Rush is on to protect oaks from pathogen

Tennessee agricultural officials are scrambling to halt the invasion of a new disease that is wiping out oak trees and other plants in the western United States. 

The fungus-like pathogen, commonly known as Sudden Oak Death, was first reported in 1995 in central coastal California after it began killing tens of thousands of tanoaks, coast live oaks and California black oaks. Despite its name, the disease also can infect a number of plants such as rhododendrons, camellias and viburnums. 

Last March, the situation took an abrupt turn for the worse when the SOD pathogen was discovered in at least two California nurseries that ship plant material nationwide. The Tennessee Department of Agriculture responded by banning imports of selected plants from California and by halting all sales and movement of susceptible nursery stock that might already have entered the state. 

Armed with a list of nurseries, greenhouses and garden centers in Tennessee that received plants from nurseries in California, teams of inspectors processed some 230 samples from 13 different sites. Last week, those tests revealed three positive cases of SOD at two Tennessee nurseries. 

"This has been a massive undertaking," said Anni Self, a plant pathologist with the Tennessee Department of Agriculture. "We put a lot of man hours into checking these nurseries and plant dealers as quickly as possible." 

Self said the infected plant material from the two nurseries in Tennessee will most likely be burned to ash or double bagged and buried six feet deep in a landfill. She said the precise protocol for dealing with the infected material will come from the U.S. Department of Agriculture. 

Tom Womack, spokesman for the Tennessee Department of Agriculture, said the situation with SOD should not deter the public from purchasing plants for gardening and landscaping this spring. 

"Whatever plant material was in question has been either tested and cleared or will be destroyed," Womack said. 

In eastern hardwood forests, SOD can easily be confused with a variety of common disorders such as oak wilt, oak decline or leaf blight. The disease - caused by a fungus-like organism called Phytophthora ramorum - was first identified in 1993 in Germany and the Netherlands on ornamental rhododendrons. Until recently, SOD was thought to be restricted to California and Oregon in the United States. 

Tennessee, Georgia and Florida are among the southeastern states believed to be likely candidates for the disease based on factors such as climate and forest type. Among nursery plants, the disease most frequently has been found in camellia. Among oaks, at least two eastern oak species, northern pin oak and northern red oak, are highly susceptible to SOD. 

Tennessee is home to at least 15 oak species that have commercial value. Oaks are included in about 85 percent of all forest types across the state, and their acorns are a major source of food to a variety of wildlife. 

George Hopper, professor and head of UT's department of forestry, wildlife and fisheries, identified oaks as the principle component of Tennessee's forested landscape. 

"Any threat to the oak species is a serious threat to Tennessee's forest resources, both economically and ecologically," Hopper said. 

So far, SOD is known to occur in the wild only in 12 coastal counties in California. Two years ago, state and federal agricultural officials and researchers from the University of Tennessee participated in a pilot survey that checked for the disease in nurseries and forested areas across the state. Federal and state scientists are about to expand that survey to include high-risk areas across 23 states, including Tennessee. 

Susan Shaw, a biologist with the Cherokee National Forest, said SOD so far has been spread mostly through nursery material. 

"We do not get any seedlings from ornamental nurseries, so in that regard, we're pretty comfortable," Shaw said. 

In oaks and tanoaks, SOD can cause cankers on the stems. Once the cankers form, the leaves turn pale yellow or brown within a few weeks, although the tree may survive for several years. A black or reddish ooze often bleeds from the cankers and is visible on the bark. 

Kurt Lamour, an assistant professor with the UT department of entomology and plant pathology, specializes at identifying the SOD pathogen through DNA samples. Lamour said the disease manifests itself in a variety of ways depending on the host plant and can be invisible to the naked eye. 

Lamour expressed confidence that the inter-agency response team has surveyed all the potentially infected plants that entered the state from California. 

"I think in this scenario, based on a historical perspective with Dutch elm disease and the chestnut blight, they responded quickly and have the proper network set up," Lamour said. 

Morgan Simmons may be reached at 865-342-6321.

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