Two agencies have begun offering to test homeowners' plant material suspected of being infected with sudden-oak death, according to information provided by the organizations.
The N.C. Department of Agriculture and Consumer Services' Plant Industry Division, in cooperation with the N.C. Cooperative Extension Service, will test the plants in question: camellias and viburnums bought between March 2003 and March 2004.
These plants originated from Monrovia Growers, a major wholesale supplier to the nurseries and garden centers in Azusa, Calif. Also, rhododendrons bought between February 2003 and May 2004 in 5- to 7-gallon containers from Lowe's Home Improvement stores may be infected.
The disease was discovered in March at the Monrovia Nursery, which had shipped plants the past year to 1,200 locations in 39 states, including North Carolina. After isolation and testing, SOD was confirmed on camellias at nine of the 67 sites where plants were shipped. Blocks of plants testing positive for the disease were destroyed.
Oaks began dying
Coastal California was the first American region to experience sudden-oak death. In 1995 large numbers of tan oaks, coast live oaks and black oaks began dying in Marin County, Calif. By 2000, University of California researchers had isolated a previously unknown species of Phytophthora, a funguslike organism, in the dying oaks. A species of Phytophthora isolated on rhododendrons in the Netherlands and Germany in 1993 matches the California species. Relatives of this organism are responsible for the Irish potato famine.
Phytophthora ramorum has two principal means of infection, foliar and bark canker hosts. In oaks the infection is exhibited by bleeding cankers on the trunk, leading to tree mortality. It is thought that oaks must be infected by nearby foliar hosts. It is these hosts that the department of agriculture is concerned about. There are 30 regulated hosts recognized by the U.S. Department of Agriculture Animal and Plant Health Inspection Service, Plant Protection Quarantine. An additional 60 species are recognized as susceptible.
The list continues to grow and to be updated. A July 30 update from the Plant Health Inspection Service says that as of July 26, 122 positive finds have been detected in 148 locales in 21 states, including nine in North Carolina.
Spread by spores
The principal plants of concern for the homeowner are rhododendrons, camellias and viburnums. These and other foliar hosts exhibit leaf spotting and some leaf drop but are not generally killed by the disease. They are however considered "reservoirs of the inoculum." That is, spores on the foliage of these plants are spread to oaks through the action of wind-blown rain. Spores are found on these foliar hosts but are not present on infested oak bark, so it is thought that the presence of these alternative hosts is an essential in oak infection. In the devastated California areas, mortality is greatest where rhododendrons and California bay laurel are present.
Though nursery inspection continues, testing of homeowner plants has begun through the cooperative-extension agencies. The disease affects the foliage or small branch tips so that only suspicious-looking leaves or twigs of camellias, viburnums and rhododendrons bought during the target period should be submitted. Samples will be tested locally at county cooperative-extension agencies. Homeowners are asked not to return plants or samples to the nursery or garden center where they were purchased.
According to the department of agriculture press release, "Plants testing positive for SOD must be destroyed to prevent further spread of the disease. Plants should not be removed from the landscape, but should be left in place until test results are complete. Additionally, if the disease has spread to other plants at the garden or landscape site, these as well as other host plants in a designated buffer area may have to be destroyed. The greater fear is that SOD could become established in the oak forests of the southern Appalachians.