History has a way of repeating itself. Forest pathologists in the East have reacted with alarm to the threat of a new blight, another nonnative pathogen introduced via nursery stock, this time affecting oaks. The new disease, known as sudden oak death, manifests as bleeding stem cankers, blighted leaves, and twig dieback, and can kill a mature tree in just a few growing seasons. The disease has killed a range of species, including tan oaks and live oaks in California and Oregon. There is no treatment yet available for infected trees; just how the disease unfolds is not completely understood.

PathoProfile

The organism that causes sudden oak death belongs to a class known as the Oomycetes or water molds, a group which includes the potato late blight, the disease responsible for the Irish potato famine in the mid-1800s. Phytophthora ramorum, or Pr for short, gets its genus name from the Greek word for “plant destroyer,” and is not to be confused with oak decline, a disease that also affects oak forests in our region. Pr’s life cycle includes an airborne phase, where spores float aloft until they land on susceptible hosts, particularly where water is available, perhaps where dew has collected in droplets on a leaf surface or shoot. Pr apparently survives in soil for long periods, probably an adaptation to survive those times when a host isn’t present. After penetrating the leaf surface or bark of its host, Pr spreads through host tissue, producing leaf spots, dying twigs, and bleeding cankers on the tree’s trunk.

The pathogen was introduced into numerous States in plant nursery shipments, mostly on camellias and rhododendrons, which can serve as sporulators—plants that aren’t killed but are sources of reproductive spores. In one of the first known and largest introductions in 2004, 1.5 million potentially infected plants were shipped from a single nursery in California; every State in the United States received potentially infected stock.

Steve Oak, plant pathologist with the Forest Service, Southern Region Forest Health Protection unit and technical coordinator for the sudden oak death early detection survey in U.S. forests, cites his informal scratch-pad estimation that perhaps 2,000 to 5,000 infected plants ended up in people’s backyards and gardens nationwide. His nightmare scenario: an infected rhododendron planted in a backyard with a stream nearby, a mature oak forest with a rhododendron understory upwind.

“If eastern oaks are susceptible, we’ll have a problem,” he says, and so he supplied seedlings of nine compass—august 2007 Steve Oak, forest health plant pathologist, is technical coordinator for the sudden oak death early detection survey in U.S. forests. (Photo by Rod Kindlund, U.S. Forest Service) eastern oak species to be tested with the pathogen at a USDA Agriculture Research Station containment greenhouse at Fort Detrick, MD, by research plant pathologist Paul Tooley. The results showed that all nine eastern oak species tested could be infected under optimal conditions— with the caution that those conditions may be hard to find in nature. The pathogen needs a host to support the production of spores; rhododendron (and perhaps other species) will do, while oaks will not.

Location and Opportunity

For the disease to infect and establish, three conditions must be present: (1) hosts, (2) pathways for introduction, and (3) the right environment. Oak, along with Bill Smith, coordinator with the SRS Eastern Forest Threat Assessment Center, Frank Koch from North Carolina State University, and members of the National Forest Health Monitoring Risk Mapping Team have produced a map of areas where all three factors are present: potential hosts, favorable environment, and pathways—in this case, nurseries that may have received infected plants. The map provides a set of target areas for field monitoring by a team of State forestry agencies trained by Oak in disease recognition and survey protocols. Smith’s data show the Southern Appalachians at high risk for infection, along with a small area on the western part of the Mississippi coast.

In addition, Pauline Spaine and Bill Otrosina of the SRS Insects, Diseases, and Invasive Plants unit in Athens, GA, have come up with a descriptive index of some of the microclimate features believed to be critical to an outbreak—a Sudden Oak Death Infection Index—to estimate when and where natural forests could be susceptible to infection. They collected hourly temperature and humidity values in natural forests, choosing sites where previously published data suggested that temperature and humidity values indicated a level of risk for Pr infection. The index can be used to highlight times when the dew point occurs at the optimum temperature for infection for at least 6 hours—times when water would be collecting on leaf surfaces where spores land. Based on their data, Spaine and Otrosina predict that certain areas in western South Carolina and along the Georgia coast, where humidity is high, may be at the greatest risk for Pr infection in the future.

Early Detection is Essential

This spring in Mississippi, rhododendron leaves deployed as Pr bait downstream from a nursery that received infected nursery stock tested positive for the pathogen. The presence of the pathogen is not the same as a disease outbreak. No such outbreak has yet occurred in wild forests in the Eastern United States. While infected vegetation has not been found in the natural environment, State and Federal forest health professionals continue to intensively monitor this site and other areas in the Southeast that might be vulnerable to sudden oak death.

“The State forestry and agriculture agencies are doing a tremendous job with survey and monitoring,” says Oak. “Regulation has been improved and strengthened, but it’s still leaky. There are two confirmed cases of Pr identified in nurseries in Florida and Mississippi this year so far, but still none found in wild forests anywhere outside of California and Oregon. Still, the takehome message for homeowners and landscapers who want to minimize the risk of introducing Pr is to buy locally propagated plants.”

If the disease is found through monitoring, current eradication measures call for ringing the affected area with a 100-foot buffer, then cutting and burning all plant material within the ring. “If it shows up simultaneously in 10 States in 100- acre blocks, there’s not much that can be done,” says Oak. “There are some key questions still to be answered. What is the latent period between introduction and establishment and between establishment and detectable infestation? How much response time do we have between detection and action?”

Though the disease hasn’t been detected in eastern oaks, forest pathologists remain vigilant.

“Are we out of the woods yet? No, the resource at risk is too great to let down our guard,” says Oak. “Our only hope is early detection, and it would be irresponsible to assume less than the worst-case scenario—another chestnut blight—until we can get more information about the epidemiology in eastern hosts.”