Ritesh Kumar
Senior Research Fellow (Plant Pathology)
ICAR Research Complex for Eastern Region-Research Centre
Senior Research Fellow (Plant Pathology)
ICAR Research Complex for Eastern Region-Research Centre
Ranchi-834010 Jharkhand, India
Email id.- ritesh.nstt@gmail.com
According to Cook and Baker (1983) ‘Biological control is the reduction of the amount of inoculum or disease producing activity of a pathogen accomplished by or through one or more organisms other than man’. The terms “biological control” and its abbreviated synonym “biocontrol” have been used in different fields of biology, most notably entomology and plant pathology. In entomology, it has been used to describe the use of live predatory insects, entomopathogenic nematodes, or microbial pathogens to suppress populations of different pest insects. In plant pathology, the term applies to the use of microbial antagonists to suppress diseases as well as the use of host specific pathogens to control weed populations (Pal et al., 2006). In both fields, the organism that suppresses the pest or pathogen is referred to as the biological control agent (BCA). More broadly, the term biological control may also be applied to the use of the natural products extracted or fermented from various sources. These formulations may be very simple mixtures of natural ingredients with specific activities or complex mixtures with multiple effects on the host as well as the target pest or pathogen.
The environmental pollution caused by excessive use and misuse of agrochemicals, as well as fear-mongering by some opponents of pesticides, has led to considerable changes in people’s attitudes towards the use of pesticides in agriculture. At the same time there is a demand from society for healthy foods with less chemical residues, and a great concern for preservation of the environment. Over the past one hundred years, research has repeatedly demonstrated that phylogenetically diverse microorganisms can act as natural antagonists of various plant pathogens. The interactions between microorganisms and plant hosts can be complex. Interactions that lead to biocontrol can include antibiosis, competition, induction of host resistance, and predation. The most common approach to biological control consists of selecting antagonistic microorganisms, studying their modes of action and developing a biological control product.
Mechanisms of biological control
Biological control may be accomplished through several approaches including mass introduction of antagonists, plant breeding, and specific cultural practices aimed at modifying the microbial balance.
Following are the mechanism on which biocontrol agents work (which are never mutually exclusive):
Antibiosis
Antibiosis has been defined as the interactions that involve a low-molecular weight compound or an antibiotic produced by a microorganism that has a direct effect on another microorganism. The may be involved and play an important role in plant disease suppression by certain bacteria and fungi.
Competition
This process is considered to be an indirect interaction whereby pathogens are excluded by depletion of a food base or by physical occupation of site. Biocontrol by nutrient competition can occur when the biocontrol agent decreases the availability of a particular substance thereby limiting the growth of the pathogen. Particularly, the biocontrol agents have a more efficient uptake or utilizing system for the substance than do the pathogens.a biocontrol agent can provide plant protection by efficient interception of these stimulating factors before pathogens can use them.
Mycoparasitism
Mycoparasitism is a process by which biocontrol fungi may attack pathogenic fungi. Fungi that are parasitic on other fungi are usually referred to as mycoparasites. Many mycoparasites occur on a wide range of fungi and some of them have been proposed to play an important role in disease control.
Cell wall degrading enzymes
Extracellular hydrolytic enzymes produced by microbes may also play a role in suppression of plant pathogenic fungi. Chitin and b-1,3-glucans are major constituents of many fungal cell walls. Several studies have demonstrated in vitro lysis of fungal cell walls either by chitinase or b-1, 3-glucanase alone or in combination. Recently, genetic evidence for the role of these enzymes in biocontrol has been obtained. A chitinase (ChiA) deficient mutant of Serratia marcescens was shown to have reduced inhibition of fungal germ tube elongation and reduced biocontrol of Fusarium wilt of pea seedling in a greenhouse assay (Lam and Gaffney, 1993).
Induced resistance
The inducible resistance in plants to a variety of pathogens is known as systemic acquired resistance (SAR) which may be induced by inoculating plants either with a necrogenic pathogen or nonpathogen or with certain natural or synthetic chemical compounds (Lam and Gaffney, 1993; Lo, 1998). These defense responses may include the physical thickening of cell walls by lignifications, deposition of callose, accumulation of antimicrobial low-molecular-weight substances (e.g., phytoalexins), and synthesis of various proteins (e.g., chitinases, glucanases, peroxidases, and other pathogenesisrelated (PR) proteins). This defense system is also triggered when plants are colonized by plant growth- promoting rhizobacteria (PGPR) and a few binucelate Rhizoctonia. Several strains of PGPR have been shown to be effective in controlling plant diseases by inducing plant systemic resistance.
