Mode of Action of Microbial Biological Control Agents Against Plant Diseases: Relevance Beyond Efficacy.

The characterization of microbial biological control agents (MBCAs) is crucial to improve their efficacy and consistency as biopesticides. Powerful approaches to characterize MBCA’s modes of action are provided by modern molecular technologies. This paper reviews improvements achieved in this subject by three “omics” approaches: namely the genomic, the transcriptomic and the proteomic approaches. The paper discusses the advantages and drawbacks of new molecular techniques and ‘discovery driven’ approaches to the study of the biocontrol properties against plant pathogens. Omics technologies are capable of: (i) identifying the genome, transcriptome or proteome features of an MBCA strain, (ii) comparing properties of strains/mutants with different biocontrol efficacy, (iii) identifying and characterizing genes, mRNAs and proteins involved in MBCA modes of action, and (iv) simultaneously studying the transcriptome or proteome of the plant host, the plant pathogen and the MBCAs in relation to their bi- or tri-trophic interactions.

Microbial biological control agents (MBCAs) against pests and diseases in crops are regarded as sustainable tools in integrated pest management. In the European Union, biological control should be preferred to conventional chemical methods if they provide satisfactory pest reduction. There is no reason to believe that all forms of biocontrol are intrinsically safe. Therefore requirements for registration to assure safety are needed. In the current registration procedure in the European Union, MBCAs are primarily treated as potentially risky organisms that not only produce toxic substances but are also dangerous because they can multiply, spread and genetically adapt. These characteristics give rise to a concern that released MBCAs will spread and become dominant in the environment, resulting in negative effects on other organisms in the natural environment or even humans. These assumption led to extensive data requirements that are a time consuming and costly hurdle for bringing MBCAs onto the market. This paper focuses on the relevance and irrelevance of the data requirements for environmental fate and persistence of MBCAs. MBCAs are naturally occurring living organisms that are numerically enhanced to reduce specific plant pathogens or pests. In contrast to chemicals, direct toxicity is not the main mode of action, but rather a variety of mechanisms is involved, including competition, parasitism and activity of secondary metabolites. Their effects and residues cannot be evaluated as done for chemical substances and their breakdown products. Populations of introduced microorganisms always decline due to the natural biological buffering of the environment to levels that are within common fluctuations and ranges without strongly affecting microbial communities. However, the currently used concept of a natural background level as a reference to which densities of MBCAs should decline is of limited value, in particular when endpoints cannot be defined from ecological theory or risk criteria. In conclusion we state that data requirements for persistence could be more freely interpreted in all cases where there is no a priori reason to assume that organisms will not be buffered by the agroecosystem. Since information is only needed ‘when relevant’, the European Union guidelines leave space for such a proportional interpretation of the data requirements on environmental fate.

Sustainable agricultural practices are essential for eradicating global hunger, especially in light of the growing world population. Utilizing natural antagonists, such as fungi and bacteria, to combat plant diseases, rather than relying solely on synthetic chemical pesticides, which pose significant risks to the environment and human health, is known as biocontrol. Microbial biological control agents (MBCAs) have proven effective against phytopathogens and are increasingly embraced in agricultural practices. MBCAs possess several beneficial traits, including antagonistic potential, rhizosphere competence, and the ability to produce lytic enzymes, antibiotics, and toxins. These biocontrol mechanisms directly target soil-borne pathogens or indirectly stimulate a plant-mediated resistance response. The effectiveness of MBCAs in managing plant diseases depends on various mechanisms, such as hyperparasitism, antibiosis, competition for nutrients or space, disruption of quorum-sensing signals, production of siderophores, generation of cell wall-degrading enzymes, and the induction and priming of plant resistance. Formulating effective biopesticides requires optimal conditions, including selecting effective strains, considering biosafety, appropriate storage methods, and ensuring a prolonged shelf life. Therefore, formulation is crucial in developing pesticide products, particularly concerning efficacy and production costs. However, several challenges must be addressed to ensure the successful application of biological control, including the shelf life of biopesticides, slower efficacy in pest management, inadequate awareness and understanding of biocontrol methods, regulatory registration for commercialization, and suitable agricultural applications. This review clarifies the principles of plant disease biocontrol, highlighting the mechanisms of action and functionality of MBCAs in biocontrol activities, the formulation of biopesticides derived from microorganisms, and the challenges and barriers associated with the development, registration, commercialization, and application of biopesticides.

Role of Bacillus tequilensis EA-CB0015 cells and lipopeptides in the biological control of black Sigatoka disease

How benthic diatoms within natural communities respond to eight common herbicides with different modes of action

This perspective examines the potential of microbial biological control agents (MBCAs) as sustainable tools for managing agricultural insect pests, set against the backdrop of growing pesticide use and climate-driven shifts in pest pressures. We highlight how Canada’s unique combination of supportive policies, dedicated research funding, and clear regulatory frameworks has enabled MBCAs to become an integral part of national pest management strategies. By focusing on regulatory innovation, market trends, and the biological and technological factors shaping MBCA adoption, we outline why Canada’s experience offers valuable insights for other countries seeking to reduce reliance on synthetic insecticides. We propose practical directions to expand the global use of MBCAs, emphasizing the importance of harmonized regulations, stronger data infrastructure, and coordinated public-private initiatives. This perspective aims to contribute to the broader discourse on sustainable pest management by showcasing how lessons from Canada can inform more resilient, climate-adapted agricultural systems worldwide.

