Here is an elaborated discussion on Biological Control Agents, highlighting:-1. Characteristics of an Effective BCA 2. Selection (Screening) of Biological Control Agents 3. Development 4. Application and Establishment 5. Commercially Available BCAs 6. Constraints 7. Future.
Random screening of large numbers of antagonistic microorganisms for suppressive effects on plant disease has often been used to select the most promising strains for further development and application.
But the chances of success can be improved by adopting a more rational approach based on ecology because, to be effective, a BCA must be able to colonize a particular habitat, or to occupy a specific niche, in sufficient numbers to interfere with the growth or survival of the target pathogen. Therefore, it is recommended to be better to screen BCAs on selective basis rather than introducing randomly selected microbial antagonists.
Characteristics of an Effective BCA:
An effective biological control agent (BCA) that results in protection against disease caused by plant pathogenic organisms must have the following characteristics:
(1) BCA must be able to control the pathogen by inhibiting its development, making it vulnerable to other members of the prevailing micro-flora or killing it. Mechanisms by which it may do this include competition for nutrients or potential sites of penetration of the host, production of antibiotics or lytic enzymes. Parasitism or inhibition is of the means by which the pathogen is able to attack its host, e.g., inhibiting enzymes necessary for penetration or destroying toxins.
(2) BCA must be able to establish itself at the appropriate location and at a sufficient density to give effective control. For air-borne pathogens, the bio-control agent must be able to compete with the naturally occurring micro-flora and withstand fluctuations in the microclimate normally associated with the crop. These may include high temperatures and high light intensities as well as wash-off by rainfall. For soil-borne pathogens, the bio-control agent must be able to compete with the soil micro-flora and to grow in the rhizosphere environment, i.e., it must be rhizosphere competent.
In aggregate, these requirements are formidable but in 2001 they were fulfilled by at least 80 available preparations, which are the successful results of various approaches to the screening, development, and application of such agents.
Selection (Screening) of Biological Control Agents:
The environment contains a multitude of potential bio-control agents. These may form part of the resident micro-flora of aerial and root surfaces of plants as well as existing independently in the soil.
In order to exploit these resources efficiently it is necessary to select promising sources of potential bio-control agents and to use appropriate screening procedures to single out the microorganisms responsible. Soils with a history of suppressiveness towards a given pathogen are obvious sources of potential bio-control agents but opinions diverge as to how screening for the effective microorganisms should be done.
However, Schisler and Slininger developed in 1997 a novel method for selecting (screening) antagonists of Fusarium dry rot of potatoes.
This method can be briefly represented as following:
1. Potato periderm, live field soil, and ϒ-irradiated soil were combined in proportions 2: 5: 93 by weight to form an antagonistic soil mix (ASM). The mixture adjusted to a moisture level of 0.2 m3m–3 and incubated in a sealed plastic bag for a week at 15°C.
2. The antagonistic soil mix (ASM) was mixed with zoospore suspensions of Fusarium sambucinum and the mixture was adjusted to a moisture level of 0.35 m3m–3 for two days at 15°C.
3. Zoospore mixed ASM was applied as a paste to wounded potato tubers of a cultivar susceptible to Fusarium dry rot 2 days later, and incubated for 4 weeks at 15°C.
4. Samples of potato tissues that remained healthy were excised and plated.
5. Microorganisms were isolated on a variety of media and checked individually in the assay for their ability to suppress lesion development on potato tubers.
6. The assay was repeated with the isolated microorganisms in order to determine those that were antagonistic to F. sambucinum.
7. 18 antagonists (12 Pseudomonas sp., 4 Enterobacter sp., and 2 Pantoea sp.) were identified and selected by routine tests.
Development of Biological Control Agents:
Bio-control agents normally possess several of the following characteristics:
1. Ability to associate sufficiently closely with the plant to exert an effect on the pathogen, i.e., to be phyllosphere or rhizosphere competent.
2. Ability to compete with the pathogen for nutrients or niches, e.g. infection courts.
3. Production of antibiotic compounds.
4. Production of lytic enzymes effective against the pathogen.
5. Ability to parasitize the pathogen.
6. Ability to interfere with the reproduction of the pathogen.
7. Ability to interfere with the virulence mechanisms of the pathogen.
8. The induction of host defence mechanisms.
Since no one BCA all above mentioned characteristics so it is often advantageous to combine BCAs that exist control by different mechanisms, for instance, it has been found that a combination of Pythium nunn and Trichoderma harzianum mutant gave greater control of Pythium damping off of cucumber.
Similarly, it has been found that combinations of Trichoderma koningii with bacteria such as Pseudomonas chlororphis and P. fluorescens gave more effective disease control than either the fungus or the bacteria alone.
Woo and co-workers showed in 2002 that a combination of Trichoderma atroviride, which produced chitinase, and Psudomonas syringae pv. syringae, which produced lipodepsipeptides, rendered better post-harvest control of infection of apple fruit by Botrytis cinerea than either separately.