Advantages of using biocontrol agents
- Safe to handle or use
- Occurs naturally
- High degree of host specificity
- Cost effective.Self perpetuation
- Eco Friendly
- Do not attack other species
Table 1. List of some biocontrol agents.
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Sl. No.
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Biocontrol Agents
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Target pathogen/pest/weed
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|
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1.
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Fungal
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Trichoderma spp.
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Rhizoctonia, Pythium, Fusarium and several soil and foliar pathogen
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Gliocladium catenulatum
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Pyhtium, Phytophthora, Rhizoctonia, Botrytis.
|
||
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Ampelomyces quisqualis
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Powdery mildews
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||
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Gliocladium spp.
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Several plant diseases
|
||
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Coniothyrium minitans
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Sclerotinia species
|
||
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Cryptococcus albidus
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Botrytis spp., Penicillium spp.
|
||
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Verticillium lecanii
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Whitefly, aphids and thrips
|
||
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Metarhizium anisopliae
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black vine weevil, spittle bugs, cockroaches, termites
|
||
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Beauveria bassiana
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whitefly, aphids, thrips, grasshoppers, locusts
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||
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Alternaria cassiae
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Weedicide (Cassia obtusifolia)
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||
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Cercospora rodmanii
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Weedicide (Eichhornia crassipes)
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||
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Colletotrichum coccodes
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Weedicide, Velvetleaf (Abutilon theophrasti) in corn and soybeans
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||
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Colletotrichum gloeosporioides f. sp cuscutae
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Weedicide (Cuscuta chinensis, C. australis)
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||
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2.
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Bacterial
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Streptomyces griseoviridis
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Fusarium, Pythium, Phytophthora.
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Streptomyces lydicus
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powdery mildew
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||
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Bacillus subtilis
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powdery mildew, Botrytis, leaf spots
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Bacillus thuringiensis
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caterpillars
|
||
Conclusion and perspectives
A successful biocontrol requires considerable understanding of cropping system; disease epidemiology; the biology, ecology, and population dynamics of biocontrol organisms; and the interactions among these variables (Lo, 1997). There has been a significant increase in the number of biological disease control agents registered or on the market worldwide in the last few years (Lo et al., 1997). For example, there currently are approximately 30 bacterial and fungal products for control of foliar, soil-borne and postharvest diseases. In recent years, the progressive developments of biocontrol to plant diseases both in research and application have been recognized in India. However, several problems have been also created and various obstacles have stalled the development of biocontrol, e.g. lack of constancy on its ffectiveness. Inconsistency in effectiveness is a commonly encountered problem when biocontrol is employed as a measure of plant disease control. Basic research is fully required before integration between all abio- and/or biofactors which are beneficial to increase efficacy of biocontrol.
Future prospects
There is a growing demand for sound, bio-based pest management practices. An upswing in commercial interests has also developed in the past few years. The future success of the biological control industry will depend on innovative business management, product marketing, extension education, and research. (Mathre et al., 1999). Increased demand for organic produce and participation in home gardening activities by pesticide-wary urban populations has enlarged the market for biocontrol products. The field of plant pathology will contribute substantially to making the 21st century the age of biotechnology by the development of innovative biocontrol strategies.
References
Cook, R. and Baker, K. F. 1983. The Nature and Practice of Biological Control of Plant Pathogens, American Phytopathological Society, St Paul, Minnesota, 539.
Lam, S. T., and Gaffney, T. D. 1993. Biological activities of bacteria used in plant pathogen control. Pages 291-320. in: Biotechnology in Plant Disease Control. I. Chet, ed., John Wiley, New York .
Lo, C. T., Nelson, E. B., and Harman, G. E. 1997. Improved biocontrol efficacy of Trichoderma harzianum 1295-22 for foliar phases of turf diseases by use of spray application. Plant Dis. 81:1132 – 1138.
Lo, C.-T. 1998. General mechanisms of action of microbial biocontrol agents. Plant Pathol. Bull. 7:155-166.
Lo, C-T. 1997. Biological control of turfgrass diseases using Trichoderma harzianum. Plant Prot. Bull. 39: 207-225.
Mathre, D. E., Cook, R. J., and Callan N. W. 1999. From discovery to use: Traversing the world of commercializing biocontrol agents for plant disease control. Plant Dis. 83:972-983.
Pal, K. K. and B. McSpadden Gardener, 2006. Biological Control of Plant Pathogens. The Plant Health Instructor DOI: 10.1094/PHI-A-2006-1117-02.
Note: This article is only a compilation of the available information on the topic and the source is cited/acknowledged in the text.
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