Groundnut (
Arachis hypogaea L.) is an important oilseed crop cultivated widely in tropical and subtropical regions. It is valued for its high oil and protein content and serves as an important source of food, animal feed and industrial raw materials. The crop also contributes to soil fertility through biological nitrogen fixation and is an important component of several agricultural production systems. According to
FAOSTAT (2023), groundnut was cultivated on approximately 30.58 million ha globally, with a total production of 54.92 million tonnes and an average yield of 1,796.1 kg ha
-1. In India, groundnut was cultivated on approximately 4.74 million ha, producing 10.40 million tonnes with an average yield of 2,192.1 kg ha
-1 in 2023 (
FAOSTAT, 2023). Groundnut is cultivated widely across several Indian states, including Gujarat, Rajasthan, Madhya Pradesh, Tamil Nadu, Karnataka and Telangana.
Groundnut production is affected by several biotic constraints, including fungal, bacterial, viral and nematode diseases. These diseases can reduce plant stand, pod development, yield and seed quality. Important diseases include foliar diseases such as early leaf spot, late leaf spot and rust, as well as several soilborne diseases, including stem rot, collar rot, dry root rot and charcoal rot
(Jadon et al., 2015; Rani et al., 2022). Soilborne diseases present particular management challenges because their causal pathogens can survive in soil and crop residues and may remain difficult to manage after infection has become established
(Mehan et al., 1994; Rani et al., 2022). However, the relative importance of individual diseases varies with cultivar, environmental conditions, production system and geographical region. Therefore, soilborne diseases should not be considered universally as the most important diseases of groundnut.
Among the soilborne diseases considered in this review, stem rot and collar rot receive primary emphasis because of their importance in groundnut production and the continuing need for effective management strategies. Stem rot is caused by
Agroathelia rolfsii (formerly
Sclerotium rolfsii), whereas collar rot is caused by
Aspergillus niger. The distinction between soilborne and seedborne inoculum is also important in collar rot because
A. niger can be transmitted through both infected seed and soilborne inoculum, contributing to disease development
(Bajaya et al., 2022). Earlier reviews have highlighted the importance and management difficulties associated with
A. rolfsii (then reported as
S. rolfsii) caused stem and pod rots of groundnut
(Mehan et al., 1994). Other soilborne pathogens of groundnut, including those associated with dry root rot and charcoal rot, are considered where they provide relevant evidence for understanding biological disease suppression.
Management of soilborne diseases commonly involves cultural practices, chemical fungicides, host resistance where available and integrated disease management approaches. However, the effectiveness of individual practices can vary with pathogen biology, environmental conditions, cropping history and application methods. Chemical fungicides may also have limitations related to application efficiency, environmental concerns, cost and the need for repeated applications. Therefore, biological approaches based on beneficial microorganisms are increasingly being considered as a complementary component of disease management rather than as a universal replacement for chemical control. Studies on groundnut have demonstrated the potential of
Trichoderma and
Pseudomonas, both individually and in combination, for reducing stem rot and collar rot. For example, combined application of
Pseudomonas fluorescens and
Trichoderma viride improved suppression of stem rot under controlled conditions
(Manjula et al., 2004), while
Trichoderma isolates have also shown antagonistic activity against
A. niger and reduced collar rot incidence in groundnut
(Gajera et al., 2011).
Microbial bioagents such as
Trichoderma,
Bacillus,
Pseudomonas and other beneficial microorganisms can suppress plant pathogens through multiple mechanisms, including antibiosis, competition for nutrients and space, mycoparasitism, production of hydrolytic enzymes and siderophores, induction of plant defense responses and colonization of the rhizosphere. Plant growth-promoting rhizobacteria (PGPR) may additionally contribute to plant growth and nutrient acquisition while influencing microbial interactions in the rhizosphere. Recent research has also increased interest in microbial consortia, rhizosphere interactions and biofilm-associated processes as approaches for improving microbial establishment, persistence and disease suppression. Recent work on microbial consortia has demonstrated the potential of compatible microbial combinations for suppression of groundnut collar rot, although further field validation is required
(Kampa et al., 2025).
