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In this article we will discuss about:- 1. Symptoms of Fusarial Wilt Disease 2. Biological Control Strategies and Perspectives of Fusarial Wilt Disease 3. Bacteria as Biocontrol Agents 4. Mechanism of Fusarial Wilt Disease Suppression 5. Antibiotics 6. Rhizosphere Competence and Colonization 7. Fungi as Biological Control Agent.
Contents:
- Symptoms of Fusarial Wilt Disease
- Biological Control Strategies and Perspectives of Fusarial Wilt Disease
- Bacteria as Biocontrol Agents in Fusarial Wilt Disease
- Mechanism of Fusarial Wilt Disease Suppression
- Antibiotics of Fusarial Wilt Disease
- Rhizosphere Competence and Colonization of Fusarial Wilt Disease
- Fungi as Biological Control Agent in Fusarial Wilt Disease
1. Symptoms of Fusarial Wilt Disease:
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Fusarial wilt is the most destructive disease and is prevalent in all chickpea growing areas in India. The pathogen, Fusarium oxysporum f. sp. ciceri is seed-borne in nature and can survive in the soil for many years. The fusarial wilt disease can be detected at the seedling stage as well as advanced stage of plant growth. The young growing tips of infected plants become limp and dropping.
The pathogen enters the root system, vessels get logged and water supply to the shoot is impaired as a result, characteristic symptoms of leaf yellowing, wilting and finally drying of the shoot occurs. Roots turn black and decompose soon. Dark brown fungal streaks can be seen in the root system near the pith region.
2. Biological Control Strategies and Perspectives of Fusarial Wilt Disease:
Biological control involves use of microorganisms to reduce the population of harmful microorganisms. Fusarial wilt disease is based on the natural phenomenon, where the nature through its own way keep the population of each microorganisms under control and prevents the Fusarial wilt disease from getting catastrophic. Fresnious (1852) was probably the first who suggested the possibility of biocontrol of plant diseases.
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This concept of disease management, however, remained neglected for centuries. The recent shift to the opinion that biological control cannot have an important role in agriculture in future, the interest and research in this area have increased since 1970’s mainly because of the realization of adverse effects of chemicals.
Biological control may be defined as the control of fusarial wilt disease through action of organisms other than use of resistant and antagonistic plants. It is defined as the reduction of the inoculum density or disease producing activities of a pathogen or parasite in its active or dormant state, by one or more organisms that occur naturally or through the manipulation of the environment or by mass production of one or more antagonists.
An antagonist is a microorganis that adversely affects another organisms. Hence technically biocontrol is reaction in pest population accomplished through introduction of antagonists or manipulating the occurring antagonists.
During antibiosis, antagonism results due to the production of toxic metabolites, lytic enzymes, volatile compounds and other substances by the origin.
Biocontrol of plant disease can also be achieved through the activity of mineral solubilizers or plant growth promoting microorganisms (PGPM). In addition to the solubilization of the vial minerals, the PGPO’s produce certain metabolites that may adversely affect the pathogenesis and produce phytohormones that may capacitate the host to defend the pathogen attack.
Numerous microorganisms have demonstrated some potential of suppressing fungal plant pathogens.
Majority of these bioagents belong to bacteria and fungi, which are discussed under the following headings:
3. Bacteria as Biocontrol Agents in Fusarial Wilt Disease:
Many bacteria have shown to exert antifungal activities which can be exploited in the control of phytopathogenic fungi. Bacterial bioagents improve plant growth by suppressing either major or minor pathogens and/or producing plant growth promoting substances like auxins, gibberellins etc.
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The biological control of root diseases can be accomplished by several means as:
i. Direct inoculation of bacterial on seed and stem cuttings.
ii. Incorporation into the soil.
iii. Indirect control by the use of cultural changes to enhance naturally occurring disease control organisms already in the soil; and
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iv. Integrated control, such as use of seed inoculum plus chemicals or any other combinations of control measures.
Agrobacteriurn radiobacter strain K-84 was the first bacterium used commercially for the control of crown gall caused by A. tumiefaciens and got worldwide success. Many bacterial genera have shown their potential for biocontrol both in vitro and in vivo conditions.
Agrobacteriurn , Arthobacter, Azotobacter, Bacillus, Escherichia coli, Enterobacter, Burkolderia, Serratia, and Stenotrophomonas were found to be potent for suppression of soil-borne fungal pathogens. Many of these biocontrol agents exhibited their effectiveness under field conditions also.
