A look at EIPD Strategic Initiative projects

Thousand cankers disease “is a rapidly emerging invasive threat to walnut orchards in California,” says Richard Bostock, plant pathologist at UC Davis and a lead researcher on a 3-year study of the disease. “Only in a few places in the state have English walnut trees died due solely to this disease,” he says, “but it’s on its way to becoming endemic in all the walnut-growing areas, and we are still not sure where this is going to go.” The fungal pathogen, Geosmithia morbida, was new to science when it was found in California in 2008 in Northern California black walnut trees growing in Davis. Its vector, the walnut twig beetle (WTB), has been known in California since 1959. The male beetles attack a tree first, releasing a pheromone that attracts thousands of beetles to the phloem tissue of the tree’s trunk and large branches, where they tunnel, feed and produce offspring. Spores of the pathogen are carried on the vector’s body and infect the host tissue; the cankers produced by the pathogen coalesce and can girdle a trunk or branch, and in severe cases can cause decline and death of the tree. The study, reaching completion now and codirected by Steven Seybold, a USDA Forest Service entomologist, has assessed the distribution of thousand cankers disease in the state’s walnut-growing areas, differences among walnut (Juglans) species in their reaction to the pathogen and attractiveness to the beetle, the genetic diversity of the fungus and the biology of the beetle. The disease appears to be present in all areas; in some orchards, there is a very low incidence of the disease, while in others, as many as 90% of the trees have WTB attacks and cankers. Thousand cankers disease has decimated eastern black walnut, Juglans nigra, in landscape and urban plantings in some Western states and parts of Colorado, where, in 2008, the association of the beetle and fungal pathogen in the disease was first discovered and reported by researchers at Colorado State University. In 2010, the disease was reported on eastern black walnut in Tennessee and subsequently in other Eastern states within the native range of this species. In California, the research team has documented the disease in the Southern California black walnut, Juglans californica, in English walnut, Juglans regia, which is the commercial orchard species, and in Northern California black walnut, Juglans hindsii, which is used as a rootstock for J. regia, and also in Paradox rootstocks. “About 70% of commercial trees are on Paradox,” says Bostock, and the study has discovered that “Paradox is highly susceptible and is very attractive to the beetles.” Investigating the population structure of the fungus in different geographic regions in the United States, other researchers have reported “a great deal of diversity in the fungal population in Arizona, California and New Mexico,” says Bostock, “and their findings suggest that these states could be the source of the thousand cankers disease epidemic.” The susceptibility of Paradox and the destruction the disease has caused in eastern black walnut are cause for concern, but “we are not raising a lot of Above, a heavily infected Southern California black walnut (Juglans californica) showing sapstaining of the bark and dieback. Inset, a canker underneath the outer bark of a Paradox hybrid walnut showing necrosis around walnut twig beetle tunnels. Top, culture of Geosmithia morbida, the fungal pathogen that causes thousand cankers disease. Middle, close-up of walnut twig beetles tunneling in the bark of an infected tree. Bottom, actual size of the beetle in relation to a penny. UC ANR‘s five strategic initiatives seek new ways of partnering within and outside the university to tackle emerging issues in California. As part of this strategic vision, the Division annually funds research and extension projects in and across all initiatives. Four recent and ongoing studies in the Endemic and Invasive Pests and Diseases Strategic Initiative are profiled here.