Despite the increasing number of studies on biological management of groundnut diseases, available information remains dispersed across individual bioagents, pathogens, mechanisms and experimental systems. A major limitation is that strong antagonistic activity observed under
in vitro conditions does not necessarily indicate effective disease suppression under pot, greenhouse or field conditions
(Kohl et al., 2019). Therefore, evaluation of bioagents requires consideration of the experimental evidence, disease response, environmental conditions, microbial colonization, formulation and method of application. In addition, the roles of PGPR, rhizosphere interactions, microbial consortia and biofilm-associated processes need to be considered in relation to actual disease management rather than as isolated biological phenomena.
Previous studies and reviews have discussed biological control of individual groundnut diseases; however, there is a need for a critical synthesis that connects bioagent selection with mechanisms of suppression, experimental evidence, rhizosphere colonization, formulation and field performance. In particular, evidence directly obtained from groundnut should be distinguished from findings derived from other crops or pathosystems when discussing PGPR, biofilms and rhizosphere-mediated disease suppression. Such a distinction is important for identifying which biological approaches are already supported by groundnut-specific evidence and which require further validation.
Therefore, this review critically synthesizes available evidence on microbial bioagents for the management of major soilborne diseases of groundnut, with primary emphasis on stem rot and collar rot, while incorporating evidence from other relevant soilborne diseases and pathogens where necessary. Particular emphasis is placed on the mechanisms underlying biological disease suppression, including antibiosis, competition, mycoparasitism, hydrolytic enzyme production, siderophore- mediated interactions, induced plant defense responses and rhizosphere colonization. The review further examines the roles of PGPR, rhizosphere interactions, microbial consortia and biofilm-associated processes and evaluates the transition from
in vitro and controlled-environment studies to field-level disease management. Finally, limitations related to strain performance, environmental conditions, microbial colonization, formulation, application and compatibility with integrated disease management are discussed to identify priorities for future research and practical deployment of microbial bioagents.
Microbial bioagents for soilborne disease management in groundnut
Microbial bioagents investigated for the management of soilborne diseases of groundnut include antagonistic fungi, bacteria, actinomycetes and combinations of beneficial microorganisms. Among these,
Trichoderma,
Pseudomonas and
Bacillus have received considerable attention, whereas actinomycetes and microbial consortia have been investigated more recently. The available evidence, however, varies substantially in terms of experimental conditions, pathogen pressure, application methods and disease parameters measured. Therefore, the efficacy of microbial bioagents should be assessed across levels of evidence, from
in vitro antagonism to disease suppression and yield response under controlled and field conditions, rather than on the basis of pathogen inhibition alone.
Fungal bioagents
Trichoderma spp. are among the most extensively investigated fungal bioagents for soilborne disease management in groundnut. Studies have examined their activity against both
Agroathelia rolfsii (formerly
Sclerotium rolfsii) and
Aspergillus niger, with considerable variation among species and isolates. Early investigations primarily demonstrated antagonistic activity in dual-culture assays, including inhibition of
A. rolfsii by
T. viride and
T. harzianum. Such studies established the potential of
Trichoderma as an antagonist but provide limited evidence for field-level disease management. The variation among isolates indicates that biocontrol activity cannot be generalized across the genus (
Jomduang and Sariah, 1997;
Pushpavati and Rao, 1998).
More direct evidence has been obtained against collar rot.
Gajera et al., (2011) evaluated
T. virens,
T. viride and
T. harzianum isolates against
A. niger and observed differences in antagonistic activity among isolates. Selected
Trichoderma treatments also reduced collar rot development in groundnut under pot conditions, indicating that pathogen inhibition could be associated with disease suppression. Similarly,
T. harzianum has been evaluated against
A. rolfsii in groundnut, with reductions in disease development reported under controlled and field-oriented experiments
(Gajera et al., 2011; Sharma et al., 2016).