Martnetti and Loper (1992) reported that a strain of Alcaligenes sp. inhibited microconidial germination and germ tube elongation of F. oxysporum f. sp. dianthi and also reduced the severity of disease presumably as a result of siderophore production.
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Enterobacter cloacae, a potent root colonizer was reported as biocontrol agent against Pythium ultimum, a prevalent phytopathogen that causes damping off of many crop plants.
Bacillus Species:
Formulations made from Bacillus spp. especially B. thuriengiensis contributes 92 per cent of the total biopesticides. The usefulness of Bacillus sp. as a source of antagonist for many plant pathogens is well known. Bacillus spp. have many-ecological advantages; their endospores are tolerant to extreme conditions of heat and desiccation.
A strain of Bacillus subtilis isolated from the lysed mycelium of Sclerotium rolfsii by Broadbent (1971), was found antagonistic to several pathogens and as a seed inoculant increased in yield. Since 1983 it is sold as a bioinoculant under the trade name of QUANTUM-400.
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The features that make Bacillus spp. as potent biocontrol agents are their abundance in the soil and the production of various potent fungicidal metabolites. B. subtilis produces bulbiformin which appears to be responsible for the management of Fusarial wilt on various crops.
A reduction of 88 per cent in the incidence of pigeon pea wilt was noticed in the autoclaved soils containing mallases, sweet clover roots and groundnut cake, when inoculated with B. subtilis. Khan and Akram, (2000) reported satisfactory control of Fusarial wilt of tomato due to the soil application of B. subtilis.
A recent study made by Khan and Khan (2001) also demonstrated the potential of another strain of B. subtilis in the management of Fusarial wilt of tomato caused by Fusarium oxysporium f. sp. lycopersici.
A few other species of Bacillus have been found antagonistic to pathogen fungi. A strain of B. cereus by Phytophtliora medicaginis, cucumber fruits from rot caused by Pythium aphanidermatum and peanut from Sclerotinia mino. B. polymyxa can also reduce the severity of Fusarial wilt of tomato caused by F. oxysporum f. sp. lycopersici.
Pseudomonas Species:
Flourescent pseudomonads have revolutionized the field of biological control of soil borne plant pathogenic fungi. During the last 25 years they have emerged as the largest potentially and most promising group of plant growth promoting rhizobacteria in the biocontrol of plant diseases. Flurescent pseudomonads have received the most attention for many compelling reasons.
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1. Flourescent pseudomonads readily colonize root in nature where they are frequently the most common of the microorganisms.
2. The simple nutritional requirement and ability to use many carbon sources that exude from root and to complete with indigenous microflora explains their ability to colonize the rhizosphere.
O’Sullivan and O’Gara (1992) have been reviewed the traits of fluorescent pseudomonads such as production of antibiotics, hydrogen cyanide, siderophores which are involved in the suppression of root pathogens. Fluorescent pseudomonads have been implicated in the control of several Fusarial wilt diseases caused by Fusarium spp. root-rot of important crops caused by Pythium spp.
4. Mechanism of Fusarial Wilt Disease Suppression:
Various biocontrol mechanisms are involved in ability of B. subtilis, P. fluorescens etc. to control the chickpea pathogenic fungi F. oxysporum f. sp. ciceri, R. bataticola and Pythium spp. The potential ones are competitions for iron and other nutrients, niche exclusion, induction of systemic resistance and production of antimicrobial metabolites.
5. Antibiotics of Fusarial Wilt Disease:
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O’Sullivan and O’Gara (1992), critically reviewed the protection mechanism in various groups of endophytes. Several strains of Pseudomonas has shown to produce wide array of antibiotics which include phenazines, pyoleutorin, pyrrolnintrin, pholoroglucinols. Burkhead (1994); reported that a strain of Pseudomonas cepacia are produced pyrolnintrin inhibitory of Fusarium sambicinum.
Another strain of P. fluorescens is known to produce phoroglucinol, which protected sugarbeet against Pythium mediated damping off.
Bacillus subitilis is known to suppress the fungal growth through production of antifungal antibiotics such as bulbiformin, agrocin-84, iturin and surfactin, inuring A, bacillomycin, mycosubtilins, mycobacillin and mycocerein.
A strain of B. subtilis produces an antifungal antibiotic which suppresses Fusarium solani. The mechanism involved in the suppression of R. solani lesions on crop by B. subtilis is also suspected to be antibiosis.
