Bioherbicides for Weed Control

Management of weeds is a necessary but expensive challenge. Chemical weed control accounts for over $14 billion spent annually (Kiely et al., 2004), excluding immense indirect costs to producers, consumers and the environment, and resulting also in the development of resistant weed biotypes. While chemical herbicides effectively control unwanted vegetation, many herbicides are no longer available due to lack of re-registration, competition from other products, and development of numerous genetically modified crops with resistance to broad-spectrum herbicides, namely glyphosate and gluphosinate. The implementation of conservation tillage practices to promote soil quality, to minimize erosion, or to simplify crop management has increased reliance on ‘burn-down’ herbicides and placed additional selection pressure on weeds to develop resistance. After years of applying herbicides, often in the presence of high weed pressure, 180 species of herbicide-resistant weeds have been identified (WeedScience, 2006). The majority of herbicide usage is for agronomic areas or turf, but few herbicides are registered for, or are being developed for, smaller markets or niche weed problems, such as invasive weeds in noncropland areas. Furthermore, chemical weed control is not an option in organic cropping systems and near to sensitive natural habitats. The high costs involved in developing and registering chemical herbicides, and recent trends in environmental awareness concerning pesticides in general, have prompted researchers to develop additional weed control tools, such as biological weed control using plant pathogens. A review of pathogen-based weed control prospects by Charles Wilson (1969) noted that ‘the idea of using plant pathogens to control weeds is almost as old as the science of plant pathology itself ’, but that the ‘seeds of the idea ... have lain dormant since their sowing’. Since that review, almost 40 years ago, numerous pathogens for weed control have been identified and a few have enjoyed limited commercial success (Hoagland, 1990, 2001). Classical pathogen-mediated biocontrol of weeds generally employs an exotic pest to manage a weed population. This is an effective weed management strategy in many systems (Bedi et al., 2002; also see Blossey, Chapter 6, this volume). An alternative method is to overwhelm the target weed with direct pathogen application, or multiple applications of a pathogen. Because this tactic uses biological agents in an application similar to chemical herbicidal applications, it is often called the ‘bioherbicidal’ approach. When the plant pathogens are fungi, these bioherbicides are often called ‘mycoherbicides’.