The weed typically encroaches on
pasture and rangeland. Unchecked until now, it has crowded out forage and
native grasses eaten by cattle and other livestock. Since its Virginia release,
the fungus has since turned up in thistle populations as far away as Wyoming
and California. Luster confirmed the fungus' identity on samples by using
genetic fingerprinting techniques that he developed.
"We've been watching this organism all across the United States,"
Bruckart says. "It has really spread through the musk thistle population
Interestingly, the largest reductions, of more than 90 percent, have
coincided with attacks by a tiny, imported seedhead weevil, Rhynocyllus
conicus. "We've found that the rust actually enhances the weevil's
feeding damage," says Bruckart.
Much of the credit for the microbial collection in Frederick should go to
collaborating scientists elsewhere in the United States and other countries,
such as Hungary, Switzerland, Croatia, the United Kingdom, and China. Last, but
certainly not least, are colleagues at ARS' European Biological Control
Laboratory in Montpellier, France. As Bruckart puts it, "They're the ones
beating the bushes for useful disease agents."
Through this network, the Frederick lab recently acquired four good
candidate pathogenstwo on Russian thistle, one on Russian knapweed, and
another on common groundsel. Chinese researchers have also sent them several
fungi for screening on "mile-a-minute," a fast-growing vine with
Fort Detrick: Biocontrol Bootcamp
On receiving a sample from abroad, Frederick researchers begin work
immediately. It's imperative a new arrival be in peak condition for the battery
of tests it will be subjected to. But first, each specimen gets an
identification number for tracking purposes. Part of it is also put in
The scientists' next step, for most pathogens, is to collect the spores.
Using an atomizer, they then spritz the spores onto the leaves of a host weed
grown inside the greenhouses. A sample of the weed's closest native U.S.
relatives is also treated. The containment facility, Luster notes, "is the
largest research facility where we can work with whole plants in a microbial
Generally, 2 to 3 weeks pass before disease symptoms appear in plants. A
300-ton air-conditioning unit helps the process along by maintaining desired
air temperatures. A high-tech filtration system also keeps tiny, floating
particles and airborne spores from leaving the greenhouse.
To further ensure that nothing escapes the facility, a waste disposal system
sterilizes any effluent generated. This includes water from plant beds,
toilets, and showers where scientists and technicians wash off any seeds,
pollen, or spores.
Putting Pathogens Through the Paces
One method of checking a weed pathogen's virulence is counting the number of
pustules or lesions that pepper a plant's leaves. Scientists also measure plant
height, weight, growth rate, mortality, and seed production.
They also determine the environmental conditions under which pathogens are
most likely to grow best and cause disease. Once a candidate meets all the
desired safety criteria, scientists apply for an APHIS permit to examine the
pathogen's weed-fighting prowess outside the greenhouse.
"When you are dealing with the ecosystem," says Bruckart,
"it's a different story" than what actually occurs in the laboratory.
For example, a weed pathogen or insect can vanish after it's been released,
only to reappear in force months or even years later. Classic biocontrol is
generally slower acting than chemical pesticides, so noticeable results may
take several years.
But once those results generally kick in and are observed, they're
permanent. One exception is if a weed marshals a natural defense against the
pathogen recruited to fight it, though evidence of potential resistance is
usually noted in greenhouse evaluations.
In Idaho, for example, scientists knew that some skeleton weed populations
were resistant to the rust fungus Puccinia chondrillina, which was
released in 1976. Additional biocontrol agents have been sought to combat these
hardy new strains.
In the end, no single weapon is likely to vanquish or eradicate such pesky,
invasive plants. Rather, management is the more likely scenario.
"We're learning that it's going to take multiple agentsdiseases
and insectsin an arsenal of strategies that includes chemicals and
management, like mowing, crop rotation, and plowing," says Bruckart.
"It's part of an integrated system, really."By
Jan Suszkiw, Agricultural
Research Service Information Staff.
This research is part of Crop Protection and Quarantine, an ARS National
Program (#304) described on the World Wide Web at
Douglas G. Luster,
William L. Bruckart, and
Norman W. Schaad are at the USDA-ARS
Foreign Diseases and Exotic
Weeds Research Laboratory, 1301 Ditto Ave., Fort Detrick, MD 21702-5023;
phone (301) 619-7340, fax (301) 619-2880.