Microorganisms in Weed Control Strategies

Alternative weed management strategies are needed to expand the capability of weed control as weed pressures continue to limit optimum yield, and as the use of synthetic chemical herbicides for weed control becomes more restricted. Biological control of weeds is based on the premise that biotic factors influence the distribution, abundance, and competitive abilities of plant species. Biological control is one alternative means of suppressing weed growth and establishment. More than 100 microorganisms have been identified as having the potential for weed biocontrol. There are, however, a number of problems with using these agents that must be solved, including limited host spectrum, lack of consistency across environments, and slow or inadequate weed suppression. It has been shown that soil microorganisms are capable of suppressing weeds in the field. It is imperative to develop an understanding of these soil microbes and their ecology so that they may be used to benefit agriculture, especially for weed management. Further study is required so that they may be produced efficiently and used effectively in weed management strategies. Research QuestionAlternative weed management is one of the critical factors limiting optimum crop yield; however, excess herbicide use can negatively affect our groundwater and environment. Alternative weed management strategies ‘are needed as the use of synthetic chemical herbicides becomes more restricted. Biological weed control arises from the fact that biotic factors significantly influence the distribution, abundance, and competitive abilities of plant species. Biological control offers an alternative means of suppressing weed growth and establishment. Biocontrol is successful when the weed, the pathogen, and environment are associated in such a manner that weed control or suppression is possible (Fig. 1). Questions arise as to the type of management systems that will favor biocontrol by increasing the successful interaction of these three components. Study ApproachCurrently, more than 100 pathogens have been identified as having the potential for weed biocontrol, and the research on biological control agents continues to increase. The three main strategies for biological control are the classical, augmentative, and integrated management approaches for weed suppression. Applied QuestionsWhat types of biological control agents exist? Most of the research on microbial control of weeds has concentrated on fungal plant pathogens for broadleaf weed control. Most notable is the use of rust fungi and mycoherbicides commercially available under the trade names of DeVine, Collego, and BioMal. The success of these commercial mycoherbicides indicates the potential for the use of microorganisms in weed control technology. Several other pathogens representing a wide range of fungal classes are being investigated for their biocontrol potential as well. The role of bacteria in weed control also is being explored, although their potential contribution often is overlooked. Will formulations enhance biocontrol? The delivery and survival of biocontrol agents are critical to the success of weed suppression. Formulations protect the microorganism, enhance their growth or survival in soil or on the leaf, and deliver viable and active biocontrol agents. There is a range of formulations using varying substrates, including wheat bran, alginate, vermiculite and montmorillonite, polyacrylimide and carrageenan pellets, and several synthetic polymers that can be used in both liquid or solid fermentation. Diversity with respect to type of biocontrol agent and its temperature and moisture requirements for germination, infection colonization, and sporulation requires that a variety of formulations be developed and used. How can interactions with other stresses be used in biocontrol? The compatibility and synergistic effects of microbial and chemical herbicide interactions will assist in the integration of biological control agents with synthetic chemical herbicides for improved weed management strategies. Combinations of biological control agents and reduced rates of chemicals may improve control and broaden the spectra of control. The concept of multiple stresses as a means of plant suppression needs further study, whether that be microbe/herbicide or microbe/other growth regulators to increase activity, selectivity, or spectrum of control. Do weed-suppressive soils exist? The changes in microbial communities readily observed with changes in management and weed-suppressive communities may exist and can be altered with management. In biologically based weed management, weed-suppressive soils can be an important component in weed control. Management of these soils could enhance weed suppression. Microbial activity of the indigenous organisms could contribute to the depletion of the weed seed bank and can be managed for greater weed seed decay. Native biocontrol agents can be used and enhanced for weed control. What are the future considerations to implementation of biocontrol? There are many challenges ahead for the use of microbes for weed control. Future research endeavors should address the discovery, ecology, mechanisms of action, molecular investigations, and fermentation and delivery technologies. The research on microbial-based biological weed control has demonstrated the potential for use of naturally occurring plant-suppressive microbes as a novel, nonchemical approach for suppressing and reducing weeds. Microbes can have a profound effect on plant growth, and thus must be considered in weed management strategies. Ecologically based approaches, such as biological weed control, which take into consideration all components, including the weed, the biocontrol agent, and the environment, will result in the greatest success. Fig. 1The interaction among the weed, the pathogen, and the environment that results in biological weed control.