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Beetle and Longtime Fungal Associate Go Rogue

Scientists with the U.S. Forest Service and Colorado State University examined a fungus native to North America, the native beetle that carries it, and their host tree and found something surprising: Geosmithia morbida (the fungus) and the walnut twig beetle co-evolved and, while the beetle/fungus complex was once the equivalent of a hang nail for a black walnut tree, it has now become lethal. Research published on November 13, 2014 in PLOS ONE by U.S. Forest Service scientist Dr. Keith Woeste, Colorado State University scientists Dr. Marcelo M. Zerillo and Dr. Jorge Ibarra Caballero, and colleagues, details the origins and spread of Thousand Cankers Disease (TCD), a fungal disease that is threatening the health of black walnut in the Eastern United States. The study provides a detailed look at the genetic diversity of the fungus and how that diversity is distributed on the landscape, allowing scientists to make much stronger conclusions about the sources of TCD spread in the past and in the future. "Black walnut is a species with tremendous economic and cultural significance," said Dr. Michael T. Rains, Director of the Northern Research Station and the Forest Products Laboratory. "To help ensure this species sustains it vibrancy, Forest Service scientists are working with state agencies, other federal agencies, and university partners to advance survey and detection efforts and to understand the genetics of the disease, as well as resistance to TCD." When black walnut trees in California and Arizona began dying of TCD two decades ago, some scientists believed that the walnut twig beetle (image) had acquired a new and probably non-native fungus that was killing the trees. "That wasn't the case," said Dr. Woeste, a research plant molecular geneticist with the U.S. Forest Service's Northern Research Station in West Layfayette, Indiana. "This is a native disease that went rogue." Analysis of isolates of Geosmithia morbida collected from 54 counties in 12 states suggest another surprise: TCD may have spread through the western U.S. and from there to eastern states, and it likely spread from multiple locations. Because the beetle's best known host, the Arizona Black Walnut, is only native to Arizona and New Mexico, it had been assumed that TCD had spread from that region. The study suggests more bad news for black walnut trees. Dr. Woeste and his collaborators found high genetic variability in Geosmithiamorbida, creating the possibility that future genetic variants could be even more virulent than the fungus is today. Study co-author Dr. Ned Tisserat previously showed that the walnut twig beetle/fungus complex will attack any walnut tree, not just black walnut, as well as trees directly related to walnut, such as butternut. In this study, scientists established that TCD affects even distant relatives of walnut called Wingnuts, an important species in Asia. The fungus isn't the only part of the complex behaving in surprising ways. As its name implies, characteristic behavior for a walnut twig beetle is to feed on twigs and small branches, resulting in very minor injury to the host tree. Trees succumbing to TCD die because the beetles attack the main stem of the tree, an unusual behavior for a twig beetle. "It sort of went wild. That was very surprising," said Woeste. Separate research by other scientists, including Northern Research Station research plant pathologist Dr. Jennifer Juzwick, has established that Geosmithia morbida is now being moved by insects other than the walnut twig beetle. Study co-authors included Dr. Andrew D. Graves, U.S. Forest Service Forest Health Protection; Dr. Colleen Hartel, Purdue University; Dr. Jay Pscheidt, Oregon State University; Dr. Jadelys Tonos, Purdue University; Dr. Kirk Broders, University of New Hampshire; Dr. Whitney Cranshaw, Colorado State University; and Dr. Steven J. Seybold, U.S. Forest Service, Pacific Southwest Research Station. [Press release] [PLOS ONE article]