An important limitation of the available literature is that many studies have focused on percentage inhibition of pathogen growth in dual-culture assays. For example, the original evidence compiled in this review includes several studies reporting inhibition values for
Trichoderma against
A. rolfsii or
A. niger, but these experiments differ in isolate, medium, incubation period and experimental conditions (Table 1). Such values are useful for identifying antagonistic potential but should not be used to rank bioagents across studies. Disease incidence or severity, plant survival and yield responses provide stronger evidence of practical efficacy.
The potential of
Trichoderma also becomes more evident when it is integrated with other microbial or non-microbial interventions.
Manjula et al., (2004) identified selected
Pseudomonas fluorescens and
T. viride isolates with antagonistic activity against
A. rolfsii and demonstrated that their combined application improved protection of groundnut seedlings compared with individual treatments. The study also showed compatibility between selected bacterial and fungal isolates, providing an early example of combining bioagents with complementary activities.
Overall, the evidence supports
Trichoderma as an important group of fungal bioagents for groundnut soilborne disease management, but its effectiveness is strain and context dependent. Selection should therefore consider disease suppression, method of application, formulation and environmental performance rather than maximum
in vitro inhibition alone.
Bacterial bioagents
Bacterial bioagents, particularly
Pseudomonas and
Bacillus, have also been investigated for suppression of soilborne pathogens of groundnut. Their potential is associated with pathogen antagonism as well as their ability to colonize the rhizosphere and influence plant responses. However, bacterial efficacy varies among strains and experimental systems.
Pseudomonas fluorescens has been evaluated against
A. rolfsii both individually and in combination with
Trichoderma.
Manjula et al., (2004) screened a large collection of bacterial isolates and identified selected
P. fluorescens isolates with antagonistic activity against
A. rolfsii. Under controlled conditions, selected isolates reduced mortality of inoculated groundnut seedlings, while their combination with
T. viride provided greater protection than the individual treatments.
Recent studies have strengthened the evidence for
Bacillus spp. against groundnut collar rot.
Hieu et al., (2024) evaluated
Bacillus siamensis and
B. velezensis isolates obtained from groundnut rhizosphere soil against
A. niger. The selected strains inhibited pathogen growth
in vitro, while a mixture of three strains subsequently reduced collar rot in pot experiments and increased yield under field conditions. Another study by
Nga et al., (2024) identified
B. velezensis and
B. siamensis isolates with antagonistic activity against
A. niger and demonstrated disease-control potential under
in vivo conditions.
These findings indicate that bacterial bioagents may contribute to disease suppression beyond direct pathogen inhibition. Nevertheless, their performance depends on establishment and persistence in the rhizosphere, compatibility with other microorganisms and environmental conditions. Consequently, bacterial bioagents should be evaluated not only for antagonistic activity but also for their ability to colonize plant-associated niches and maintain activity under field conditions.
Actinomycetes and other beneficial microorganisms
Actinomycetes represent a comparatively less explored group of microbial bioagents for groundnut soilborne disease management.
Streptomyces spp. are of particular interest because of their capacity to produce antimicrobial metabolites and other compounds associated with pathogen suppression.
Jacob et al., (2016) isolated actinomycetes from groundnut rhizosphere soil and evaluated them against
A. rolfsii.
Streptomyces sp. RP1A-12 showed antagonistic activity and was further evaluated for antifungal and plant-growth-promoting properties, demonstrating the potential of actinomycetes as additional microbial resources for stem-rot management.
Arbuscular mycorrhizal (AM) fungi and other root-associated microorganisms have also been considered in relation to soilborne disease suppression. However, the evidence directly demonstrating their effectiveness against the primary diseases considered in this review is comparatively limited. Therefore, such microorganisms are better considered in relation to broader rhizosphere interactions and plant-mediated disease suppression rather than being ranked alongside bioagents with stronger direct groundnut evidence.
Microbial consortia and combined bioagents
The use of microbial consortia has emerged as an approach for combining complementary functions of different microorganisms. A combination may potentially provide pathogen antagonism together with improved rhizosphere colonization, nutrient mobilization or plant-growth promotion. However, the success of a consortium depends on compatibility among its constituent microorganisms and cannot be assumed from the performance of individual strains.
