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United States Department of Agriculture

Agricultural Research Service

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The Heart of the Matter
Tree Heartwood Can Halt the Spread of a Fatal Oak Disease

What do you do when you've got a cold? Many of us turn to natural remedies like hot tea with plenty of honey and lemon. Natural remedies can't cure every illness, but they can be very effective for some problems––and not just human problems. Scientists with the Agricultural Research Service (ARS) are investigating the use of a natural remedy to protect trees against a serious illness.

The illness is called sudden oak death (SOD), and the remedy involves using "heartwood" (wood from the center of a tree) to prevent the disease from spreading.

SOD spreads quickly and can harm a variety of oak species and other plants. Most of the plants that are infected don't die, but they do suffer leaf and twig damage. Unfortunately, SOD is a much more serious problem for many oaks and tanoaks. Since the disease first appeared in the United States, it has killed around 1 million trees.

Once an oak has been infected with SOD, its fate is grim. Its leaves turn pale, then brown. The bark splits and breaks and begins to ooze a dark-red, saplike substance. New shoots wilt and die. Beetles begin to bore into the wood and eventually––often between 6 months to 2 years––the tree will die.

The source of this destruction is smaller than an acorn: a fungus-like microorganism known as Phytophthora ramorum. It's pronounced fight-OFF-thor-uh rum-OR-um, but most people call it P. ramorum for short. P. ramorum infects over 30 plant species, including rhododendron, maple, and honeysuckle. Scientists have not yet found a cure for it, but help may be on the way.

For years, scientists have known that tree heartwood produces protective compounds that destroy microorganisms. The compounds found in the heartwood of Alaska yellow cedar, for example, can prevent decay for up to a century after the tree has died! Compounds from the heartwood of several other conifers (cone-bearing trees) are known to have similar antimicrobial properties. Could any of these compounds offer protection against P. ramorum?

Daniel Manter thinks heartwood could be used to prevent P. ramorum from growing and spreading. Manter is a plant physiologist in the ARS Soil Plant Nutrient Research Unit at Fort Collins, Colorado.

Spores Away!

With Rick Kelsey, of the USDA Forest Service, and Joe Karchesy at Oregon State University, Manter exposed P. ramorum spores to various compounds, wood chips, and essential oils extracted from heartwood. Spores are the microscopic reproductive cells that P. ramorum and similar organisms use to move to new locations. They can hitch a ride on automobiles, animals­­, and human clothes. What the scientists found was exciting.

"Under a microscope you can actually see the spores explode," Manter says. "The outer membrane breaks and the cell contents come tumbling out."

Heartwood extracts taken from western redcedar and incense cedar were the most effective. They damaged twice as many spores as the extracts taken from Alaskan yellow cedar, western juniper, and Port Orford cedar. Douglas fir and redwood extracts, which were also examined in the study, were less effective.

"With further research, individual heartwood compounds might be developed into environmentally friendly treatments that could protect plants against P. ramorum infection," Manter says.

How would this work? One solution might involve making wood chips from the heartwood. Chips are light and easy to move, so they could be scattered in areas with high human activities––such as park trails, walkways, and bike paths. That way, humans would be less likely to pick up spores on their shoes and carry them to a new area. That would be good news for oaks and tanoaks everywhere.—By Laura McGinnis, Agricultural Research Service.

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Last Modified: 6/9/2008
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