Researchers discover how to stop spread

Images courtesy of USDA ARS. Photos by Peggy Greb.

Sudden oak death (SOD), which, at the moment, is confined to the West Coast, has been compared to the chestnut blight in its potential to devastate oak and tan oak trees. The invasive mold that causes SOD, Phytophthora ramorum, (a close relative of potato late blight), is spreading quickly throughout California and is now in 14 coastal counties from Monterey to Humboldt and in a small portion of southwest Oregon; it has also been found in nursery stocks in Washington State and British Columbia. It is believed the mold was introduced to California in the 1990s through nursery stock from Asia.

Once an oak tree gets infected with SOD, death is quick—between six months and two years. The leaves first turn pale, and then brown, followed by oozing bark cankers. The tree will either die from drought because the mold spores block water flow, or by starvation if the spores infest the leaves and inhibit photosynthesis.

Early signs (dying leaves) of sudden oak death on an oak tree.

To date, the mold has killed over 1 million western oak; it is spread via contaminated soil and windblown rain. SOD has also spread to the United Kingdom and the Netherlands, possibly through infected nursery stock.

So far, only chemical treatments (systemic fungicides and surfactants) have been available to help combat the disease. However, these chemicals are only effective as a preventative measure for high-value trees in yards and landscapes, they do not cure SOD, and are useless for trees devastated by the disease.

Heartwood of cedar trees kills spores

Help is underway from a surprising source: the heartwood of cedar trees, which contains chemicals lethal to the mold that causes SOD. “There’s quite an array of chemical content in the heartwood of cedar trees that have been shown in the past to be antimicrobial and active against insects,” said Agricultural Research Service (ARS) plant physiologist David Manter. “So, that seemed to be a good place to start to find a chemical that would work,” said Manter, who conducted the study with the ARS Soil Plant Nutrient Research Unit in Fort Collins, Colo., along with Dr. Rick Kelsey of the U.S. Forest Service and Oregon State University’s Dr. Joe Karchesy.

The researchers found that when the P. ramorum spores were exposed to extracts from the heartwood of incense cedar, western red cedar, Alaskan yellow cedar, western juniper or Port Orford cedar, the spores were destroyed and fungal cell growth was stopped. “When we put the heartwood chips and spores together in the lab, we saw under the microscope that the spores themselves exploded,” said Manter. “There was something leaching out of the chips that caused this to happen.”

The research team identified several compounds in the cedar that were good candidates. They also found that heartwood could be processed into shavings, wood chips or liquid extracts, according to Manter, who added that the extracts provide an easy-to-use, environmentally friendly tool to control SOD.

In wood chip form, field trials showed that applying the cedar wood chips down where the spores may land, such as on a trail, was effective in controlling the disease. “Spores are being spread by landing on the ground and moving to new areas through hikers’ shoes and bicycle tires,” he said. “This is how the disease is being spread to new parks.” Manter would like to see cedar wood chips put down on the ground in all infected areas. “Spreading cedar wood chips can be a simple, but very important step to help reduce spore movement and prevent the spread of disease to new areas,” he said.

The next goal for the researchers is to develop a sprayable fungicide using the heartwood compound to kill the spores in the wood and leaves of a tree; the researchers are conducting greenhouse trials at the moment to determine the best way to apply the fungicide and how often.

The hope is that these cedar-based fungicides will kill the mold and block its ability to grow. “If the compound can get past the membrane and into the cells, as a systemic fungicide, it can reduce the disease and offer protection against new infection,” he said. This conclusion is not yet proven, but Manter said he would know the result after a few more tests in the greenhouse. Manter said that based on laboratory work, it looks like the cedar extract can be taken up by oak trees. “So, this looks good,” he said.

Plant Physiologist Daniel Manter takes a closer look at a Phytophthora culture. P. ramorumis the microorganism that causes sudden oak death.

Fear of spread to the East

The disease has not yet spread to the oak-rich forests in the East; still, the USDA Forest Service laboratory research has shown that Eastern forest trees (such as red oak and pin oak) are susceptible, but is unclear to what extent. There is justifiable fear that it is only a matter of time until old eastern oak forests will be under siege. The USDA and U.S. Forest Service have stepped in to take aggressive steps to quarantine the fungus to prevent spread eastward. “Any nursery stock (from California) has to be screened before it is transported,” said Manter. Nurseries in certain counties of California cannot even ship outside the infected area. Inspections are difficult, said Manter. “There are not always visual symptoms on the trees,” he said, adding that only a molecular diagnosis is accurate.

The goal, at the moment, is to slow down the spread, said Manter. “The important thing is that SOD is on the radar screen.” Treatment options, up until now, have been limited. Burning is a last resort, but has been done for infected nursery stock.

Commercial availability

A few companies have contacted the researchers for information on their cedar heartwood research, but are still waiting to see the data before making any financial commitment. “They want to see really good efficacy data, because it is a costly enterprise,” said Manter.

While the cedar heartwood extracts are organic, Manter is careful about making too much of that statement. “Organic doesn’t mean nontoxic to other organisms, but from what we have seen, they are benign to nontargeted organisms,” he said. When it becomes commercially available, cedar heartwood extract would be the first organic product available to slow down SOD.

Manter adds, “I’m not a huge fan of the ‘organic’ label—when it comes down to chemistry there is no difference between organic and synthetic.” He envisions the cedar compounds would eventually be made in a laboratory, where it is cheaper to synthesize and will not sacrifice cedar trees. “We would never be able to extract enough of the compound from the trees to meet the demand,” he said.

When will either organic or synthetic cedar heartwood become available commercially? The best-case scenario would be in the next one to two years, Manter said. “Unfortunately, this is not a fast process.”

The author is a freelance writer from Keene, N.H.