Manjula et al., (2004) provided early evidence for this approach by demonstrating improved suppression of groundnut stem rot when compatible
P. fluorescens and
T. viride isolates were applied together. More recently,
Kampa et al., (2025) evaluated compatible fungal and bacterial combinations against groundnut collar rot and identified a consortium comprising
Trichoderma asperellum and
Bacillus subtilis isolates that reduced collar rot incidence and promoted plant growth under experimental conditions.
The development of microbial consortia therefore requires more than combining strains with high individual antagonistic activity. Compatibility, relative growth rates, nutrient requirements, colonization ability, formulation stability and persistence after application must be considered during consortium development. These factors become particularly important during field application.
Comparative evidence and critical assessment
The available evidence does not support ranking
Trichoderma,
Pseudomonas,
Bacillus or
Streptomyces as universally superior bioagents for soilborne disease management in groundnut. Studies compiled in Table 1 demonstrate that the performance of microbial bioagents varies with the pathogen, bioagent strain, experimental system and application method. Therefore, the strength of evidence should be considered according to the experimental level and outcome measured, with greater emphasis placed on studies demonstrating disease reduction and plant or yield responses under controlled and field conditions. This distinction is particularly important because a bioagent that strongly inhibits pathogen growth in vitro may not establish effectively in soil or the rhizosphere and therefore may not provide comparable disease control in the field.
Taken together, the available evidence indicates that microbial bioagents have substantial potential for managing soilborne diseases of groundnut, but their performance is dependent on strain characteristics, pathogen, formulation, application method and environmental conditions. The next section therefore examines the mechanisms through which these microorganisms suppress pathogens and influence plant health, providing a basis for understanding why different bioagents perform differently across experimental systems.
Mechanisms of biological disease suppression
Microbial bioagents suppress soilborne pathogens through multiple, often interacting mechanisms rather than through a single mode of action. These include direct antagonism through antibiosis, competition and mycoparasitism, together with hydrolytic enzyme production, siderophore-mediated nutrient competition, induction of plant defense responses, nutrient mobilization and rhizosphere colonization. The relative contribution of these mechanisms depends on the bioagent strain, pathogen, host plant and environmental conditions and several mechanisms may operate simultaneously
(Benitez et al., 2004; Dimkic et al., 2022; Woo et al., 2023). Mechanistic evidence should therefore be interpreted together with the level of experimental validation, because a mechanism demonstrated under culture conditions does not necessarily establish its contribution to disease suppression in plants or under field conditions.
Antibiosis and secondary metabolites constitute important components of direct microbial antagonism.
Trichoderma spp. produce diverse metabolites, including polyketides, terpenoids, pyrones and peptaibols, while
Bacillus,
Pseudomonas and
Streptomyces produce lipopeptides, phloroglucinol derivatives and other specialised metabolites with antimicrobial properties
(Benitez et al., 2004; Ongena and Jacques, 2008;
Biessy and Filion, 2021;
Dow et al., 2023). Groundnut-specific evidence provides a direct link between this mechanism and disease suppression:
Ayyandurai et al., (2023) detected antifungal volatile compounds and 6-pentyl-2H-pyran-2-one during the interaction of
Trichoderma spp. with
Agroathelia rolfsii and reported reduced stem-rot incidence following application of a
Trichoderma consortium. However, antimicrobial activity detected in vitro should not be interpreted as proof that antibiosis is the principal mechanism operating in the field, because metabolite production, diffusion and persistence are strongly influenced by the soil and rhizosphere environment. Similar limitations apply to
Streptomyces, for which recent work emphasizes substantial differences between specialised-metabolite activity observed in vitro and that expressed in planta
(Dow et al., 2023).
