Sushma Verma, Kiran Netam and Tripti Verma

Biofertilizers are eco-friendly alternatives to chemical fertilizers that not only enhance nutrient availability but also play a significant role in plant disease suppression. They promote plant growth through nitrogen fixation, phosphate solubilization, and phytohormone production while simultaneously inhibiting phytopathogens via mechanisms such as antibiosis, competition, and induced systemic resistance. This article reviews the types of biofertilizers, their mechanisms in disease suppression, recent advances in microbial consortia development, and their integration into sustainable disease management systems.

Introduction
Agriculture today faces significant challenges due to declining soil health, excessive use of chemical fertilizers and pesticides, and the emergence of resistant pathogens. Chemical control methods, though effective, have led to environmental contamination, human health risks, and disturbance of soil microbial balance. Therefore, there is a growing emphasis on eco-friendly strategies that maintain productivity while ensuring sustainability.

Biofertilizers, traditionally used for nutrient enrichment, have now gained attention for their potential role in disease suppression. They consist of beneficial microorganisms that improve soil fertility and plant growth through biological nitrogen fixation, phosphate solubilization, and phytohormone production. Moreover, many of these microbes produce antibiotics, siderophores, enzymes, and secondary metabolites that inhibit plant pathogens. Thus, biofertilizers contribute both to nutrient management and plant disease control, making them an integral component of sustainable agriculture.

Major types of biofertilizers involved in disease suppression

Type

Microorganism(s)

Key Functions

Disease-Suppressive Role

Rhizobium

Rhizobium leguminosarum, R. japonicum

Symbiotic nitrogen fixation in legumes

Induces systemic resistance; reduces root rot and damping-off

Azospirillum

Azospirillum brasilense

Nitrogen fixation and phytohormone production

Promotes root growth and enhances plant vigor against pathogens

Azotobacter

Azotobacter chroococcum

Free-living nitrogen fixation, production of vitamins and siderophores

Produces antibiotics that inhibit Fusarium and Pythium

Phosphate Solubilizing Bacteria (PSB)

Bacillus megaterium, Pseudomonas striata

Solubilizes insoluble phosphorus compounds

Competes with soil pathogens for space and nutrients

Mycorrhizal Fungi

Glomus mosseae, Gigaspora margarita

Improves nutrient and water uptake

Enhances resistance to Fusarium and Phytophthora

Cyanobacteria

Anabaena, Nostoc

Nitrogen fixation in paddy fields

Produces bioactive compounds with antifungal effects

Trichoderma spp.

T. harzianum, T. viride

Mycoparasitism, enzyme production

Degrades cell walls of fungal pathogens

Plant Growth-Promoting Rhizobacteria (PGPR)

Pseudomonas fluorescens, Bacillus subtilis

Enhances plant growth and defense

Produces antibiotics, HCN, and siderophores

Mechanisms of disease suppression
Biofertilizers suppress diseases through several direct and indirect mechanisms that affect both the pathogen and the host plant.

Antibiosis
Many biofertilizer microbes secrete antibiotics and antifungal compounds (e.g., pyoluteorin, 2,4-diacetylphloroglucinol, and iturins) that inhibit pathogen growth. Pseudomonas fluorescens and Bacillus subtilis are well-known antibiotic producers.

Competition for nutrients and space
Beneficial microbes colonize the rhizosphere more efficiently than pathogens, depriving them of essential nutrients like carbon and iron. Effective colonization prevents pathogen establishment near root surfaces.

Siderophore production
Siderophores are iron-chelating compounds that bind Fe³⁺ ions in the soil, making them unavailable to pathogens. This limits the growth of iron-dependent pathogens such as Fusarium oxysporum.

Production of lytic enzymes
Biofertilizer organisms like Trichoderma and Bacillus produce enzymes such as chitinase, glucanase, and protease that degrade fungal cell walls, resulting in pathogen lysis.

Induced systemic resistance (ISR)
Certain rhizobacteria trigger plant defense pathways similar to systemic acquired resistance (SAR). ISR enhances the plant’s innate immunity through the production of defense enzymes like peroxidase, polyphenol oxidase, and phenylalanine ammonia-lyase.

Enhancement of nutrient uptake and plant vigor
Healthier and more vigorous plants have greater tolerance against pathogens. Biofertilizers improve nutrient uptake (especially N, P, and K), enhancing overall plant resilience.

Example of biofertilizers used in major crop groups
  • Pseudomonas fluorescens and Bacillus subtilis have been reported to suppress sheath blight and bacterial leaf blight in rice by producing antibiotics and triggering ISR.
  • Cyanobacterial biofertilizers like Anabaena azollae release antimicrobial compounds that reduce fungal infection in paddy soils.
  • Trichoderma harzianum effectively controls damping-off in tomato and cucumber caused by Pythium spp.
  • Azotobacter chroococcum reduces wilt incidence in tomato and chili.
  • Rhizobium inoculation enhances nodulation and reduces root rot in chickpea and soybean.
  • Combined use of Rhizobium and Trichoderma improves both yield and disease resistance.
  • Mycorrhizal inoculation improves phosphorus uptake in papaya and reduces root rot.
  • PGPR application in banana suppresses Fusarium oxysporum f. sp. cubense causing Panama wilt.

Integration of biofertilizers into disease management
The success of biofertilizers in field conditions depends on their integration into comprehensive Integrated Disease Management (IDM) strategies. IDM combines biological, cultural, and chemical methods to achieve effective and sustainable control.
  • Combination with organic amendments: Adding compost or vermicompost enhances microbial activity and persistence.
  • Consortia formulation: Multi-strain formulations combining Trichoderma, Pseudomonas, and Bacillus offer synergistic benefits.
  • Seed and soil treatment: Biofertilizers can be applied as seed coats, soil amendments, or root dips for preventive protection.
  • Reduced chemical inputs: Combining biofertilizers with half-dose fungicides reduces chemical residues while maintaining disease control efficacy.

Limitations and challenges
Despite their benefits, several challenges limit the widespread adoption of biofertilizers:
  • Variability in field performance: Efficacy depends on soil type, temperature, pH, and crop species.
  • Short shelf-life: Many formulations lose viability during storage.
  • Compatibility issues: Not all microbial strains coexist well in consortia.
  • Lack of farmer awareness and quality control: Inconsistent commercial products reduce user confidence.
  • Regulatory and commercialization barriers: Need for standardized quality certification and field testing.