TURF SCIENCE


Sudden Oak Death on the Rise

Infected nursery stock being blamed for spread of the disease




This picture illustrates the tip droop symptom of sudden oak death on tanoak.
Photos by Joseph O'Brien, USDA Forest Service, www.Bugwood.org .

The water mold Phytophthora ramorum is the causal agent of several plant diseases, including ramorum leaf blight, ramorum dieback, Phytophthora canker diseases, and the most well known, sudden oak death. The disease was first observed in 1993 in Germany and The Netherlands on ornamental rhododendrons and viburnum. In the United States, P. ramorum was first detected in Mill Valley, Calif., on tanoak in the mid-1990s.

Since its discovery in the United States, P. ramorum has been confirmed in forests in California and Oregon and in nurseries in California, Oregon, Washington and British Columbia. In 2004, distribution of infected nursery stock resulted in detections in 21 states.

Origin

The geographic origin of P. ramorum is unknown. Before the mid-1990s, there were no reports of this species in the United States or Europe. P. ramorum's limited known geographical distribution in relation to its hosts' distribution and genetic makeup suggests it was recently introduced into the United States. The European and North American populations are thought to be distinct populations transported independently from another location, perhaps the site of origin. Host range P. ramorum causes two types of diseases, bark cankers that may kill the host and foliar blights that may serve as a reservoir for the pathogen. In response to the identification of P. ramorum, the United States Department of Agriculture Animal and Plant Health Inspection Service (USDA-APHIS) has developed a list of regulated and associated hosts. As of January 2005, 31 plant species or genera are considered hosts and another 37 plant species are awaiting experimental confirmation.



The sapwood decay fungus (Hypoxylon thouarsianum) may also colonize trees infected with sudden oak death. The presence of the half-dome-shaped fruiting bodies are indicative of advanced stages of sudden oak death.

Transmission

The spread of P. ramorum likely occurs by movement of infected plant material, wind-blown rain and contaminated irrigation water. Dispersal by soil and potting mix is under investigation. Moist, cool, windy conditions are conducive to pathogen spread via dispersing spores from the leaves of foliar hosts. Certain foliar hosts support abundant spore production, and these plants may play an important role in P. ramorum transmission to bark canker hosts. Shipments of infected nursery stock have inadvertently disseminated P. ramorum throughout the United States.

Symptoms and identification

Bark canker hosts infected with P. ramorum are distinguished by droplets of dark reddish brown liquid on the bark ("bleeding"), usually not associated with cracks or insect holes. If the outer bark is scraped away, a dark brown canker surrounded by a dark line is visible in the inner bark. The bleeding canker can girdle the tree and then the leaves turn brown and the entire crown dies. Tree death may occur within several months to several years after initial infection.

Infected trees are attractive to Monarthrum scutellare and M. dentiger and bark beetles (Pseudopityophthorus pubipennis). Sapwood decay fungus (Hypoxylon thouarsianum) may also colonize infected trees.



Newly formed fruiting bodies are olive greenish or shiny black and release powdery green spores upon contact. Older fruiting bodies are dull black.

Infection on foliar hosts is indicated by dark grey-to-brown lesions with indistinct edges. These lesions can occur anywhere on the leaf blade, in vascular tissue or on the petiole. Stem lesions often accompany petiole lesions. Some hosts with leaf lesions defoliate and eventually show twig dieback.

Management recommendations

In 2004, P. ramorum was confirmed in several large West Coast production nurseries and in several smaller production or wholesale nurseries. These facilities shipped 2.3 million potentially infected plants to 49 states and the District of Columbia. Only Hawaii was spared.

USDA-APHIS, along with the Integrated Pest Management Centers, National Plant Diagnostic Network, U.S. Forest Service, State Departments of Agriculture, Cooperative Extension Services and Master Gardeners launched a massive program to educate the public, track the shipments, test plants for P. ramorum and destroy infected plant material.

By the end of 2004, inspectors had found infected plants at 171 nurseries in 20 states. The goal of APHIS-Plant Protection and Quarantine (PPQ) is to determine the distribution of infected plant material in the United States and to limit the spread beyond infected areas through quarantines and a public education program. Symptoms on affected hosts vary considerably by species, and it is difficult to differentiate P. ramorum-infected plants from those infected by other pathogens.

If you suspect trees or plants are infected with P. ramorum, contact your state's university diagnostic laboratory or Department of Agriculture diagnostic laboratory immediately (contact university extension personnel for the address of the diagnostic laboratory). Plants infected with P. ramorum should be destroyed because no chemical control measures are currently available.

Because P. ramorum is a regulated organism, its destruction and disposal protocols are coordinated by state regulatory officials. If diagnosticians confirm P. ramorum infestation of plants at nurseries or other commercial landscape facilities, an Emergency Action Order from APHIS may be issued. The order may require that P. ramorum-infected plants and all susceptible plants within 2 meters of infected plants be destroyed, and that all susceptible plants within 10 meters be held for 90 days until inspected.

This information was taken in part from a publication by the USDA-CSREES Integrated Pest Management Centers. For more information, go to http://www.ncipmc.org.