology from the University of Minnesota, described the
beetle as reddish-brown bark beetle, about 1.5 to 1.9 millimeters long.
"It's much smaller in size than a grain of rice," he said. The entrance
holes into the black walnut tree look like pin pricks.
"But if
you peel back the bark, you'll see the well-developed beetle galleries
and blotches of fungal-stained wood and bark that look like a thousand
cankers,"said Graves, who is researching the host colonization behavior
of the beetle. He described some of the coalescing cankers as
"enormous." The cankers widen and girdle twigs and branches, resulting
in die back of the tree crown.
Disease symptoms include dark
stains on the outer bark tissue that extend into the cambium; yellowing
and thinning of the upper crown; wilting of leaves; flagging branches;
die back and eventual death, all within three years. Seybold said that
the disease is so recently discovered that specialists have not had
time to develop and test integrated pest management tools to address
the issue. The natural system of attraction of the beetles to the
trees and to each other might form the basis of a future monitoring and
tree protection toolkit.
"The impact of these beetles and their
fungus," Kimsey said, "may be devastating to yet another of our native
trees. When I think of the possibility of losing all of the magnificent
black walnuts in Davis, it makes me very sad."
The disease
complex first gained notice in the Española Valley of New Mexico in
2001 when walnut trees declined and died. Scientists initially
attributed the mortality to drought stress. However, when the drought
subsided, the massive dieoffs did not.
The beetle-disease
complex is associated with widespread deaths of black walnuts planted
as street or highway trees in Boulder, Co., Portland, Ore., Prosser,
Wash., and several counties in California, including Los Angeles,
Sutter, Ventura, and Yolo. It was first noted by scientists in
California in 2008.
UC Davis walnut specialist Charles Leslie,
a member of the Davis-based thousand cankers disease research team,
says two species of black walnut are native to California: Juglans
californica (a southern California shrublike black walnut) and Juglans
hindsii (the northern California black walnut).
Northern
California black walnut is widely planted in Yolo County as an
ornamental tree, lining roads and ranches, Leslie said. "These black
walnuts are different from the commercial walnuts grown in the Central
Valley, which are Persian, commonly called "English" walnut trees grown
on black walnut root stock."
California black walnut "is prized
more as a shade tree than for its nuts," Leslie said. "To crack the
nut, you need to run over it with the family Hummer or hit it with a
sledgehammer," he quipped.
However, eastern black walnut is a
favorite in the ice cream industry, and the wood is especially prized
for furniture and guitars.
To confirm the extent of the disease
in the state, the Davis researchers are participating in a federally
funded project to collect diseased branches throughout California,
particularly in the native ranges of Juglans californica (Los Angeles
and Ventura counties) and Juglans hindsii (Mt. Diablo and elsewhere in
Contra Costa and Yolo counties. They are also rearing the beetles and
studying host colonization behavior. "The beetle appears to pump out
at least two generations a year in California," Graves said.
Colorado
State University plant sciences professor Whitney Cranshaw, who is on
the front lines of the research in Boulder and Denver, said people
continually ask him "How can a little twig beetle be killing healthy
trees?"
"With Geosmithia," he said. "The fungus is carried into
the tree when the beetle tunnels into and wounds the tree. The fungus
produces large cankers."
The aggressive fungus girdles the tree and "it's death by 1000 cankers," Cranshaw said.
The
attacks generally occur from mid-April through mid-September. At the
end of summer, the beetles and the fungus that they carry move into the
lower part of the trunk to hibernate.
In their continuing
research, scientists hope to establish a baseline of the beetle and
fungal populations to understand the full extent of the problem.
Native black walnut trees in the western U.S. are important components
of the vegetation along streams and riparian zones, Seybold said, so
their "loss may have significant ecological implications."
The
scientists also advocate research on vector transmission, overwintering
biology, an estimation of the risk and threat to the walnut-growing
industry in California and to commercially valuable native black walnut
trees in the eastern U.S., development of attractive baits, and an
insecticide treatment.
Insecticides may prove useful, but only
if used prior to the beetle arriving at the tree, Graves said.
"Insecticide sprays are of limited effectiveness due to the extended
period when the beetles are active, and because the beetles are feeding
beneath the bark, insecticides will not be useful in killing beetles
that have already entered the tree. Even if the insecticide kills the
adult beetles and larvae, the Geosmithia may continue to colonize the
bark and phloem."
The scientists also discussed their research
this past spring at meetings in Savannah, Georgia (National Forest
Health Monitoring Workshop) Spokane, Wash. (Western Forest Insect Work
Conference); and San Diego (Pacific Branch ESA Meeting).