Developing sustainable agriculture by identifying non-chemical alternative Plant Protection Products (PPP) is a cornerstone in achieving long-sought environmental friendliness. Despite significant legislative and political efforts to promote biocontrol solutions and Integrated Pest Management (IPM), the literature points out the disadvantages posed by European Union’s (EU) two-tier system for Microbial Biological Control Agents (MBCA) approval and subsequent Microbial Biological Control Products (MBCP) authorization by each EU Member State (MS). Despite the disadvantages, in a recent article, we showed that the EU had outcompeted the US and other countries in approved MBCA in the last decades; however, MBCP approval at the national level lags. Achieving the EU Green Deal’s aim set out in the ‘Farm to Fork Strategy’ to reduce the use and risk of pesticides by 50% by 2030 is difficult without developing viable alternatives. Why do we not have higher MBCP availability and usage in the EU? Is it the current legislation, its poor application, or some other factors? The current legislative framework stimulated MBCA approval. Thus, we compare MBCA approval and MBCP authorization procedure to evaluate if MBCP authorization is more difficult and thus causes a bottleneck. We find that requirements for MBCP authorization are unnecessarily more complex. We recommend simplifying the MBCP dossier requirements and making them as similar to MBCA as possible to accelerate the MBCP authorization in more EU MS to increase their availability and integration in agronomic crops’ pest management plans.

Seed coating technologies are used to cover seeds with external materials including propagules of microbial biological control agents (MBCA). Protection of MBCA during seed processing and handling is essential to achieve long shelf-life of coated MBCA. The viability of MBCA added to slurries prepared before use for seed coating should thus not be affected. The survival of Cladosporium cladosporioides H39 and Lysobacter enzymogenes 3.1T8 in slurries provided by seed companies was quantified by previously designed strain-specific viability-qPCR assays. The effects of storage temperature and storage duration of slurries on survival of the added microorganisms were also quantified. Viability of added inocula decreased by more than 90% within a few days in most slurries, even if stored at low temperature (5°C). However, for certain slurry-MBCA combinations, reduction in viability was less than 90% during the initial days of storage. Selection of MBCA with long shelflife on coated seeds and adaptations of seed processing technologies to protect MBCA during seed processing and handling are essential for a change from chemical to biological seed treatments for control of seedling diseases.

The microbial antagonism that is seen in biological control of plant pathogens is broadly based on the categories of competition (for nutrients and space), parasitism (which may be by the production of volatile or nonvolatile antibiotics), and hyperparasitism. A dual culture system on agar plates of the target and the potential antagonist can detect several different classes of antagonism, including antibiosis, competition, and hyperparasitism. A great number of saprophytic microorganisms compete for nutrients and space in the rhizosphere, and can give a significant degree of biological control of soil-borne plant pathogens. Microbial antagonism has often been considered as occurring mainly externally to the plant, in the phylloplane, rhizosphere, or in the bulk soil. Competition for nutrients and space, the production of antibiotics, and hyperparasitism all play important roles in the antagonism of pathogens arriving and persisting in the phyllosphere. Epiphytic microorganisms take up nutrients rapidly, resulting in rapid reductions in the amounts of nutrients available to pathogens.

The root–microbe–soil interface: new tools for sustainable plant production

Behavior is increasingly reported as a sensitive and early indicator of toxicant stress in aquatic organisms. However, the systematic understanding of behavioral effects and comparisons between effect profiles is hampered because the available studies are limited to few chemicals and differ in the exposure conditions and effect parameters examined. The aims of the present study were 1) to explore behavioral responses of Daphnia magna exposed to different toxicants, 2) to compare behavioral effect profiles with regard to chemical modes of action, and 3) to determine the sensitivity and response time of behavioral parameters in a new multi-cell exposure system named Multi-DaphTrack compared with currently utilized tests. Twelve compounds covering different modes of toxic action were selected to sample a wide range of potential effect profiles. Acute standard immobilization tests and 48 h of behavioral tracking were performed in the customized Multi-DaphTrack system and a single-cell commercialized biological early warning system. Contrasting behavioral profiles were observed for average speed (i.e., intensity, time of effect onset, effect duration), but no distinct behavioral profiles could be drawn from the chemical mode of action. Most compounds tested in the Multi-DaphTrack system induced an early and significant average speed increase at concentrations near or below the 10% effective concentration (48 h) of the acute immobilization test, demonstrating that the Multi-DaphTrack system is fast and sensitive. To conclude, behavior endpoints could be used as an alternative or complement to the current acute standard test or chemical analysis for the predictive evaluation of ecotoxic effects of effluents or water bodies.

