RIVERSIDE, Calif. -- A research team led by University of California, Riverside’s Leonard Nunney, a professor of biology, has received a grant of $1.75 million from the U.S. Department of Agriculture to battle a bioterrorist less than half an inch long and bearing a lethal weapon that is microscopic in size.
Nunney’s team will study Xylella, a pathogen that has potential, along with its vector, the glassy-winged sharpshooter, to wipe out California’s grape, peach and almond industry, as well as much of the state’s ornamental bushes and trees. In time, the state’s citrus industry also could be destroyed by the sharpshooter and the Xylella pathogen.
“Xylella causes damaging diseases in a wide range of economically important crops,” said Nunney, who also is director of the Graduate Research Unit in Evolution and Ecology at UCR. “The North American varieties of Xylella attack grapes, almonds, and peaches, and have devastated the ornamental oleander bushes that line Southern California freeways—at enormous replacement cost.”
There are four main Xylella subspecies, three in North America and one in South America. The South American variety feeds on citrus. “But it has not yet crossed our border,” Nunney said.
Among the North American varieties, subspecies fastidiosa, the one that causes Pierce’s disease, is found on grapes and almonds; subspecies sandyi on oleanders, day lilies, magnolias, and jacarandas; subspecies multiplex on almonds, brittlebushes, sages, olives, oaks, plums, and peaches.
“There are several puzzles about this bacterium,” Nunney said. “If you find Xylella on a certain plant, you can’t predict what else it might be found on. Most bacteria will attack a group of related organisms. If they attack a peach tree, they might also attack apricots. But with Xylella there’s little phylogenetic pattern as to which species each type of Xylella will be found on. We need to understand what it is about these plants that makes each type of Xylella favor them.”
According to Nunney, new Xylella strains are evolving, thus complicating the research being done on the bacterium. “We started to pick up some strains that showed a mixture of all three North American subspecies. They may have picked up pieces of DNA from dead bacteria. We think this interchange of genetic material is relatively recent—probably during the last 100 years or so.”
Each of the new strains of Xylella identified so far attacks different plants than do the original Xylella subspecies “That’s worrisome,” Nunney said. “Clearly, we need to know more about the genetic basis of host specificity.”
In the next four years, Nunney and his research group will develop a nationwide map of different subspecies of Xylella, and an effective monitoring system both to catch foreign forms introduced into the United States and new forms evolving within the country. With its South American partners, Nunney’s group will explore the geographical origins of the different forms of Xylella.
“Are there yet more unknown forms of Xylella in the native plants of Central and South America"” Nunney asked. “Citrus is not native to South America, so Xylella must have been living in something else before citrus was introduced, since that form is older than the citrus introduction on that continent. We have to understand the interrelationship with all forms before we can understand the current and future dangers posed by Xylella to grapevines and to other plants as well.”
Nunney’s interdisciplinary group includes co-PIs Richard Stouthamer, Bob Luck, Don Cooksey and Frank Wong of UCR. Additional collaborators are Lisa Morano, the University of Houston-Downtown; Don Hopkins, Mid-Florida Research and Education Center of the University of Florida; Erin Schuenzel and Norm Schaad of the Foreign Disease-Weed Science Research Unit, U.S. Department of Agriculture - Agricultural Research Service (USDA-ARS), Fort Detrick, Md.; Guillermo Logarzo, the South American Biological Control Laboratory, USDA-ARS, Hurlingham, Argentina; and Carlos Coviella, Universidad Nacional de Lujan, Argentina.
Funding for the research is being provided by the USDA’s Cooperative State Research, Education and Extension Service (CSREES). CSREES advances knowledge for agriculture, the environment, human health and well-being, and communities by supporting research, education, and extension programs in the Land-Grant University System and other partner organizations.
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