SJU Scientists Study Bacterial Leaf Scorch on Campus

Monday, November 3, 2008

t's fall foliage season on campus. After a late start, the leaves on our numerous trees are turning brilliant, jewel-like shades of ruby, garnet, citrine and topaz. But mixed in with the autumnal palette, there are trees on main campus with leaves that have already turned brown. And if it seems too early in the season for foliage to be desiccated and brown, in many cases, it is. Unfortunately, some trees on campus are infected with a pathogen that caused their leaves to start dying as early as June.

Susan Jackson, M.S. '07, an adjunct professor in the biology department, knows exactly where to find four species of oak at Saint Joseph's with leaves, branches and xylem, or transport tissue, that is prematurely dead. For her biology thesis, she surveyed 66 oaks on main campus during October of 2005 and 2006 to determine the plants that suffer from bacterial leaf scorch disease. She found that an astounding 41 percent of them are infected, and will almost certainly die within five to 10 years.

"It's a major problem," says Jackson. "At this time, there are no known antibiotics that can treat the disease. Over the past two years, some of the affected oaks on campus have been cut down, but I am sure new trees – and not only oaks – have been infected during this time."

Bacterial leaf scorch is caused by Xylella fastidiosa (Xf), a pathogen that is spread among woody plants and crops by an insect called a leafhopper. The bacterium has forged a speedy path of destruction in more than the mighty oak. It has devastated other arboreal trees, as well as citrus groves and grape vines all over the world, resulting in multi-billion dollar losses in agriculture. It has also destroyed the priceless aesthetic beauty of tree canopies in parks, forests and suburban landscapes.

With her advisor, Andrew McElrone, Ph.D., a former biology faculty member who now researches the problem for the U.S.D.A., and Piotr Hadbas, Ph.D., assistant professor of physics, Jackson published research in the October 2008 Journal of Experimental Botany that describes the complex mechanisms by which the disease spreads rampantly throughout the xylem of the infected plant. The pathogen moves through the veins that carry water and nutrients, colonizing and clogging them so that water transport is disrupted in its roots and tissues. Because the disease is systemic, in effect, the plant eventually starves and dies of thirst.

Jackson hopes that her work provides another glimpse into the pathology of Xf, so that in time, researchers will discover how to isolate and remove the diseased tissues before the bacterium has an opportunity to spread. But for now, because Xf is systemic, she says, "It is impossible to pinpoint specifically where it is located in an infected plant."

Jackson adds that while many people involved in horticulture and the scientific communities are aware of the problem, the general public is not. "I would like people to understand that when they take a walk on campus or anywhere to enjoy the beauty of the scenery, that our trees are at risk of being infected with Xf. And I hope they will ask for more to be done to study and eventually stop this disease."

--Patricia Allen

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