Competition for nutrients and ecological niches provides another important mechanism of pathogen suppression. Beneficial microorganisms can rapidly utilize root-derived carbon sources and occupy available colonization sites, thereby limiting resources accessible to competing pathogens. Competition for iron may be particularly important, as siderophores produced by beneficial bacteria can chelate ferric iron and influence its availability to other microorganisms (
Whipps, 2001;
Kohl et al., 2019). Mycoparasitism provides a more direct form of antagonism, particularly for
Trichoderma, involving recognition, attachment, penetration and degradation of pathogen hyphae or survival structures. These interactions are frequently associated with hydrolytic enzymes such as chitinases, β-1,3-glucanases and proteases, which degrade components of fungal cell walls
(Benitez et al., 2004; Guzman-Guzman et al., 2023). However, the presence of an enzyme or siderophore
in vitro should not by itself be regarded as evidence of disease suppression; its contribution depends on whether it is produced and remains active at the plant-pathogen interface.
Beneficial microorganisms can also suppress disease indirectly by altering host physiology and plant defense. PGPR and other root-associated microbes may activate induced systemic resistance (ISR), leading to enhanced expression or priming of defense responses following pathogen challenge. Microbial signals and metabolites can influence hormonal pathways involving jasmonic acid, ethylene, salicylic acid and other signaling components, thereby increasing the plant’s capacity to restrict pathogen development
(Pieterse et al., 2014; Backer et al., 2018). Nutrient mobilization, including phosphate solubilization and improved micronutrient acquisition, may further promote root development and plant vigor, potentially increasing tolerance to disease. Nevertheless, improved plant growth should be distinguished from direct disease suppression because a larger or more vigorous plant does not necessarily indicate that pathogen populations or disease severity have been reduced.
The effectiveness of these mechanisms ultimately depends on the ability of the bioagent to establish and persist in the rhizosphere. Root exudates, soil properties, indigenous microbial communities and interactions with other introduced microorganisms can determine whether an applied bioagent successfully colonizes the root environment
(Berendsen et al., 2012; Kohl et al., 2019). Thus, biological disease suppression should be viewed as an outcome of interacting microbial, plant and environmental processes rather than as the consequence of a single antagonistic trait. For groundnut soilborne diseases, direct evidence exists for several individual mechanisms, particularly antagonism by
Trichoderma,
Pseudomonas,
Bacillus and
Streptomyces, but the relative contribution of these mechanisms under field conditions remains insufficiently resolved. Future studies should therefore combine mechanistic measurements with pathogen population, disease severity, colonization and yield assessments to establish which mechanisms are most relevant under practical production conditions.
PGPR and rhizosphere-mediated disease suppression
Plant growth-promoting rhizobacteria (PGPR) are root-associated bacteria that can influence plant growth and health through a combination of direct and indirect mechanisms. Their disease-suppressive effects may involve antimicrobial metabolite production, competition for nutrients and colonization sites, siderophore-mediated interactions, nutrient mobilization and induction of plant defense responses
(Meena et al., 2020; Backer et al., 2018). However, the effectiveness of PGPR depends not only on the presence of these traits but also on their ability to establish and maintain populations in the rhizosphere.
Groundnut provides direct evidence that PGPR can contribute to both disease suppression and plant growth promotion.
Dey et al., (2004) evaluated fluorescent
Pseudomonas isolates from groundnut and reported that selected isolates suppressed collar rot caused by
Aspergillus niger and stem rot caused by
Agroathelia rolfsii, while also improving several growth and yield-related traits. The study further demonstrated differences among isolates in their ability to utilize groundnut seed exudates and colonize the rhizosphere, illustrating that rhizosphere competence can be an important determinant of PGPR performance rather than an incidental trait.
PGPR-mediated disease suppression can also occur through plant-mediated mechanisms. Root-associated bacteria may prime systemic defense responses, resulting in enhanced expression of defense-related enzymes and genes following pathogen challenge. Microbial signals such as lipopolysaccharides, lipopeptides, siderophores and certain volatile or antimicrobial compounds can participate in the induction of systemic resistance
(Pieterse et al., 2014; Meena et al., 2020). The outcome is not necessarily direct killing of the pathogen; rather, PGPR may increase the capacity of the plant to restrict pathogen establishment or disease development. Consequently, disease suppression attributed to PGPR should be distinguished from simple growth promotion, because increased biomass or yield does not by itself demonstrate suppression of the causal pathogen.
