By Nancy Gutierrez
In 1995 residents of Mill Valley in Marin County observed large numbers of tanoak trees dying. At the same time, reports from Santa Cruz and Monterey counties confirm the deaths of more tanoaks.
Nine years, 19 states and millions of dollars later Sudden Oak Death (SOD) is spreading in epidemic proportions. The newest state to experience the invasive disease is New York. A positive sample was confirmed by the county on June 25 in a mature red oak tree located in an oak forest in a county park.
SOD, also known as Ramorum Canker and Blight, is a forest disease caused by the fungus-like pathogen Phytophthora ramorum, that, at its worst, infects woody portions of trees. This infected area, called a canker, expands into the trunk disrupting its physiological function killing the tree. This previously unknown pathogen was first discovered on U.S. soil in California coastal counties and has since caused widespread dieback of tanoak and several oak species in the central and northern coastal counties of California.
In March, Monrovia Nurseries in Azusa, Calif. shipped infected plants to several states throughout the nation. Trace-forward surveys from Monrovia Nursery reported infestations in Florida, Washington, Oregon, Georgia, Louisiana, North Carolina, New Mexico, Tennessee, Texas, Colorado, and Virginia.
Because the shipments were to nurseries the diseased plants were contained, however growing panic over the severity of the disease lead to restrictions and quarantines regarding infected plants.
Professional nurseries, a $3 billion industry, the third largest in California, have been hit hard. State to state movement of plants that are considered hosts for the pathogen has been restricted there are even restrictions on intercounty shipments of plants. In April there were four states refusing to take in any California nursery stock.
"Nurseries have been impacted the most. This has affected California's ability to ship plant material to the east coast and Canada," said Agricultural Biologist for the Tulare County Ag Commissioners Office, Dennis Haines. "It varies with states. A few are not accepting any plants from California. Some states are restricting close and associated plant hosts [of SOD]. Others are just buying from nurseries that have been inspected and certified free from SOD."
The disease thrives in coastally influenced forests with a mix of oaks and many foliar-host plants, especially Bay laurel trees. Katie Palmieri, a representative with the Center for Forestry at the University of California, Berkley, said SOD is also more likely to appear in areas with less diversity in plant species and a prevalence of hosts for the disease like Bay laurel.
"[SOD] needs a humid, cool environment, with a lot of rainfall," Haines said. "you have to have the disease organism, in conjunction with a potential host under the right environmental conditions."
The climate in the Central Valley is too dry and hot for SOD to survive and have any fatal effects on local native plants. But Nurseries can create these environments in greenhouses and hothouses. In May several nurseries in Tulare County tested positive for SOD in plants shipped from Southern California nurseries including the Monrovia Nursery in Azusa.
Nurseries have artificial humid and damp environments. The nursery environment created a habitat for the disease," Haines said. "Plants that were shipped to retail outlets in Tulare County tested positive [for SOD]. These plants were destroyed."
"Nurseries sampled and tested plants that showed symptoms of SOD. Samples were sent to a lab for testing. If tests returned with a positive result the entire lot that the plant came from was destroyed and the nursery was not certified. Haines said nurseries that had negative test results were placed under a certification program and are continually monitored.
"We have four nurseries that have been inspected and certified as not producing hosts for SOD," he said.
Haines said the Monrovia Nursery in Woodlake was sampled and found clean. What is alarming for nursery owners is the increasing number of plants that are being confirmed as hosts for the pathogen.
SOD is found on other species like Douglas fir trees, rhododendron's, California Bay laurel and camellias. While several oak species can die from SOD other plants sustain less fatal infections on foliage and twigs. It is these carriers who spread the disease through wind blown rain. The California Oak Mortality Task Force, a coalition of research/educational institutions, public agencies, non-profit organizations, and private interest groups, was formed in 2000 to coordinate research, management, monitoring, education, and public policy efforts addressing elevated levels of oak mortality from SOD in California. The COMTF web site reports that there are 60 species known to be susceptible to the Phytophthora ramorum fungus. Of those 60 species, 30 have been recognized as regulated hosts by the United States Department of Agriculture, Animal and Plant Health Inspection Service – Plant Protection and Quarantine (USDA-APHIS-PPQ), and by the California Department of Food and Agriculture (CDFA). The web site reports that, "Regulated plants are those adapted from other regulated lists or were added upon completion, documentation and review of traditional Koch’s postulates." Koch's postulates is a system of questions designed to prove that a specific organism is the cause of a disease.
The 30 identified hosts include: Japanese camellia, Sasanqua camellia, Witch hazel, Toyon, Tanoak, California honeysuckle, Himalaya pieris, Pieris ‘Forest Flame’, Pieris ‘Brouwer’s Beauty, Japanese pieris, Douglas fir, Coast live oak, Canyon live oak, California black oak, Shreve oak, California coffeeberry, Rhododendron (including azaleas), Wood rose, Coast redwood, Western starflower, California bay laurel, Oregon myrtle, pepperwood, Evergreen huckleberry, Bodnant viburnum, Doublefile viburnum, Laurustinus.
Though many of these plants are sold in this area, Haines reinforces the fact that the disease itself cannot thrive here. The disease is widespread in Marin, Santa Cruz and parts of Monterey and Sonoma counties. The disease is patchy in Humboldt, Mendocino, Napa, Solano, Contra Costa, Alameda, Santa Clara, and San Mateo. This supports the the notion that it tends to thrive in moist micro-climates like those found along California's coast.
"I have talked to pathologists from the forest service and the likelihood of a natural infestation in the foothills is highly unlikely," Haines said. "In Stanislaus [county] there was a nursery growing camellias under a shade cloth with overhead sprinklers. SOD was confirmed there. But under those conditions, which are the most optimum for the valley, the disease was just barely hanging on, it was not killing any plants."
For more information visit www.suddenoakdeath.org.