Introduction
Chemical pesticides used to suppress Fusarium wilt have raised issues with the environment and human safety. For the past 70 years, biological management of plant pathogens has been employed; yet, there are numerous conflicts around its efficacy. Because FWB is harmful, several researchers have gathered methods to regulate it, such as fumigation, which sterilizes soil and can contaminate ecosystems and soil. While tetraploid banana varieties resistant to FWB have been successfully created through conventional breeding procedures, genetic engineering advancements have not progressed far. Additionally, a transgenic Cavendish strain resistant to TR4 has been discovered. Organic additives, intercropping, and crop rotation and biofumigation, biological soil disinfection, and soil solarization are further techniques. Nonetheless, it continues to be difficult for them to effectively manage the illness and offer them commercial advice.
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Biocontrol using endophytic and root associated rhizospheric microbes
Antagonistic microbes, which protect and promote plant growth by colonizing and multiplying in both the rhizosphere and plant system, could be an alternative approach for controlling fungus-borne diseases (FWB) in banana cultivation. Biological control using endophytic antifungal microorganisms can prevent problems while improving soil quality. The richness and diversity of soil microorganisms in banana rhizospheres are crucial for acceptable FWB control. The more antagonistic microorganisms in the soil, the better the FWB control will be.
Recent research has shown that manipulating banana rhizosphere microbiota by introducing well-characterized antagonistic soil microorganisms alone or in combination with organic amendments can reduce the incidence of FWB to an acceptable level. This changes the structure and composition of the microbial community, which can be used to control FWB more effectively.
Several beneficial microorganisms have been isolated to suppress soil-borne diseases, but their survival, stability, and impact on the emerging microbial community are largely unknown, resulting in conflicting field results. Different species of rhizospheric and endophytic microorganisms, such as Trichoderma, Penicillium, Bacillus, and Pseudomonas, have been used as biocontrol agents against FWB under glasshouse and field conditions. Field evaluations have demonstrated that soil application of these bacterial combinations gives effective suppression of FWB in cv. Grand Naine, increasing banana yield parameters and decreasing the percent of FWB.
Some microorganisms can also be used as indicator species of healthy banana plants on Fusarium wilt-infested areas. Sucker dipping and soil application with P. fluorescens spore suspension resulted in the highest wilt disease suppression in two field trials in India. Combining botanical fungicides with biocontrol agents significantly decreased wilt disease incidence under greenhouse and field conditions.
In recent years, the most important endophytic fungi and bacteria isolated from different parts of bananas have been used to control FWB. One successful example is the Jiangcheng Shenglong banana plantation, which had over 300 ha under banana cultivation without Fusarium wilt disease symptoms while over 200 plants showed signs of TR4 during 2008-2009.
Colonization of microbes in banana tissue
The successful development of Fusarium wilt tolerance in banana plants is based upon the efficient biological management of soil-borne fungi. This is due to the fact that plant tissue and the rhizosphere are the sites of multiplication activity and colonization for Fusarium wilt (FWB). The potential of beneficial microorganisms related to plants to strengthen resistance in the host against FWB has been investigated. Because antagonistic bacteria can easily colonize roots, it has been discovered that they are more successful at suppressing root infections.
For up to 35 days following treatment, endophytic bacteria can colonize plant tissues, such as petioles, corms, roots, and pseudostems. Tissues from banana roots, corms, and pseudostems contained Bacillus subtilis strain KY-21 up to 20 days following inoculation. The majority of endophytic colonization cases are found in intercellular gaps, particularly in roots.
It has also been documented that arbuscular mycorrhizal fungi (AMF) can increase a plant’s resilience to disease. Under greenhouse circumstances, applying 2 a.m. fungi (Glomus spp.) to banana plantlets (Grand Naine) improved nutrient uptake and plant development while lowering internal and exterior Fusarium wilt symptoms.
The growth characteristics of micropropagated Grand Naine banana plantlets at nursery stage phase were dramatically boosted by combining AMF with Bacillus consortium, which included three strains of Bacillus. In Lakatan banana seedlings, Glomus spp. demonstrated the most effective therapy for reducing the Fusarium wilt TR4 infection and increasing plant growth characteristics. It would also be feasible to use T. harzianum in conjunction with Glomus species.
Injecting endophytes and root associated rhizospheric microorganisms into tissue cultured banana plantlets to control FWB
It has been noted that endophytic bacterial colonization occurs in the tissue of the banana shoot tip, exhibiting mutualistic interaction and no pathogenic impact on the host banana tissue. It’s now possible to prevent Foc infection in bananas by adding endophytic microorganisms to the roots of tissue-cultured plants. In banana plantlets, this approach has been demonstrated to decrease the severity of wilt and boost plant growth characteristics. When endophytes from naturally occurring, healthy banana plants were artificially injected into banana tissue culture plantlets, the result was a 67% suppression of FWB in plantlets after five months of pathogen infection.
Numerous investigations have documented the enhancement of host plantlet growth, whereby bacterial treatments augment growth and confer resistance against wilt pathogens. On banana plantlets grown in tissue culture, Bacillus species can encourage growth and the development of resistance against wilt pathogens. Plantlets grown from tissue culture bananas with rhizobacteria that promote plant growth (PGPR) showed a high rate of development and vigor. strains of Pseudomonas 84 and 4B were injected into tissue-cultured banana plantlets during the hardening stage, which enhanced plant development and decreased Foc infections in rhizomes. A strain of P. aeruginosa (FP10) from the banana rhizosphere was encapsulated in the tips of banana shoots, and this increased the rate of plantlet regeneration in comparison to the untreated controls. The most successful method for raising the caliber of micropropagated food was bio-inoculation with microbes which fix nitrogen, solubilize nitrogen, and produce IAA.
Antagonistic mechanisms of bio-control agents
Controlling diseases and comprehending their mechanisms are greatly aided by antagonistic endophytes. Plant infections have been managed by them using a range of strategies, such as nutritional competition, antibiotic synthesis, biochemical and structural processes, stress tolerance, and the creation of disease resistance. While certain microorganisms work directly with the disease to cause resistance, others compete with it for nutrition and space, weakening and eventually killing the pathogen. Moreover, antagonists can function as antibiosis or hyperparasitism, targeting and eliminating fungal pathogen mycelium, spores, and resting structures. Certain antagonists generate secondary metabolites with antimicrobial properties to prevent germs from causing harm.
Certain bacteria that are hostile produce enzymes that break down pathogen structures and stop the formation of fungal mycelia. One important tool for controlling a broad range of plant diseases in different plant species in greenhouse and field settings is induced systemic resistance, or ISR. Nonetheless, further investigation is required to comprehend the efficacy of antagonists and their method of action, which involves intricate, natural, and highly controlled interactions between microbiological cells and plant on site. When designating an antagonistic microorganism to control infections, tolerance and ecotoxicological dangers of these complicated interactions to human beings and the environment must be taken into account.