An important limitation is that many PGPR traits are evaluated independently under laboratory conditions, whereas their expression in the rhizosphere is influenced by root exudates, soil properties, indigenous microbial communities and competition with other microorganisms. Thus, the ability of an isolate to produce a siderophore, solubilize phosphate or exhibit antagonism in vitro does not necessarily predict its performance after application to soil. Effective PGPR-based disease management therefore requires simultaneous consideration of functional traits, rhizosphere colonization, plant response and disease outcome. This provides a direct link between PGPR and the broader concept of rhizosphere-mediated disease suppression considered in the following section.
Rhizosphere engineering for disease management
Rhizosphere engineering refers to deliberate manipulation of the plant–microbe–soil environment to favor microbial communities and functions associated with plant health and disease suppression. Unlike the application of a single microbial bioagent, this approach considers the rhizosphere as an ecological system in which plant genotype, root exudates, soil properties, resident microorganisms and introduced beneficial microbes interact to determine disease outcomes. Strategies proposed for rhizosphere engineering include the use of microbial inoculants, organic and inorganic amendments, crop rotation, manipulation of root-associated microbial recruitment and development of synthetic microbial communities
(Yang et al., 2023; Andargie et al., 2023).
For soilborne diseases, an important objective of rhizosphere engineering is the development or restoration of disease-suppressive soils, in which microbial communities and soil conditions collectively restrict pathogen establishment or disease development. Such suppressiveness may arise through antagonistic microorganisms, competition for resources, antimicrobial metabolites, changes in nutrient availability and other microbial interactions. However, suppressiveness is highly context dependent and can be difficult to predict because soil type, cropping history, plant genotype, nutrient status and indigenous microbial communities influence microbiome assembly (
Sagova-Mareckova et al., 2023;
Andargie et al., 2023).
Evidence from groundnut provides an important example of this approach. A recent study using field, greenhouse and laboratory experiments demonstrated that crop rotation and inoculation with native microbiomes could influence the capacity of soil and rhizosphere microbial communities to suppress peanut root-rot disease
(Zhou et al., 2023). Such findings suggest that disease management may be improved not only by introducing selected antagonists but also by manipulating the resident microbial community and cropping environment. Organic amendments represent another potential strategy because they can modify substrate availability, microbial diversity and soil properties while introducing or stimulating microorganisms associated with disease suppression (
De Corato, 2023).
Emerging approaches include the use of synthetic microbial communities, beneficial root-associated consortia and prebiotic or plant-mediated approaches designed to favor particular microbial functions. Root exudates can influence microbial recruitment and provide substrates that selectively support particular microbial populations, while plant genotype and immune signaling can influence the assembly of disease-suppressive microbiomes
(Spooren et al., 2024; Yang et al., 2023). However, the ability to predictably manipulate a complex rhizosphere remains limited. Introducing a beneficial microorganism or changing a single soil property may produce different outcomes depending on the resident microbiome and environmental conditions.
For groundnut soilborne disease management, rhizosphere engineering therefore represents a complementary strategy rather than a standalone replacement for conventional disease management. Future research should identify groundnut-specific microbial communities and soil conditions associated with suppression of
Agroathelia rolfsii,
Aspergillus niger and other important soilborne pathogens and determine whether these communities can be reproducibly established through crop rotation, organic amendments, compatible microbial inoculants or synthetic consortia. Such approaches could shift biological control from the application of individual antagonists toward management of the broader rhizosphere environment in which disease suppression occurs (Fig 1).
Biofilms and biofilm-associated microbial interactions
Biofilm formation represents an important ecological strategy by which beneficial microorganisms establish and persist on plant roots. In a biofilm, microbial cells are associated with a self-produced extracellular polymeric substance (EPS) matrix that facilitates attachment to root surfaces and provides a structured microenvironment for microbial growth and interaction. Biofilm development is therefore relevant to biological disease management because successful disease suppression depends not only on the antagonistic traits of a bioagent but also on its ability to reach, colonize and persist at the plant-soil interface (Fig 2). Root exudates and other rhizodeposits can influence microbial attachment, multiplication and biofilm development, linking plant physiology with the establishment of beneficial microbial populations in the rhizosphere
(Hassan et al., 2019; Liu et al., 2024).
