Sudden Oak Death Does Not Affect Greenhouse-Grown Crops

March 26, 2004 - 11:31

Florida Department of Agriculture and Consumer Services (FDACS) has banned the importation of all plant nursery stock from California following the discovery of Sudden Oak Death (SOD) disease in several large commercial nurseries that ship nationwide. With the recent announcement of the Proven Winners/ColorChoice partnership, this information may prove more valuable to greenhouse growers than it did in the past.

"We are very concerned about the potential impact this serious disease could have in Florida if it were to become established," said Charles H. Bronson, commissioner for FDACS. "It could rapidly kill many of the plant species that make up Florida landscapes and the department is taking every measure possible to protect the nursery industry, Florida’s natural areas and residential gardens."

Phytopthora ramorum, or SOD, is a serious fungal disease that affects not only oaks but also other plants including azaleas, rhododendrons, camellias, viburnum and maples. At least 40 different plants species can act as hosts for the fungus. However, no reports, to date, have confirmed that SOD affects greenhouse-grown crops. To date, it has only had an immediate impact on nursery stock.

First discovered in Europe in 1993 and in the San Francisco Bay area in 1995, SOD killed tens of thousands of oak trees along the California coast. There have also been a few localized outbreaks in Oregon, Washington and Canada. Although it does not kill most of the species, scientists say it is unprecedented to have a pathogen spread across so many native plant species so quickly.

According to the California Department of Food and Agriculture (CDFA), the disease is presumably spread through: the movement of infected plant material from one nursery to another; the natural environmental movement of spores from one nursery to another; or the transmission of the disease from non-plant pathways to plant material (infected soil, water, equipment, etc.)

According to the FDACS division of plant industry, symptoms on the foliage of SOD-infected plants look like leaf scorch or sunburn. Lesions have dark brown centers and lighter edges. The edges of the lesions are diffuse and fuzzy. In older infections, the leaf lesions can progress down the leaf shoot and into the stems, causing leaf-drop, stem lesions and dieback.

Samples of SOD-infected plants are tested by the DNA by Polymerase Chain Reaction method. The actual determination of the SOD disease is made by a culturing method, which may take several weeks to grow and determine if the plants are positive.

In 2001, a Federal Domestic Quarantine and a State Interior Quarantine were established to prevent the spread of SOD from 12 counties in California. Recently, CDFA has been conducting a statewide survey to determine if SOD is present in nurseries throughout the state. The survey is part of a national survey to determine if the disease exists in other states. Samples of host plants are collected at the nurseries and are tested by the CDFA’s plant pest diagnostics laboratory.

During a routine sampling at Monrovia Nursery in Azusa, Calif., six varieties of camellia were detected with SOD: ‘Kumasaka’, ‘Betty Sette’, ‘Charles Cobb’, ‘Kramer Supreme’, ‘Shiro Chan’ and ‘Jean May’. A total of 398,000 potential host plants have been put on hold. At press time, it was only found on camellias and had not been found on any perennials or bedding plants.

According to Katie Bloom, media contact for Monrovia, "We are under order from the USDA and CDFA to hold the material exactly where it is. They’ve been testing, taking samples, and they don’t want to disturb the location. The material has been isolated and is on hold, and the material that has tested positive will eventually be destroyed." Monrovia ships plants to more than 5,000 retailers nationwide.

According to the CDFA, all SOD-host plants and associated hosts within 10 meters of the infected lots will remain on hold for 90 days. They will be inspected twice for symptoms of SOD prior to release. All other SOD-hosts and associated hosts will be inspected once and released if no symptoms are found. The USDA and CDFA will conduct a trace forward investigation for all SOD-host material from Monrovia during the past 12 months.

In November 2003, Monrovia was tested and found to be free of Phytopthora ramorum. According to Bloom, the USDA and CDFA do not know how Monrovia now tested positive for SOD. "They are doing a trace-back and trace-forward effort on plants to see if they can get a better grip on where the material came from and where it went to."

According to the California Oak Mortality Task Force, the federal government is providing $7.4 million in funding for SOD in 2004. $1.5 million goes to the USDA Agricultural Research Service (ARS) for research on horticultural aspects of the disease. The USDA Animal Plant Health Inspection Service received $2 million for Phytopthora ramorum quarantine enforcement, inspection and monitoring. Funding has also been provided to the Forest Service Research and the Forest Service, State and Private Forestry for Sudden Oak Death management, monitoring and education.

In addition to banning the importation of nursery stock from California, the FDACS has also halted the sale of products that have already reached Florida nurseries. Officials are currently locating and regulating any plant material that may have entered the state. Monrovia has advised garden centers that have received camellia shipments from any California growers within the last 12 months to contact their local or state agriculture inspectors.

After contacting several sources, no one has been able to positively identify greenhouse-grown crops as being infected with SOD. As far as the green industry is concerned, this disease does not affect perennials, annuals or bedding plants, and therefore does not pose a problem. It has only been confirmed, at this point, that SOD affects nursery stock and woodies.

The USDA has established a toll-free hotline to answer questions regarding the discovery of Sudden Oak Death in Southern California nurseries. For more information, call (888) 703-4457.

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