Tomato (Solanumlycopersicum L.) is one of the world’s most important vegetable crops. Annual tomato production is estimated at 180 million tons, with a market value exceeding $80 billion [1]. However, the production and marketing of fresh tomatoes are severely affected (or threatened) by numerous plant pathogens, particularly fungi and bacteria. These pathogens cause significant yield losses both during cultivation and during post-harvest preservation and storage [2]. Among the major diseases affecting tomatoes worldwide, collar rot caused by Sclerotiumrolfsii arise among the most destructive one [3][4]. S.rolfsii is considered one of the most aggressive soil-borne fungal pathogens affecting tomatoes. With an extremely broad host range comprising more than 500 species, it is capable of infecting the plant at all development stages of its. These infections can result in complete yield loss (100%), particularly in warm, humid environments conducive to the pathogen’s development [5]. Over the past few decades, plant disease control has relied primarily on the use of chemical pesticides. Although these products offer, in most cases, immediate crop protection, their intensive and prolonged use can lead to numerous undesirable effects, including toxicity to non-target organisms, environmental pollution, loss of biodiversity, increased pathogen resistance due to selection pressure, higher economic costs, and even chronic or acute toxicity to humans [6].

In this context, the search for sustainable alternatives has led to growing interest in biological control based on the use of plant-growth-promoting bacteria (PGPB). The most frequently reported PGPB genera include Bacillus, Azoarcus, Pseudomonas, Serratia, Enterobacter, Streptomyces, Herbaspirillum, Azospirillum, Azotobacter, Klebsiella, Alcaligenes, Arthrobacter, Burkholderia, Microbacterium, Micrococcus, Pantoea, and Stenotrophomonas[7]-[15]. These bacteria can exert an antagonistic effect against plant pathogens through various mechanisms, such as the production of antibiotic lipopeptides, the induction of systemic resistance, the release of hydrolytic enzymes and volatile organic compounds, as well as the modulation of the indigenous microbiota of the soil or the plants into which they are inoculated [16].

This study adopts an ecological and sustainable approach and aims to evaluate the antifungal potential of bacterial strains isolated from Opuntiadillenii against S.rolfsii. These strains had previously been isolated, identified, and characterized for their PGP traits, as well as various extracellular enzymatic activities, by Brun et al. [17].

Worldwide, tomato cultivation represents a crop of major nutritional, economic, and public health importance. However, its productivity is severely limited by soil-borne diseases such as collar rot caused by S.rolfsii. Given the limitations of conventional control methods, this study explored a biocontrol approach based on indigenous bacteria isolated from the rhizosphere and endosphere of Opuntiadillenii.

In this context, in vitro trials showed that the highest levels of inhibition were observed with Bacillus subtilis S7 (PIR = 100%) and P.flexa S6 (PIR = 61.77%), indicating strong antifungal activity. This efficacy is consistent with their high production of proteases and lipases previously described by Brun et al. [17]. Under in vitro conditions, direct enzymatic degradation of the mycelium is the central mechanism: proteases alter the protein structures of the fungal cell wall, whilst lipases disrupt membrane integrity, leading to the arrest of mycelial growth [21][22]. Recent studies show that several strains of B. subtilis, B. cereus,or B.velezensis can induce inhibitions of over 60% - 100% of S.rolfsii mycelium, directly linked to the production of extracellular hydrolytic enzymes [23]-[25].

Beyond these in vitro observations, the results obtained under pathogen-associated conditions highlight an even greater potential for biocontrol, particularly for Bacillus subtilis C7, whose high efficacy suggests a particularly effective direct antifungal interaction. This performance is likely due to the simultaneous mobilizations of several functional traits, including strong enzymatic activity and the ability to compete for nutrients. This type of response is well documented in species of the genus Bacillus, which are known for their ability to produce a variety of antifungal metabolites, notably lipopeptides from the iturin, fengycin and surfactin families, as well as hydrolytic enzymes capable of damaging the cell wall, mycelium and survival structures of S.rolfsii [26].These compounds act by disrupting the cell membrane, inhibiting mycelial growth and limiting sclerotium germination, thereby leading to a significant reduction in the expression of symptoms at the collar and in the root system [27].

In addition to biocontrol effects, results obtained in the absence of the pathogen reveal growth-promoting effects in P.flexa C6, P.flexa S6 and B. subtilis S7, suggesting direct stimulation of plant development. This ability may be linked to their functional traits, notably the production of phytohormones such as IAA, phosphorus solubilization and the production of siderophores, which promote improved plant nutrition and metabolic activity [17]. Thus, inoculation with Priestiamegaterium led to a significant increase in above-ground and root biomass, leaf length and weight, as well as chlorophyll content, after four months of cultivation under non-stressful conditions; these effects were directly correlated with AIA production and phosphorus solubilization (a significant increase in growth parameters compared with the non-inoculated control) [28]. Similarly, strains of Bacillus subtilis showed a marked improvement in phosphorus uptake in tomatoes, associated with stimulation of root growth and the elongation of root hairs, leading to a significant increase in plant biomass and P content in plants grown in uninfected soil [29].

