Verticillium wilt, caused by the soilborne pathogen Verticillium dahliae, poses a severe threat to olive trees, spreading rapidly and lacking effective curative treatments. In response to this urgent agricultural challenge, Andrea Crespo-Barreiro, Jorge Cara-Jiménez, and Fernando González-Andrés explored sustainable management strategies in their recent study published in Industrial Crops & Products. Their research focused on the individual and combined effects of Bacillus siamensis as a biological control agent (BCA) and biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More on infected ‘Picual’ olive plants.

The study found that all treatments— Bacillus siamensis (M+), biochar (B+), and their combination (MB+)—significantly reduced disease severity compared to the untreated control. Among these, the Bacillus siamensis (M+) treatment proved most effective, decreasing disease incidence to 16.7%, disease severity to 2.1%, and relative area under the disease progress curve (RAUDPC) to 1.5%. These figures were comparable to healthy, uninfected plants (C-), demonstrating a remarkable reduction in disease progression by over 68% compared to infected controls. The pathogen-infected control (C+) exhibited a high disease severity of 70.8% and an RAUDPC of 61.0%.

The Bacillus siamensis (M+) treatment also notably increased fresh aerial biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More by 7% and dry aerial biomass by 3% compared to the pathogen-free control (C-). This suggests that Bacillus siamensis not only controls the disease directly but also promotes plant growth, possibly through the production of phytohormones like indole-3-acetic acid (IAA). Biochar alone (B+) also showed positive effects, increasing fresh weight by 23% and dry weight by 26% compared to the pathogen-free control, making it the most effective for biomass production despite being less effective at controlling disease progression than M+.

Further investigation into biochemical stress markers revealed that the Bacillus siamensis (M+) treatment significantly alleviated pathogen-induced stress. It resulted in the lowest concentrations of malondialdehyde (MDA), a marker for oxidative damage, in leaves and hydrogen peroxide (H2​O2​) levels in both leaves and roots, mirroring those in pathogen-free control plants. This reduction in oxidative stress was corroborated by the downregulation of the catalase (CAT) gene, an enzyme responsible for eliminating H2​O2​, indicating the plant’s reduced need to activate its stress response.

While Bacillus siamensis (M+) downregulated genes associated with salicylic acid (SA) pathways (WRKY5 and ACL), suggesting its protective effects do not primarily rely on these mechanisms, biochar alone (B+) activated the jasmonic acid (JA) pathway, evidenced by the upregulation of the bHLH gene. This indicates that biochar induces systemic resistance, a defense mechanism in plants. However, the combined treatment (MB+) showed reduced efficacy compared to individual applications, suggesting a lack of synergy between the two. This could be because the bacterium counteracted biochar’s effect on JA signaling. Despite this, the combined treatment still offered some biocontrol, likely due to biochar’s role in improving soil health and microbial activity.

Overall, this research highlights the significant potential of Bacillus siamensis as a stand-alone biological control agent for managing Verticillium wilt in olive cultivation. While biochar also contributes to plant defense and growth, the optimal strategies for combining these elements require further exploration to harness their full potential for sustainable disease management in agriculture.

Source: Crespo-Barreiro, A., Cara-Jiménez, J., & González-Andrés, F. (2025). Boosting olive plant defense systems against Verticillium dahliae: Integrating biochar and Bacillus siamensis for sustainable disease management. Industrial Crops & Products, 233, 121395.