Antibiotic production was clearly indicated as the mode of action of biocontrol against damping-off. Biocontrol of the damping off of tomato through the application of B. subtilis was brought about by the production of two antibiotics, iturin and surfactin (Table 6.1).
Production of lytic enzymes like chitinasses and 1, 3-? glucanases by certain bacteria may also form the basis of control of plant pathogenic fungi in the rhizosphere. Production of hydrogen cyanide has been reported as a mechanism of Fusarial wilt disease suppression by certain bacteria.
Species of Bacillus and Pseudomonas produce a range of metabolites including biosurfactants, volatiles and compounds which elicit plant resistance mechanisms.
Baker (1990) reported that the siderophores likes pseudobactines or pyoverdins being very efficient competitors of iron may help in the control of Fusarium wilt. Competition for iron has been reported as one of the mechanisms responsible for soil suppressiveness to Fusarium wilts. The involvement of competition for iron as a mechanism of the antagonism is expressed by fluorescent.
Pseudomonas against F. oxysporum. The intensity of the competition for iron depends on several environmental factors, of which pH is the most important. Indeed, most soils that are suppressive to Fusarial wilts exhibited a pH higher or equal to 7, making iron not easily available for microorganisms.
Fluorescent pseudomonads produce different types of metabolites but until now competition for iron has been linked with suppression of Fusarial wilts; Phenazines, phloroglucinols or any other antibiotics (e.g. oomycin, pyrrolnintrin, pyolterorin) have not been implicated in the control of Fusarial diseases.
6. Rhizosphere Competence and Colonization of Fusarial Wilt Disease:
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Root colonization is defined as the process whereby introduced bacteria become distributed along roots in non-sterile soil, multiply and survive for several weeks in the presence of indigenous soil micro-biota. Root colonization involves two phases; phase-1 is the attachment to roots and phase-II is the multiplication on roots.
Rhizosphere competence describes the relative root colonizing ability of a rhizobacteria. Bacterial traits that are linked to weak rhizosphere competence are poorly understood but some that may be important are characterized into three classes.
Class-I involves cell surface polysaccharides, fimbraie, flagella and chemotaxis towards seed or root exudates. Class-II involves growth rate and ability to utilize complex carbohydrates, and the Class-Ill involves osmotolerance, which is necessary for survival.
A chickpea rhizosphere-competent bacterial strain of P. fluorescens that suppressed F. oxysporum f. sp. ciceri, R. bataticola and Pythium spp. was developed with the help of spontaneous chromosomal Rifr strain method. This is the first report of a single biocontrol bacterium active against three most devastating pathogenic fungi of chickpea.
Seed bacterization with this strain of P. fluorescens increased the germination of seed by 25 per cent reduced the number of diseased plants by 45 per cent, compared to non-bacterized controls. The seedling dry weight, shoot length and root length were increased by 16-18 per cent.
A rifampicin resistant mutant of P. fluorescens, used to monitor chickpea root colonization confined the rapid aggressive colonization by the bacteria, making it a potential biocontrol agent against chickpea phytopathogenic fungi.
7. Fungi as Biological Control Agent in Fusarial Wilt Disease:
There are a large number of fungi, which possess considerable ability to antagonize plant pathogenic fungi. Among them mycoparasites constitute a major group of fungi, which can have potential role in biological control. Mycoparasites are the fungi which parasite another fungus and the phenomenon is referred to as mycoparasitism.
Mycoparasitism involves the following four stages:
1. Chemotrophic growth, in which a chemical stimulus from the pathogenic fungus attracts the parasite.
2. Recognition, as an antagonist attacks only a few fungi.
3. Attachment, in which the hyphae of the antagonist can either grow alongside the host hyphae or coil around it.
4. Degradation of the host wall by production of lytic enzymes such as chitinases and glucose 1, 3,-b glucosinases.
Barnett and Binder (1973) recognized two broad types of mycoparasitism i.e. necrotrophic and biotrophic. Sneh (1977) reported a number of microorganisms parasitizing oospores of the Phytophthora spp. Deacon (1993) found Pythium oligandrum to be an aggressive, necrotrophic parasite on the plant pathogenic fungi Gaumanomyces graminis var. tritici and Phialophora radidicola.
The hyperparasite produces numerous thin haustorial threads and coils around the host hyphae. Species of Trichoderma represent a group of efficient mycoparasites of soil borne plant pathogen fungi. During the recent past they have been the topic of intense biocontrol researches.
