Although plant pathogens are typically viewed as detrimental, plant pathologists with The American Phytopathological Society (APS) say plant pathogens may be a successful, eco-friendly tool for managing weeds.

“The use of plant pathogens to suppress weeds is considered as one of the alternative weed control options for areas or production systems where the use of chemical herbicides is not permitted or feasible,” said Erin Rosskopf, USDA, Agricultural Research Service, Fort Pierce, Florida. “Plant pathogens may also be used when the herbicide selection or usage must be rotated with other control methods in order to prevent the development of resistant weeds or lessen the impact of herbicides on the environment,” she said.

Weed management is important due to the amount of damage weeds can cause to agricultural productivity. Weeds can reduce crop yields by as much as 12 percent (causing up to $32 billion in losses), based on the potential value of all U.S. crops of approximately $267 billion/year. Weeds also pose serious ecological problems. Invasive weeds are capable of altering ecosystem processes and displacing native plant and animal species. In addition, weeds serve as reservoirs for plant pathogens that impact crops.

According to Rosskopf, there are two approaches used for managing weeds with plant pathogens–the classical biological control approach and the bioherbicides approach. The classical biocontrol approach uses a pathogen imported from a foreign location to control a native or naturalized weed with minimal technological manipulations.

“Classical biological control using imported pathogens has an overall success rate of 57 percent and has been just as successful as the use of imported insects, with no instances of unexpected or undesirable effects,” says Raghavan Charudattan, University of Florida, Gainesville, Florida.

The bioherbicide approach utilizes native plant pathogens that are isolated from weeds and are grown to produce large numbers of infective propagules (such as spores). Infective propagules are applied at rates that will cause high levels of infection, which will greatly reduce the growth of, or kill the target weed before economic losses are incurred. Annual applications are required since the pathogen does not generally survive between growing seasons. It is estimated that there are more than 200 plant pathogens that have been or are under evaluation for their potential as bioherbicides.

More information is available in the first of a two-part series on using plant pathogens for weed biocontrol, located at www.apsnet.org/online/feature/weed1/. APS is a non-profit, professional scientific organization. The research of the organization’s 5,000 worldwide members advances the understanding of the science of plant pathology and its application to plant health.