ANNAPOLIS, MD (August 11, 2004) - Maryland may have an elusive plant disease lurking in unsuspecting neighborhoods. Sudden oak death, as it is commonly known, is caused by the fungus-like pathogen, Phytophthora ramorum, and has been designated as the Maryland Invasive Species Council's "Invader of the Month" for August. Responsible for the deaths of thousands of oaks and tanoaks in California over the past decade, P. ramorum has now headed East.
Many popular landscape plants such as Rhododendron, azaleas, camellias,
Viburnum, and lilacs purchased and planted in
Maryland since March 2003 could be providing safe harbor for the pathogen.
Carol Holko, Maryland Department of Agriculture, says, "Establishment of
this disease in Maryland would have far-reaching economic and environmental
implications." If P. ramorum were to survive and spread, over
60% of Maryland's hardwood forests could be at risk. Maryland's $1.15 billion nursery
and landscape industry has already been negatively impacted by recent events.
The name "sudden oak death" is somewhat misleading. Although oaks in the Pacific
northwest have been visible and primary victims, the known host range
continues to grow and now includes over 60 plants,
many of them common in the forest understory and urban landscapes. While
oaks and other tree hosts develop bleeding cankers on the trunk or main
stem that girdle and kill the trees, many other hosts suffer symptoms such
as tip dieback and leaf spots. The disease is increasingly
being referred to as "ramorum blight", or "ramorum dieback".
These foliar hosts may not die from the
disease, but can serve to move the pathogen around. P.
ramorum can spread via infected plant tissue, soil, and water.
P. ramorum is a relative newcomer. As early
as 1993, oaks and tanoaks in California and rhododendrons in Europe were dying mysteriously.
It wasn't until 2000 that the pathogen responsible for both
was identified as P. ramorum. Since the mid-1990's, researchers, regulators,
and stakeholders in Europe and North America have been working at a feverish pace to
understand and contain the disease.
Until 2003, P. ramorum was believed to be confined
to native plants in the environment in northern coastal California, Oregon,
and British Columbia in North America. In 2002, USDA imposed a quarantine
that now includes 13 counties in California and an area in Oregon to
prevent its artificial spread through commerce. In 2003, plants in Oregon and Washington
nurseries tested positive for the pathogen. Although the introduction
was contained and eradicated, concerns about the role of nursery plants
in spreading the disease mounted.
In March of 2004, the California Department of Food
and Agriculture reported that Monrovia Nursery in southern California,
far from the quarantine area, had plants that tested positive for P. ramorum.
In April 2004 the USDA imposed an Executive Order restricting any California
nursery from shipping plants outside the regulated area without being
first declared to be free of P. ramorum. Lists of "trace-forward"
nurseries, those known to have received potentially infected material from
California, were sent to the affected states. Of over 500 camellias, viburnums,
and lilacs known to have been shipped to 11 nurseries in Maryland starting
in March of 2003, less than 50 remained in stock. One camellia tested positive for
P. ramorum. The remaining plants had made their way into Maryland
In April of 2004, in the course of conducting routine
surveys for P. ramorum, MDA discovered rhododendrons testing positive
for the pathogen in a retail garden center. Those rhododendrons were traced back
to a nursery in Columbia County, Oregon which was found to have infected plants.
USDA again distributed trace-forward lists to the states.
In Maryland, 18 Lowe's Home Improvement Centers had received suspect
material. Of more than 25,000 plants shipped, the vast majority had been
sold. "We are deeply concerned about the likelihood that infected plants
were purchased and planted by consumers," says Holko.
Surveys for P. ramorum in nurseries and the environment
are ongoing nationwide. To date, P. ramorum has been detected in
148 sites in 21 states. Remarkable progress has been made by the research
community in the four years since the pathogen was identified. Much more
work lies ahead.
Maryland and many other states share the goal of finding and eradicating P. ramorum
in the East, through cooperation between and within all states concerned.
In Maryland, anyone who thinks they may have purchased an infected plant is urged to contact the University of Maryland
Home and Garden Information Center, 410-531-5556 or
for assistance and instructions.
For more information about Phytophthora ramorum and other Invasive Species of Concern, visit www.mdinvasivesp.org or call MDA at 410-841-5920.
photos available electronically on request.