Direct evidence from groundnut illustrates the potential importance of this trait. Haggag and Timmusk (2008) isolated two biofilm-forming
Paenibacillus polymyxa strains from the rhizosphere of peanut grown in soils suppressive to
Aspergillus niger. The strains were evaluated for rhizosphere colonization, biofilm formation and suppression of crown rot under greenhouse and field conditions. Both strains suppressed the disease, while the strain with greater biofilm-forming capacity provided better protection, supporting a relationship between biofilm-associated root colonization and biocontrol performance. Importantly, this study provides direct evidence in groundnut that biofilm formation can contribute to disease management rather than merely representing a laboratory characteristic of a potential bioagent. However, the observed association does not establish that biofilm formation alone caused the superior disease suppression, because the strains may also have differed in antimicrobial activity, growth rate, root colonization or other functional traits.
Additional evidence from peanut further indicates that biofilm formation and rhizosphere colonization are influenced by plant-microbe interactions.
Li et al., (2019) isolated several bacterial endophytes from peanut root tips, including
Bacillus strains capable of biofilm formation, demonstrating that this trait occurs among naturally associated peanut bacteria. More direct evidence for host influence was provided by a study of
Burkholderia pyrrocinia P10, in which peanut root exudates altered bacterial gene expression associated with biofilm formation and quorum sensing and enhanced processes related to rhizosphere colonization
(Han et al., 2023). In parallel, field-based characterization of the peanut rhizosphere has shown that bacterial community composition changes across crop developmental stages, with shifts in the abundance of potentially beneficial groups such as
Pseudomonas and
Bacillus (Hinsu et al., 2021). Similar developmental changes in rhizobacterial communities have also been documented in groundnut, indicating that microbial colonization and community structure are dynamic properties influenced by plant developmental stage rather than fixed characteristics of an applied bioagent.
Biofilms may enhance the persistence of beneficial microorganisms by maintaining cells within a localized extracellular matrix and promoting interactions among neighbouring cells. This organization can facilitate the continued expression of functions associated with biological control, including antimicrobial metabolite production, nutrient competition and interactions with the plant. In some bacteria, cell-to-cell signalling and quorum-sensing processes are also associated with biofilm development and regulation of collective microbial behaviours. However, these processes should not be interpreted as evidence that biofilm formation automatically results in improved disease suppression. The ecological outcome depends on the composition of the microbial community, root environment, soil conditions and interactions with resident microorganisms. Efficient colonization of a root surface may therefore be a prerequisite for activity, but it is not necessarily sufficient for consistent disease control
(Liu et al., 2024).
A major limitation in the current literature is that biofilm formation is often measured under simplified laboratory conditions, whereas agricultural soils present heterogeneous physical, chemical and biological environments. Laboratory biofilm biomass or surface-attachment assays consequently provide evidence of colonization potential rather than direct evidence of field persistence or disease suppression. For groundnut soilborne diseases, future studies should combine quantitative measurements of biofilm formation and root colonization with pathogen population dynamics, disease severity and plant or yield responses. Particular attention should also be given to whether biofilm formation improves the persistence of applied
Trichoderma,
Bacillus,
Pseudomonas or other compatible microorganisms under field conditions. Such evidence would help determine whether biofilm-associated traits can be incorporated into the selection, formulation and delivery of microbial bioagents for reliable management of soilborne diseases.
From experimental efficacy to field-level disease management
The major challenge in microbial disease management is translating antagonistic activity observed
in vitro or under controlled conditions into consistent field-level disease suppression. Laboratory assays are useful for identifying promising strains and investigating mechanisms, but field performance is influenced by soil properties, temperature, moisture, pathogen pressure, cultivar, indigenous microbiota, application method and the ability of the introduced microorganism to establish and persist in the rhizosphere
(Kohl et al., 2019; Ptaszek et al., 2023). Thus, high
in vitro antagonism should not be considered a reliable predictor of field efficacy.
