Threat Of Sudden Oak Death A Worry For Forests And Wildlife

A disease that kills oaks in California has been confirmed in 13 Georgia nurseries, and officials are awaiting the results of tests on plants bought and planted by homeowners.

Daryl Kirby | April 6, 2006

A disease that is killing oak trees in California has been shipped to Georgia through infected nursery plants, and officials are fearful that if the fungus escapes into the Georgia woodlands it could be devastating to oak forests here.

Monrovia, a major nursery supplier in California, shipped about 28,000 plants to Georgia from January of 2003 to March of 2004 that could have been infected with Sudden Oak Death (SOD), a fungus that has killed tens of thousands of oak trees in 13 California counties and one county in Oregon.

So far, testing has confirmed plants infected with SOD at 13 Georgia nurseries — spread across the state from Dalton and Athens in the north to Atlanta and Augusta and south to Douglas and St. Simons — that received plants from Monrovia. Those plants were destroyed, but by the time testing was started, a large number of plants possibly infected with SOD were gone from the nurseries, likely sold to homeowners.
Just how many plants carrying the disease have been placed into yards and landscapes is unknown. A number of plants from homeowners that were possibly infected are now being tested, and officials are awaiting confirmation of results.

Dr. Jean Williams-Woodward, a plant pathologist with the University of Georgia, is heading up the testing of suspected SOD-infected plants in Georgia. Her lab has processed more than 800 samples, mostly from nurseries that received plants from Monrovia, and the only confirmed positives have been from 13 nurseries. The UGA lab has also tested samples from homeowners who bought Monrovia plants, particularly camellias, rhododendron and viburnum (snowball bush), which have been shown to be highly susceptible to the fungus.

Any of the lab’s tests that come back positive for SOD must be verified by the United States Department of Agriculture (USDA) through the Animal and Plant Health Inspection Service (APHIS). Dr. Woodward would not comment on whether any of the UGA lab’s tests of homeowner samples had come back positive for SOD, but GON learned that two plant samples taken from homeowners in the Atlanta area have in fact shown positive results for SOD.

“Those results are not confirmed yet. They are not considered positive,” said Commissioner Tommy Irvin of the Georgia Department of Agriculture. “Every sample must be sent on to confirmation from the USDA. We don’t want to over react, and we don’t want to under react either, which is why we went ahead and destroyed those plants in question.”

One of the potentially infected plants was still in the pot it was sold in, while the other had been planted in the landscape.

Georgia’s battle to prevent SOD from infecting and killing our oak forests began on March 15. That’s when Commissioner Irvin issued a total quarantine on all nursery-plant shipments from California after word that Sudden Oak Death fungus had been discovered on camellias at the Monrovia Nursery in California. Four days later the Georgia Department of Agriculture partially lifted the quarantine.

“We are still blocking shipments on any genus of plants that we know can serve as a host for this fungus,” said Commissioner Irvin.

The list of plants that are known to serve as hosts for SOD that are banned from import from California is a long one, and it includes: camellia, rhododendron, viburnum, maple, buckeye, strawberry tree, bearberry, witch hazel, honeysuckle, rose, blueberry, beech, mountain laurel, fir, yew, blackberry and raspberry. A total of at least 59 plants and trees have been confirmed as hosts for the fungus.

In a press release dated March 19, Commissioner Irvin said, “Sudden Oak Death is an extremely serious disease. This has the potential to be more devastating than chestnut blight, which wiped out virtually all of the native American chestnut in the 1930s. The cost to lumber companies, homeowners, gardeners, and cities would be overwhelming and the damage to wildlife and our landscape would be heartbreaking.”

Sudden Oak Death was first detected in the United States in Marin County, Ca. on a tanoak tree in 1995. It has since spread through 13 coastal counties of northern California and in one county of southwest Oregon along the California border. It is thought that the disease prefers wet, cool climates, however the discovery of the fungus on camellia plants at the Monrovia Nursery occurred in Los Angelas County, well away from any area previously infected by SOD. It is not known how the plants in the nursery were infected.

There is also little information on the life history and characteristics of how Sudden Oak Death is spread, even in California. In the Southeast, no research is being done yet to find out which trees will be susceptible to SOD. Researchers believe that infection propagates on the leaves of understory host plants, such as rhododendron and buckeye, causing a rapid build-up of the fungus in the environment, which then can infect oaks.

Anyone concerned about SOD in Georgia should not look for dead or dying oak trees at this time, according to Dr. Woodward.

“It would take several years for an oak tree to show signs of Sudden Oak Death,” she said.

A bleeding canker — an oozing sore on the lower trunk of an oak tree — is a sign of SOD infection in oaks, however SOD is not the only cause of an oak canker. The only way to positively identify SOD on an oak tree or on the leaves of host plants is through laboratory testing.

“It would take several years for an infected plant to transfer the disease to an oak. It could spread the disease this year, but it would then take another year to really show the first symptoms on an oak — branch die-back, bleeding cankers. And those symptoms depend on the oak species, and we really only know about what has happened in California. We don’t know what symptoms might be in the South or what trees might be affected.”

Once an oak tree that is infected with SOD shows the symptom of a bleeding canker, the tree could be dead as quickly as six months later.
On shrub species like camellia, the symptoms of SOD can range from leaf spots to twig girdling, and often do not result in the death of the plant.

According to James Johnson, the forest health coordinator for the Georgia Forestry Commission, only those who bought Monrovia plants since January 2003 should be concerned about SOD right now.

“If it spreads through our forests, it could be catastrophic,” James said, “but there are a couple of bright spots. One, it takes several years of building understory infections before mature oaks are at risk. Two, in the places in California that have gotten this stuff, it’s not killing all of the oaks. Some oaks die, then it will end at that hillside. There have been comparisons to chestnut blight, but we don’t think it would happen that way.”

There is no cure for a tree infected with SOD, although last week researchers announced a fungicide that could help protect oaks from being infected by SOD. However, large-scale application of the fungicide is not feasible.

To protect against SOD or other plant and tree diseases, buy plants grown in Georgia by nursery suppliers.

If you purchased a plant since 2003 that originated from California, particularly Monrovia Nurseries, contact your County Extension Service office for instructions.

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