pramorum

Maintaining a safe and healthy nursery industry in the Northwest
A comprehensive resource about P. ramorum

ABOUT P. RAMORUM

As part of our commitment to providing and assuring healthy plants and Christmas trees, we offer the following basic information about P. ramorum and its effects on the nursery industry. In addition to this public information material, we support scientific research, testing, regulatory oversight and removal of any diseased plant materials to ensure that all materials shipped from the Northwest are healthy. This is ensured through a cooperative program between nursery producers and state and federal regulatory agencies.

What is P. ramorum?
Phytophthora ramorum (fi -TOFF- thora ram-OR-um) is a fungus-like organism that can cause leaf spotting, stem canker, or leaf blight on a variety of woody ornamentals. It can also kill several species of oak, such as tanbark oak and live oak very rapidly, hence its common name, sudden oak death. Even though for the vast majority of host plants, it is no more serious a pest than many of the other species of aerial Phytophthora, the nursery industry is taking the threat of P. ramorum very seriously.

The fungus does not harm animals or humans. Rather than call this disease sudden oak death, most experts are now calling this disease ramorum leaf spot, or ramorum die-back. Because it is not recognized as being widely distributed in the U.S., the USDA has established a quarantine against this organism.

P. ramorum prefers cool, damp weather and is suppressed by hot, dry weather, though it still can survive in these conditions. It can be spread by irrigation water, rain-splash and probably by blowing wind. Scientists have discovered two specific forms of this organism, a European form (A1) and a North American form (A2). The North American form has been shown through research to be generally less damaging and less prolific than its European cousin.

The genus Phytophthora is made up of many species in addition to P.ramorum. Other species, with habits similar to P. ramorum are common in the U.S. These are generally referred to as ‘aerial’ Phytophthora.

How does P. ramorum impact plants?
Symptoms vary by plant species and often resemble symptoms caused by other species of Phytophthora. Laboratory tests are required to make a diagnosis. For most host species, the symptoms range from leaf spots to twig lesions. Some plant species suffer a die-back of stems or branches. A few species of plants can be killed by the fungus, including tanbark oaks and some live oaks. It is generally confined to attacking above-ground parts of plants, and is rarely found in soil or associated with roots. Thus, it is considered an aerial Phytophthora (as opposed to being a root-rotting form of Phytophthora.) It can persist either as an active, vegetative infection, or as a dormant spore.

Below are some examples of infected plants (photos courtesy of the Oregon Department of Agriculture):

Infected Camellia showing leaf browning Infected Rhododendron leaf showing diffuse
and collapse browning around mid-vein

Additional pictures and description of the fungus and its hosts can be seen at the diagnostic guide link at: http://www.suddenoakdeath.org.

On some plants, including camellia, the fungus generally causes few symptoms, and it reproduces at a very low level. This combination of factors makes it difficult to detect on camellia and has thus set the unfortunate conditions that allowed the shipment of infected camellia plants to nurseries across the U.S.

Which plants can be affected?
Any plant that is capable of being infected by a pathogen is called a host plant. It is important to note that all plants that are hosts (i.e. capable of being infected) are not necessarily infected with P. ramorum. P. ramorum only infects certain plants and the vast majority of plant species do not harbor P. ramorum.

Currently, there are 28 regulated host species listed by the USDA. An additional 36 species are suspected hosts, but conclusive scientific studies are not yet complete. All the species and crosses of the genus Rhododendron and Camellia are considered hosts. The following three genera have multiple host species: Pieris, Quercus and Viburnum. New hosts are being found almost monthly, however, most are minimally impacted from a disease standpoint.

For the most current list of hosts classified by the USDA, click here: http://www.aphis.usda.gov/ppq/ispm/sod/usdasodlist.html

Is P. ramorum preventable?
The best way to avoid infection by P. ramorum is to exclude the pathogen from a production facility or growing site. A panel of nursery stock growers, plant pathologists, consultants and regulators from California, Oregon and Washington met over the summer and fall of 2004 and have drafted a set of grower practices that will effectively prevent any new introductions to a production facility. By using a combination of these “best management practices” and effective regulations (which include a thorough inspection and testing program), high quality and healthy plants are assured.

Where is P. ramorum found?
Phytophthora ramorum was originally detected in Germany in 1993. A few years later, it was found in Marin County, California. It has been detected in the European Union, in Canada and in at least 21 states of the U.S. including Alabama, Arizona, Arkansas, California, Colorado, Florida, Georgia, Louisiana, Maryland, New Jersey, New Mexico, New York, North Carolina, Oklahoma, Oregon, Pennsylvania, South Carolina, Texas, Tennessee, Virginia and Washington state.

How is P. ramorum spread?
Researchers are studying how P. ramorum spreads. The pathogen has been isolated from host debris (e.g. firewood and leaves), nursery stock, soil, stream water and rain-splash. Human movement of infested host materials is considered the most likely means of long distance spread. Infested soil and water-splash may contribute to local and/or tree-to-tree spread of the disease. Researchers are also investigating insects and birds as possible vectors for this disease.

How can P. ramorum be controlled and eliminated?
Once a plant is infected, there is no treatment for P. ramorum. Plants may be protected from the infection with labeled fungicides. The best way to protect plants from P. ramorum is to utilize specific best management practices that lower the risk of receiving infected material. Because it is a regulated disease, all infected plants must be incinerated to ash or subjected to deep burial (to a depth of a least 6 feet) and double plastic bagged.

P. RAMORUM IN OREGON

What is the situation in Oregon nurseries?
Oregon has participated in all annual surveys for Phytophthora ramorum since the inception of the USDA inspection program in 2001. To date, over 70,000 samples have been collected and processed in Oregon, and the pathogen has been detected to a very limited extent in a few nurseries. In these cases, the Oregon Department of Agriculture (ODA) took immediate action to suspend shipments and to intensively survey the nurseries and surrounding environs for the pathogen. The nurseries fully cooperated with state and federal authorities and provided all shipping documents necessary to conduct trace back and trace forward surveys.

Have Christmas trees been affected by P. ramorum?
To date, no Christmas trees in the Pacific Northwest have been found infected with P. ramorum. Christmas trees in Oregon have been intensively surveyed and not a single infected tree has been found.

How has Oregon responded to this issue?
The Oregon Association of Nurseries asked the ODA to immediately implement a Phytophthora ramorum-free nursery stock certification program. This is a mandatory program for all Oregon nurseries that produce host material and requires annual testing and inspection of the facility. In addition, all host and associated host materials shipped into Oregon must either have been produced in a nursery with an ODA-approved Phytophthora ramorum-free nursery stock certification program, or the material must be safeguarded, segregated, held, inspected and tested prior to release. This is a state-of-the-art program using the most current sampling and testing procedures and will set the benchmark for Phytophthora ramorum host stock production in the industry.

In Oregon, the nursery industry members, their association and the state regulators have always taken an aggressive stand with respect to Phytophthora ramorum. We have constantly urged the USDA to add more resources for survey efforts and to take a strong position with respect to international and interstate shipments of high-risk stock. We believe our aggressive response is directly related to the negligible level of Phytophthora ramorum present in our state. However, we are prepared to go further to assure customers and regulators that Oregon offers high quality, Phytophthora ramorum-free nursery stock.