Downy mildew (DM) is a destructive disease that significantly reduces the yield and quality of important pulses (legumes) and horticultural crops, particularly during humid and cool seasons. This disease is caused by obligate and host‐specific oomycete pathogens. Controlling the pathogen is challenging due to its long‐term spore survival and rapid mutation. Although chemical pesticides have been the most effective method to control DM pathogens, their environmental hazards remain a global concern. Current research is focused on exploring the potential of microbial biological control agents (MBCA), particularly rhizobacteria strains of the genera Bacillus and Pseudomonas , which have shown suppression of plant pathogens. However, to date, no MBCA has been reported to be effective against DM pathogens in pulses. We investigated the effectiveness of Bacillus and Pseudomonas strains as potential biopesticides against the pea downy mildew pathogen Peronospora viciae f. sp. pisi (Pvp). In vitro bioassays showed 100% inhibition of Pvp spore germination compared to the control. In planta antagonism assays further demonstrated significant suppression (> 80%) of Pvp sporulation in pea plants sprayed with strains of Bacillus velezensis or Pseudomonas fluorescens or their filtrates. The drench application also showed significant effects where either a Pseudomonas or cold‐adapted Bacillus strain was used. We observed a synergistic effect for the dual foliar application of the microbes compared to individual application (40%–78% suppression). Molecular biomass analysis supported these findings. Based on these results, we conclude that Bacillus and Pseudomonas MBCAs could be highly effective in combating Pvp infections in the field.

Plant diseases and pests are risk factors that threaten global food security. Excessive chemical pesticide applications are commonly used to reduce the effects of plant diseases caused by bacterial and fungal pathogens. A major concern, as we strive toward more sustainable agriculture, is to increase crop yields for the increasing population. Microbial biological control agents (MBCAs) have proved their efficacy to be a green strategy to manage plant diseases, stimulate plant growth and performance, and increase yield. Besides their role in growth enhancement, plant growth-promoting rhizobacteria/fungi (PGPR/PGPF) could suppress plant diseases by producing inhibitory chemicals and inducing immune responses in plants against phytopathogens. As biofertilizers and biopesticides, PGPR and PGPF are considered as feasible, attractive economic approach for sustainable agriculture; thus, resulting in a “win-win” situation. Several PGPR and PGPF strains have been identified as effective BCAs under environmentally controlled conditions. In general, any MBCA must overcome certain challenges before it can be registered or widely utilized to control diseases/pests. Successful MBCAs offer a practical solution to improve greenhouse crop performance with reduced fertilizer inputs and chemical pesticide applications. This current review aims to fill the gap in the current knowledge of plant growth-promoting microorganisms (PGPM), provide attention about the scientific basis for policy development, and recommend further research related to the applications of PGPM used for commercial purposes.

In a world with constant population growth, and in the context of climate change, the need to supply the demand of safe crops has stimulated an interest in ecological products that can increase agricultural productivity. This implies the use of beneficial organisms and natural products to improve crop performance and control pests and diseases, replacing chemical compounds that can affect the environment and human health. Microbial biological control agents (MBCAs) interact with pathogens directly or by inducing a physiological state of resistance in the plant. This involves several mechanisms, like interference with phytohormone pathways and priming defensive compounds. In Argentina, one of the world's main maize exporters, yield is restricted by several limitations, including foliar diseases such as common rust and northern corn leaf blight (NCLB). Here, we discuss the impact of pathogen infection on important food crops and MBCA interactions with the plant's immune system, and its biochemical indicators such as phytohormones, reactive oxygen species, phenolic compounds and lytic enzymes, focused mainly on the maize-NCLB pathosystem. MBCA could be integrated into disease management as a mechanism to improve the plant's inducible defences against foliar diseases. However, there is still much to elucidate regarding plant responses when exposed to hemibiotrophic pathogens.

There is considerable interest in the exploitation of microbial biological control agents (MBCAs) for the control of crop pests, weeds and diseases. MBCAs can be used where chemical pesticides are banned or being phased out or where pests have developed resistance to standard chemicals. The use of MBCAs can play an important role in crop protection, as a key element in integrated pest management (IPM) programmes. However, despite considerable research efforts on the development of new biological control agents the number of such products on the market in the European Union (EU) is still extremely low compared with the USA or Canada. In areas that previously constrained the commercialization of MBCAs, discovery, fermentation, formulation and application, significant progress has been made. The low number of products is mainly due to the slow registration process. In the EU, MBCAs are regulated by and follow Directive 91/414/EEC for placing plant protection products in the market. Once an active ingredient is listed in Annex I, national registrations for the formulated product have to follow. This time consuming and expensive process has forced most companies to suspend their efforts in research and development. Initiatives by stakeholders from industry, science, regulatory authorities, policy and environment are underway to accelerate market introduction of MBCAs.