Groundnut studies demonstrate that microbial bioagents can contribute to effective field disease management when appropriately delivered. Combined application of
Pseudomonas fluorescens and
Trichoderma viride showed improved protection against stem rot compared with individual applications under controlled conditions, supporting the potential value of compatible microbial combinations
(Manjula et al., 2004). Field studies on collar rot have further demonstrated that
T. viride, when applied as seed treatment and through FYM-enriched furrow application, reduced disease incidence and increased pod yield. Importantly, integration of bioagents with other management components, including fungicides, also proved effective, highlighting the practical relevance of integrated disease management rather than complete reliance on a single biological treatment (
Data et al., 2016).
The field environment can strongly influence bioagent establishment and activity. Soil texture, pH, organic matter, nutrient availability and moisture affect microbial survival, while indigenous microorganisms may compete with or interact positively with introduced strains. Application method is equally important: seed treatment can position the bioagent near emerging roots, whereas soil application may provide greater spatial coverage but requires adequate survival and distribution. Consequently, formulation, viable cell or propagule density, application timing and delivery method should be considered as integral components of bioagent efficacy rather than as separate technical considerations
(Ptaszek et al., 2023).
Compatibility with existing crop-protection practices is another important consideration. Biological agents should not necessarily be regarded as alternatives to fungicides in all situations. Instead, compatible bioagents, reduced chemical inputs and agronomic practices can be combined within integrated disease-management programmes. Such compatibility, however, is strain- and chemical-specific and should be established experimentally rather than assumed.
Future field research should therefore move beyond simple comparisons of disease reduction and include information on soil conditions, pathogen pressure, bioagent formulation, application method, rhizosphere establishment, disease severity and yield. Multi-location and multi-season validation will be particularly important for identifying the environmental conditions under which microbial bioagents provide consistent benefits. This approach can help shift biological control from strain selection based mainly on laboratory antagonism toward context-specific, field-validated disease-management strategies.
Critical knowledge gaps and future directions
Despite considerable evidence for the potential of microbial bioagents against soilborne diseases of groundnut, several gaps limit their consistent field application. First, mechanistic studies are often conducted under simplified laboratory conditions, while relatively few studies establish how specific mechanisms contribute to disease suppression under field conditions. Future research should therefore link microbial traits such as antibiosis, mycoparasitism, siderophore production, induced resistance and biofilm formation with rhizosphere colonization, pathogen populations, disease development and yield responses.
Second, greater emphasis is needed on strain-level selection and formulation. The performance of a microbial species cannot be assumed to represent all strains of that species and traits demonstrated
in vitro may not be expressed consistently after field application. Selection should therefore consider antagonistic activity together with rhizosphere competence, environmental adaptability, formulation stability and compatibility with other microorganisms and crop-protection inputs. Recent groundnut studies combining
Trichoderma,
Bacillus and fungicides demonstrate the potential of such integrated approaches, but their reproducibility across locations and seasons requires further validation
(Vamshi et al., 2025).
Third, microbial consortia and rhizosphere engineering require more systematic investigation. Compatible combinations of fungi and bacteria may provide complementary functions, but compatibility, formulation complexity, persistence and interactions with indigenous microbiota remain important constraints. Future studies should move beyond simply combining promising isolates and instead use mechanistic and ecological approaches to identify combinations that are functionally complementary and capable of establishing under field conditions.
Finally, field research should increasingly adopt multi-location, multi-season and farmer-participatory evaluation. Standardized reporting of soil characteristics, cultivar, pathogen pressure, bioagent formulation, application method, environmental conditions, disease severity and yield would improve comparison among studies. Research should also evaluate biological treatments within integrated disease-management programmes rather than assuming complete replacement of conventional inputs. A stronger connection between microbial ecology, formulation technology, agronomy and field validation will be essential for converting promising bioagents into reliable disease-management tools for groundnut.
