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Genetic Sleuthing To Track Microscopic Weed Warriors

By Jan Suszkiw
September 15, 2000

Certain fungi and other microbes that attack invasive weeds offer an environmentally friendly method of controlling the pesky plants without resorting to traditional chemical herbicides. Now, Agricultural Research Service scientists have developed molecular sleuthing techniques to monitor these biocontrol agents once released into the environment.

ARS plant physiologist Doug Luster says their approach can both detect and identify a weed pathogen’s unique genetic “fingerprint” using polymerase chain reaction (PCR), amplified fragment length poymorphism (AFLP), DNA sequencing, molecular marking, and other sensitive technologies.

This has already resulted in DNA fingerprints for several isolates, or types, of Myrothecium verucarria, a soil fungus that kills morning glories. Morning glory is a weed that plagues sugarcane and other crops.

In field studies, spraying redroot- and smallflower-morning glories with an oil-based carrier containing Myrothecium spores proved as lethal to these weeds as the herbicide atrazine. Luster conducted the study with ARS plant pathologist Dana Berner and agronomist Rex Millhollon.

Though used in the lab, DNA fingerprinting is intended to help scientists keep close tabs on the spore growth and spread, host range and effectiveness of biocontrol pathogens like Myrothecium once they’ve been released to control weeds. The technology also allows researchers to pinpoint and analyze particular DNA regions that can differentiate strains of the same fungal family, such as Puccinia carduorum and P. jacea, which attack musk thistle and yellow starthistle, respectively.

In weed-infested crop fields or pastures, for example, scientists hand-collect spore samples to identify weed pathogens and their whereabouts. They also examine the spores under a microscope, subject them to biochemical tests, and scrutinize infected plants for tell-tale disease symptoms. But the results can be ambiguous.

DNA fingerprinting offers genetic evidence linking a specific microbial release to a specific disease seen in target weeds. It also reveals the spread of biocontrol microbes and demonstrates their effectiveness in reducing invasive weed populations. ARS is the U.S. Department of Agriculture's principal research agency.

Scientific contacts: Douglas G. Luster and Dana K. Berner, ARS Foreign Diseases-Weed Science Research Unit, Fort Detrick, Md., phone (301) 619-7344 [Luster], (301) 619-7339 [Berner], fax (301) 619- 2880,; and Rex W. Millhollon, ARS Sugarcane Research Unit, Houma, La., phone (504) 853-3174, fax (504) 868-8369,