New Utah Pests Fact Sheets:

New Threat to Walnuts: Walnut Twig Beetle and Thousand Cankers Disease

Walnut trees are dying in Utah and Dr. C. Reed Funk, retired Rutgers University plant breeder, was among the first to notice.  Dr. Funk started a non-profit organization called “Improving Perennial Plants for Food and Bioenergy” (IPPFB) with research field sites in Dayton, Idaho, and his hometown of Richmond, Utah.  Walnuts, namely Persian, English, and black, are one of their primary research crops, and the organization now grows several thousand walnuts representing a worldwide progeny.  In 2004, IPPFB noticed that several research walnuts were in decline or dying, some within 2 years.  Dr. Funk also began to take notice of landscape trees in Richmond and northern Utah, and saw majestic specimens, some centuries old, dying before his eyes.

Some digging for answers on the Internet led him to articles about a mysterious walnut decline observed in Boulder, Colorado.  There, Parks and Recreation Urban Forestry staff noticed a rapid decline and death of walnuts starting in 2003 and identified the walnut twig beetle (Pityophthorus juglandis) as a possible cause.  Colorado State University Plant Pathologist, Ned Tisserat, observed that a pathogen was also involved, but had not yet identified it.  Fearing a similar diagnosis, Dr. Funk sent samples to the city forester in Boulder in fall 2007, who in turn sent them to Tisserat.

Bark beetle galleries on the trunk of a walnut caused by walnut twig beetle.	Dr. Funk and assistant, Ken Turner, examining the health of a black walnut.

Tisserat initially identified the fungus Fusarium solani forming long, vertical cankers on the dying trees on Boulder. “This pathogen, however, is typically associated with trees already in decline, and we had not found it associated with the walnut twig beetle or the beetles’ galleries,” he said.  In summer 2008, after months of investigation using the IPPFB sample and samples from Boulder, Tisserat consistently isolated a fungus from branch and twig cankers surrounding the beetle galleries, and from the beetle itself.  He identified it to genus as a Geosmithia sp.  Tisserat noted that “several Geosmithia species are known to be associated with bark beetles on hardwoods, but it has never been seen on Juglans, nor associated with walnut twig beetle.”

The complex (the beetle, Geosmithia, and Fusarium solani) that is killing walnuts has been named thousand cankers disease.  “We’re calling it ‘thousand cankers’ because the number of cankers found within an individual tree is enormous,” says Tisserat.  “Each canker is associated with an individual beetle attack.  So it’s death by 1,000 cankers.”  Geosmithia cankers expand rapidly vertically, often coalescing to one large canker.  Fusarium cankers are much larger—they may extend from the ground to large scaffold limbs—and are only associated with trees in advanced stages of decline.  Symptoms of the decline include yellowing and thinning of the upper crown, death of large limbs, and rapid foliage wilt followed by tree death.

The walnut twig beetle is native to North America, but was historically only associated with the native Arizona walnut (Juglans major) located in southwest U.S., and Chihuahua, Mexico.  It is believed to have at least two generations.  This tiny beetle overwinters as an adult in tree bark, emerges in spring, feeds on bark and wood, and lays eggs in a nuptial gallery.  Bark beetle galleries can be found in small twigs, large limbs, and sometimes in the main trunks.  Exit holes are almost too small to see with the naked eye.

The beetle’s 2003 identification from Boulder was the first known report in Colorado, but in Utah, the beetle was first recorded in 1988 from Provo by Brigham Young University staff.  UPPDL also identified the beetle from Iron, Sanpete, and Cache counties starting in the mid-90s.  We presume that the beetle now occurs in Utah whereever black walnuts grow.

	Geosmithia cankers associated with galleries

Previous reports of black walnut deaths in the West have been attributed to drought stress, with the beetle as a secondary pest.  Black walnut is native to eastern U.S. and not adapted to the desert environment.  However, the thousand cankers complex has been associated with trees on irrigated sites, over a wide area (CO, UT, ID, and southern CA—but not in English walnut production areas), and kills trees in a matter of 1 to 3 years, suggesting no relationship to environmental stress.

There are many questions about thousand cankers disease that Tisserat and CSU Entomologist Whitney Cranshaw plan to address, including the origin of this Geosmithia species, control options, the distribution and biology of the insect and the pathogen, resistance, and prevention of spread to native walnuts in the East, and to production farms.  And IPPFB is finding that it must now shift focus on breeding walnuts for their nuts to finding specimens with resistance.  “We have already seen some trees that are showing promise,” notes Dr. Funk. “The black walnuts seem to be the most susceptible, while the Persian and English walnuts are a bit more tolerant.”

As it stands now in Colorado, over 500 walnut trees have been killed by thousand cankers disease in Boulder, and none remain in Colorado Springs.  In Utah, black walnuts are not widely planted, and a mortality estimate has not been determined.

Keep an eye on your walnut trees and report any sudden dieback or death to the UPPDL.  Unfortunately, there are no identified controls; rapid detection and removal may be the only option to prevent spread.

-Marion Murray, IPM Project Leader

 Picture of the Quarter 

Dark hyphae of the fungus that causes necrotic ring spot (Ophiosphaerella korrae) can be seen growing between two turf roots. The fungus is thought to move from plant to plant by growing along the surface of the roots and rhizomes. -Erin Frank, Plant Disease Diagnostician (No longer at USU)