The relentless battle against weeds consumes significant resources in agriculture and natural resource management. While chemical herbicides have long been the dominant weapon, concerns about their environmental impact and the rise of herbicide-resistant weeds have fueled the search for alternative solutions. One promising avenue is biological weed control, which harnesses the power of plant pathogens – nature’s own weed killers – to suppress or eliminate unwanted vegetation.
Classical Biocontrol: A Long-Term Strategy:
Classical biocontrol involves introducing a host-specific pathogen from a weed’s native range into the area where the weed has become invasive. This strategy aims to restore the ecological balance disrupted when a weed escapes its natural enemies. A classic success story is the control of rush skeleton weed (Chondrilla juncea) in Australia with the rust fungus Puccinia chondrillina. This fungus, originating from the weed’s Mediterranean homeland, decimated the dominant biotype of rush skeleton weed, demonstrating the remarkable potential of biocontrol.
However, classical biocontrol isn’t without its challenges. The introduction of new pathogen strains may be necessary to combat the emergence of resistant weed biotypes, as seen with rush skeleton weed. The success of classical biocontrol depends on the pathogen’s ability to establish, persist, and spread within the new environment. Autoecious rusts, which complete their life cycle on a single host, offer a distinct advantage in this regard, ensuring perpetuation without reliance on an alternate host, as some examples show. In contrast, work with heteroecious rusts (requiring two hosts) offers new control options such as establishing primary infection foci of the rust at the beginning of the growing season by planting the alternate host mixed with teliospore-bearing straw of the primary host. While the alternate host in such cases may require targeted weed management after its purpose is complete, the approach offers a potentially powerful tool.
Rust fungi are not the only successful biocontrol agents. The smut fungus, Entyloma ageratinae, has proven effective in areas against the Asteraceae Hamakua pamakani (Ageratina riparia), in Hawaiian forests and rangelands. Further, examples of the rust fungi Puccinia carduorum (against musk thistle, Carduus thoermeri), Phragmidium violaceum (against weedy Rubus species), and Uromycladium tepperianum (against the invasive tree Acacia saligna) highlight the versatility and efficacy of these natural enemies. Another striking success involves U. tepperianum: introduced from Australia to South Africa, it devastated invasive A. saligna populations, causing widespread gall formation and ultimately killing many trees.
Bioherbicides: Targeted Microbial Warfare:
Bioherbicides represent a more targeted approach, employing inundative applications of plant pathogens to control weeds in specific agricultural settings. These microbial weed control agents are subject to regulatory oversight, similar to chemical pesticides. Currently, several bioherbicides are registered for use against weeds affecting economically important crops, e.g., citrus (DeVinet, utilizing Phytophthora palmivora against Morrenia odorata), rice and soybeans (Colleto, using Colletotrichum gloeosporioides f.sp. aeschynomene against Aeschynomene virginica), and turfgrass (CAMPERICO, employing Xanthomonas campestris pv. poae, registered solely within Japan). It is interesting that while various bioherbicides based on fungal and bacterial pathogens are registered and/or used on regional bases in different parts of the world, there are none based on viruses, even though a considerable amount of research on viral pathogens is being carried out worldwide. In spite of their inherent challenges, bio-herbicides have demonstrable strengths. For example, unlike chemical pesticides, biological agents can be active beyond their initial monocyclic development after inoculation. This biological trait of self-perpetuation could allow the agent to continue controlling weeds throughout the growing season, but this concept has not yet been fully utilized. Hence, the proper exploitation of secondary and tertiary disease cycles that may develop from the introduction and establishment of initial infection foci may resolve issues related to the capabilities of pathogens in terms of quantities of application and area coverage needed, but the technical and regulatory hurdles to this usage remain to be overcome.
The Epidemiological Edge:
Plant pathogens, known for their ability to cause widespread epidemics, offer a unique advantage in weed control. Understanding the factors that influence disease epidemics, including favorable environmental conditions and the presence of susceptible host populations, enables strategic manipulation of the disease cycle for optimal weed suppression. The timing and placement of inoculum, for example, can be tailored to maximize disease spread and impact on the target weed.
System Management: Integrating Biocontrol into Pest Management Strategies:
A promising approach to harnessing pathogen potential is system management, which incorporates biological weed control into a broader pest strategy. This method recognizes that pathogens can act as stress factors, weakening weeds and making them more susceptible to other control measures, such as low doses of herbicides, necrotrophic pathogens, or biochemicals that disrupt plant defenses. By integrating multiple control tactics, system management aims to achieve sustainable weed control while minimizing environmental impact and reducing reliance on harsh chemicals.
Overcoming Limitations:
Despite its potential, biological weed control faces significant hurdles. Limited commercial interest, due to small and specialized markets for biocontrol agents, poses a major challenge. Technical difficulties in producing, formulating, and maintaining viable inoculum can also hinder development. Furthermore, regulatory restrictions on the use of genetically modified pathogens may limit the application of advanced biotechnological strategies. Fortunately, new production and formulation strategies are being tested to reduce technological limitations with bioherbicides including new technologies for storage such as dry storage methods.
Another substantial area of constraint lies in classical biocontrol. The time often required by the process, along with potential nontarget risks and complex regulatory regimes, is often a strong deterrent to support. These concerns arise despite an increasingly prominent track record of success and attractive cost:benefit ratios. There are numerous successful cases of classi-cal biocontrol being applied to various species or ecosystems from terrestrial plants to forest and aquatic situations. However, a lack of understanding of their economic and ecological benefits and a lack of will from some regulators and research and funding institutions to develop biocontrol technologies for use in publically managed lands have led to under-use of this technology.
Looking Ahead: A Sustainable Future for Weed Management:
Plant pathogens hold tremendous promise as eco-friendly weed control agents. Continued research and development are essential to overcome technical and commercial limitations and unlock the full potential of these natural enemies. Integrating biological control into broader system management strategies will be key to achieving sustainable weed management. The ongoing exploration of pathogen-derived genes, gene products, and genetic mechanisms of herbicide activity or plant cell death will enable more targeted and effective biocontrol approaches while mitigating risks inherent in earlier products. By embracing nature’s own arsenal, we can move towards a future where weed control is both effective and environmentally responsible.