| Martin, who's with the ARS Crop
Improvement and Protection Research Unit, and Tooley, at the Foreign Disease-Weed
Science Research Unit, are developing more assays to further clarify who's
who among Phytophthora's many microbe members. Details are on the
web at http://pwa.ars.usda.gov/salinas/cipru/frank/phyto.htm.
Resistant or Susceptible?
Researchers such as Tooley and ARS plant pathologist Robert G. Linderman
in Corvallis, Oregon, are probing P. ramorum's differing effects
on various kinds of popular woody ornamental plantsand some small
fruits, as well.
Based at the ARS Horticultural Crops Research Unit, Linderman works
with ARS plant pathologist Niklaus J. Grunwald and Oregon State University
post-doc Steven Scheuerell, investigating how western U.S. nurseries
typically grow familiar plantsmainly rhododendrons, viburnums,
and lilacs. Tooley and collaborators delineate the pathogen's prowess
in attacking East Coast nursery crops.
Linderman's experiments showed that many more western nursery crops
than expected are susceptible. His group determines a plant's vulnerability
by inoculating detached leaves with the pathogen.
"We've seen how easy it could be to overlook mild or minimal symptoms,"
Linderman points out, "or to misdiagnose plants that have symptoms
similar to those of this newly emerging disease."
The East Coast team of Tooley, research associate Kerrie L. Kyde of
the Maryland Department of Natural Resources, and Larry Englander of
the University of Rhode Island have tested more than 50 members of the
Ericaceae family, which includes some small berries as well as rhododendrons
and azaleas. They ranked cranberries and highbush blueberries among
the most resistant and published their findings in the September 2004
issue of Plant Disease.
Color photos taken by Tooley and Kyde offer a first-time glimpse of
sudden oak death disease symptoms on many popular East Coast species.
The images are available on CD-ROM and have also been posted by the
North Central Integrated Pest Management Center on the World Wide Web
Finding the Best Fungicides
At both Corvallis and Fort Detrick, finding the best fungicides to
quell P. ramorum is a top priority. Linderman, for example, was
among several researchers to find that a familiar fungicide, Subdue
Maxx, could help control P. ramorum.
"While it doesn't kill the microbe outright," says Linderman,
"it does a dramatically good job of suppressing it when applied
as a soil drench." That's unlike most other fungicides, which are
applied as sprays. He found that Subdue Maxx is effective against several
other Phytophthora species as well and considers it "the
fungicide of choice until some of the newer chemicals can be more thoroughly
"Most of the systemic fungicides now used as sprays don't kill
P. ramorum," agrees ARS plant pathologist Nina Shishkoff
at Fort Detrick. "They just suspend the microbe's growth and perhaps
mask its symptoms."
For example, tiny dark spots on leavescalled lesionsmight
go unnoticed until later in the infection, when they expand. "We
need to understand what happens to P. ramorum when you spray
these chemicals," says Shishkoff. To that end, she's spraying rhododendrons
with three different kinds of systemic fungicides and monitoring the
chemicals' effects on the pathogen's persistence and infectivity.
Pots and Planting Mixes: Pathogen's Playground?
Besides eluding certain fungicides, P. ramorum may also be able
to survive in nursery containers and planting mixesalso called
potting soilthus boosting its ability to spread from one locale
to the next. "In some jurisdictions," explains Linderman,
"plant-health inspectors will destroy infected plants but will
keep the containers. But even after the pots are washed, the pathogen
could still survive in them."
His team's experiments with air-steam combinations may determine which
temperatures are hot enough to kill the pathogen without damaging the
containerswhich are generally made out of plastic.
Shishkoff, meanwhile, intends to unearth new information about P.
ramorum's life underground, particularly its potential to infect
Every detail that these researchers uncover will go a long way toward
protecting prized trees, shrubs, and berry plants against this problematic
Wood, David Elstein, and
Agricultural Research Service Information Staff.
This research is part of Plant Diseases, an ARS National Program
(#303) described on the World Wide Web at www.nps.ars.usda.gov.
Frank N. Martin is
with the USDA-ARS Crop
Improvement and Protection Research Unit, 1636 E. Alisal St., Salinas,
CA 93905; phone (831) 755-2873, fax (831) 755-2814.
Paul W. Tooley and
Nina Shishkoff are
in the USDA-ARS Foreign
Disease-Weed Science Research Unit, 1301 Ditto Ave., Fort Detrick,
MD 21702; phone (301) 619-2632 [Tooley], (301) 619-2877 [Shishkoff],
fax (301) 619-2880.
Robert G. Linderman
is in the USDA-ARS Horticultural
Crops Research Unit, 3420 N.W. Orchard Ave., Corvallis, OR 97330;
phone (541) 738-4062, fax (541) 738-4025.
"Sudden Oak Death: ARS Scientists Fight Back!" was
published in the February
2005 issue of Agricultural Research magazine.