In the presence of the pathogen, the marked reduction in plant growth observed indicates that fungal infection is the predominant factor driving the plant’s physiological response, severely limiting the expression of growth-promoting traits. Under intense pressure from necrotrophic soil-borne pathogens, the plant engages in active trade-offs between growth and defence, during which metabolic resources are prioritized for immune responses. It has been shown that the activation of hormonal pathways associated with jasmonate, ethylene and salicylate leads to direct repression of processes related to cell elongation, division and carbon assimilation, resulting in a marked inhibition of vegetative growth [30][31]. Furthermore, infection by aggressive soil pathogens is accompanied by a profound disruption of root function, including tissue damage, reduced water and mineral uptake, and qualitative and quantitative changes in root exudates. These changes have been identified as major factors in the restructuring of the rhizosphere, negatively affecting the recruitment and persistence of beneficial microorganisms and limiting the effective expression of their PGP traits, even when these are present at the genetic level [32][33]. In this context of high biotic stress and rhizosphere dysfunction, Bacillus subtilis C7 stood out as the only strain capable of maintaining a measurable improvement in above-ground growth. This response suggests that the observed effect is primarily based on a sufficient reduction in pathogen pressure, allowing the plant’s physiological functions to be partially preserved, rather than on direct stimulation of plant metabolism. This type of indirect effect, based on limiting the establishment and spread of the pathogen in the rhizosphere, is now recognized as a key characteristic of effective biocontrol agents [34]-[36].

The results also show that seedlings inoculated with the necrotrophic pathogen Sclerotiumrolfsii Sacc. have an average root weight approximately twice that of the control seedlings inoculated with sterilised water. This may be explained by the fact that the infected plant develops a compensatory mechanism in the presence of S. rolfsii to ensure a continuous supply of nutrients. Indeed, S.rolfsii is a basidiomycete fungus that typically causes vascular necrosis, collar rot and significant yield losses, accompanied by an increase in root weight during the early or moderate stages of infection, notably through root overcompensation [37]. The theory of overcompensation suggests that pathogen pressure can paradoxically increase the biomass of the host or its organs. Preston et al. [38], in a study modelling structured host-pathogen interactions, reported that biomass increases proportionally with the prevalence of infection. In this study, it appears that the partial destruction of root tissues by the pathogen triggers an apical inhibition response on lateral root growth, thereby releasing compensatory growth potential. Rivera-Vega et al. [39], following their research into plant-fungus interactions, demonstrated that the host can redirect its resources towards compensatory vegetative growth in response to an attack. Thus, when S.rolfsii preferentially attacks the collar and proximal tissues, the plant can respond by stimulating the production of adventitious lateral roots in areas not yet colonized, thereby increasing root biomass.

The discrepancy observed between the in vitro and greenhouse performance of biocontrol strains is widely reported and is mainly explained by the complexity of plant-microorganism-environment interactions, which are not fully replicated under controlled laboratory conditions [40]. In particular, the efficacy of biocontrol agents in vivodepends largely on their ability to effectively colonise plant tissues and establish themselves within the microbial community of the rhizosphere, which constitutes a major limiting factor under real-world conditions [41][42]. The superior performance of B. subtilis C7 under greenhouse conditions may therefore be linked to a greater ability to colonise roots, facilitated by high production of exopolysaccharides (EPS). EPS play a key role in bacterial adhesion to roots, the formation of protective biofilms, and the enhancement of the stability of plant-microorganism interactions [43][44]. This ability, combined with better adaptation to biotic and abiotic soil conditions (microbial competition, water stress, fluctuating nutrient availability), would enable B. subtilis C7 to establish itself sustainably in the rhizosphere and effectively express its PGP traits in vivo. Conversely, although B. subtilis S7 performs well in vitro, its efficacy appears to be largely restricted to controlled laboratory conditions, suggesting a limited ability to maintain stable root colonisation and to mobilise its PGP traits particularly those related to physical and ecological interactions with the plant under real-world conditions.

This study revealed marked functional diversity among the indigenous bacteria associated with Opuntiadillenii, both under pathogen pressure and in the absence of it. When exposed to Sclerotiumrolfsii, the strains tested exhibited varying levels of efficacy, reflecting the specificity of plant-bacteria-pathogen interactions. Although some strains showed strong antagonistic potential in vitro, particularly via mechanisms linked to extracellular enzymatic activity, this performance was not consistently replicated under greenhouse conditions.

In this regard, Bacillus subtilis C7 stood out for its ability to significantly reduce disease incidence whilst maintaining the above-ground growth of tomato plants and promoting root development. In the absence of pathogen pressure, strain-dependent growth-promoting effects were also observed, highlighting the complementary nature of the mechanisms involved.

Overall, these results confirm the potential of indigenous bacteria as biocontrol agents and growth promoters, whilst highlighting the importance of selecting strains adapted to the targeted phytopathological context for effective agronomic application.

The authors gratefully thank Ms. Vigan Dossi Murielle, laboratory technician at the Laboratory of Analysis and Research for Crop Protection (LAR-DC), for valuable technical assistance during the experimental work and data collection.