Soil has abundant microbial population and species diversity. Coexistence and interaction of these different species of soil microorganism with plant root affect plant development and soil microbial structure. Plant disease caused by plant pathogen can be controlled by biological control agents. Biocontrol may operate through antibiosis or competition for space or nutrients in the rhizosphere and parasitism. Antibiotic production is a mode of action for suppression of disease. Inhibition of pathogens by antimicrobial compound is known as antibiosis. Different volatile and non-volatile secondary metabolite have been characterized in biocontrol agents. Soil microorganisms such as fungi, bacteria and actinomycetes can produce natural bioactive compound the antibiotic.
Antibiosis is a biological interaction between organisms. During antagonistic interaction the secondary metabolite is released by one organisms that inhibits the growth of the other organisms. Biocontrol agent act through antibiosis, substrate competition, mycoparasitism and produce cell wall degrading enzyme (glucanases and chitinases) and diverse antibiotic (gliotoxin, peptaibols or gliovirin.) against the fungal phytopathogen (Howell 2003). Patulin a mycotoxin inhibits three important rice pathogen Pyricularia oryzae, Drechslera oryzae and Gerlachia oryzae (Okeke et al., 1993). Patulin is produced by Aspergillus, Penicillium and Byssochlamys (Puel et al., 2010). The species producing patulin are:
- Aspergillus clavatus and giganteus (Varga et al., 2007).
- Penicillium carneum, P. clavigerum, P. concentricum, P. coprobium, P. dipodomyicola, P. expansum, P. glandicola, P. gladioli, P. griseofulvum, P. marinum, P. paneum, P. sclerotigenum, P. vulpinum (Frisvad et al., 2004).
- Byssochlamys nivea and some strains of Paecilomyces saturatus (Samson et al., 2009).
Most of the plant disease is caused by fungus and many soil borne biocontrol agent have the potential of controlling these plant pathogen. A few example of antagonists, target pathogen and host plant (Ghisalberti 2000) are:
- Chaetomium globosum inhibits Pythium ultimum and Sclerotium cepivorum that infect sugarbeet and onion respectively.
- Gliocladium roseum suppresses Fusarium culmorum infecting wheat, Bipolaris sorokiniana that infects barley and Fusarium monoliformae that infects maize.
- Idriella bolleys controls pathogen Fusarium culmorum infecting wheat.
- Paecilomyces lilacinus suppresses fungi Macrophomina phaesolina infecting mungbean and sunflower.
- Trichoderma virens (“Q”) produce the antibiotic, gliotoxin, that is active against Rhizoctonia solani, but less active against Pythium ultimum (Howell et al., 1993), other strains (“P”) of fungus produce antibiotic gliovirin which is highly active against ultimum but has no effect on R. solani (Howell and Stipanovic, 1983). Gliocladium virens produce phytotoxin, viridiol (Howell and Stipanovic, 1984). Howell and Puckhaber (2005) studied the efficacy of the “P” and “Q” strains of Trichoderma virens. “P” Strains of T. virens are ineffective biocontrol agents of seedling disease in cotton but are pathogenic to susceptible seed lots. While the “Q” strain with high level of phytoalexins are effective biocontrol agents of cotton seedling disease and are not pathogenic to cotton.
The antagonistic potential of Trichoderma species makes it the best biocontrol agent. This involves production of extracellular cell wall degrading enzyme β-1,3-glucanases and chitinases by Trichoderma harzianum which suppresses the fungal pathogen showing antibiosis (Chet and Inbar 1994).
Antifungal metabolites produced by bacteria include ammonia, butyrolactones, 2,4-diacetylphloroglucinol, HCN, kanosamine, Oligomycin A, Oomycin A, phenazine-1-carboxylic acid, pyoluterin, pyrrolnitrin, viscosinamide, xanthobaccin, zwittermycin A and several other uncharacterized moieties (Laville et al., 1992; Milner et al., 1996; Nielson et al., 1998; Kang et al., 1998; Kim et al., 1999; Thrane et al., 1999; Nakayama et al., 1999). Fluorescent Pseudomonas population produce antifungal metabolite 2,4-diacetylphloroglucinol (Weller et al., 2002). Streptomyces synthesize many siderophores and degradative enzymes (e.g., chitinases) to break down complex substrates (Chater et al., 2010; Hjort et al., 2010) and have the ability to produce a vast array of antibiotic compounds which can inhibit the growth of competitors (Watve et al., 2001).
Suppressive soil provide information about natural microbe based plant defense. Indigenous soil microorganisms can reduce disease (Schlatter et al., 2017). Multiple microbial interactions in the rhizosphere are shown to provide enhanced biocontrol in many cases as compared to the biocontrol agents used singly (Whipps 2001). Diverse soil microorganisms producing range of antimicrobial molecules serve to be a better biopesticide.
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BIOLOGICAL PROCESSES- A NATURE'S GIFT 