Delany and others (2001) genetically modified strain of Pseudomonas fluorescens and demonstrated that the GE-modified strains produced significant and substantial increases in the antibiotic compared with the wild-type strain and were as effective as a proprietary fungicide in controlling damping off of sugar-beet seedlings caused by Pythium ultimum.
Application and Establishment of BCAs:
Biological control agents provide effective control only when they are applied at the right place at the appropriate time in sufficient amount and successfully establish there. This programme is not so simple and faces major difficulty. Application as seed dressing is very attractive but it is necessary for the shelf-life of the BCA to be sufficiently long and perfectly to match that of the seed itself.
Development of the BCA plays important role here. In the initial field trials of Bacillus cereus UW85, the bacterium was added to alfalfa seeds in one percent methylcellulose, but later it was found that clay-granule formulations of this BCA applied in furrow provided the most consistent results.
Environmental impact, as another aspect, that needs care before the widespread application of any biological control agent. Gullino and co-workers (1995) applied the strains of Fusarium oxysporum isolated from rhizospheres of carnation grown in a Fusarium suppressive soil against wilt diseases in many plant species.
This BCA was developed because the strains were potentially excellent antagonists to wilt causing pathogen. However, the results of application of BCA were discouraging because the populations of introduced strains of BCA generally decreased after 6-10 weeks and then remained stable. The BCA could not affect population dynamics of other microorganisms and no transfer of genetic material was obtained as the BCA could not reproduce sexually in this changed environment.
Commercially Available BCAs:
Hundreds, possibly thousands, of biological control agents have been demonstrated to reduce disease severity in the laboratory, greenhouse, or field. Many of them have shown very promising effect in their experimental trials, but have yet to make a significant impact in the market place. To date, however, only eight BCAs have been registered and are commercially available for use. Four of them are fungi and four of them are bacteria.
Although the actual use of commercially available BCAs in rather limited, it is expected that these and other such products will find wide acceptance in the not too distant future.
The main constraints in the slow progress in use of these products are the apprehension of the fanning community (users) about both its efficacy and safety. It is definite and has been emphasized time and again that the future of plant disease management cannot rest on the shoulders of hazardous chemicals and therefore biological control has to play significant role in days to come.
Constraints of BCAs:
There are many constraints with respect to biological control and some important ones are the following:
1. One of the major difficulties is the application of BCAs in getting them to the right place at the right time in sufficient density to be effective and then maintaining them there. For example, use of BCAs as seed dressings is very attractive but it is necessary for the self- life of the BCA to be sufficiently long to be practicable and preferably to matches that of the seed itself. In initial field trials of Bacillus cereus UW85, the bacterium was added to alfalfa seeds in 1% methylcellulose but later, it was found that clay-granule formulations applied in-furrow provided the most consistent results.
2. Other difficulty is the apprehension of the growers about the efficacy of BCAs. For convenience, Trichoderma harzianum was formulated by a Japanese company in the form of dry powder spore and mycelia in rice bran and was named as ‘Trichoderma Powder’.
The latter is still available in Japanese market and is used by growers but, as per estimate, only 0.2 ton of formulation was produced by the company in 1994. Similarly, only 0.6 ton of the formulation of Agrobacterium radiobacter was produced in 1992 for control of crown gall.
However, these market potentials stand nowhere in comparison to fungicide market. This may have been due to the fact that the grower’s confidence in such new products remains very less in-spite of the fact that over 35% control of the disease was found under field conditions and no adverse effect of BCAs was observed on plants.
3. Safety is not least important. A bio-control agent may be pathogenic to humans and other members of the biota. For instance, bacteria belonging to the complex Burkholderia cepacia are used as bio-control agents and in bioremediation but some strains are plant pathogens and others are opportunistic pathogens of humans with cystic fibrosis.
4. Industries have to play a major role for developing a marketable product of BCAs but, most of them are showing least interest probably because high expenditure in comparison to low profit. This can overcome only if the government institutions bear main responsibility to make this novel approach available by providing subsidies to producers as well as convince the growers to use BCA products.
Future of BCAs:
In some respects the record of the formulations of BCAs and their practical applications has been disappointing. It is evident from the assessment that even after more than half a century of intensive research, only few commercial products have reached the market place and much of the early promise has not been fulfilled in practical terms as the BCAs have not been able to replace or minimize the use of fungicides and bactericides in crop production system.
Therefore, to make the future of this novel approach secure, there needs to be a more subtle approach in which BCAs are seen as part of an integrated strategy for disease management. There are indications that mixtures of BCAs with different modes of action are more effective in comparison to the application of a single type of BCA. Such mixtures can combine strains that show variations in environmental tolerance thereby extending the range of conditions under which the formulations will work.
Further, the traditional concept of biological control (i.e., use of either a specific antagonist or their combination directly) is now being superceded by a wider vision. Scientists are turning towards discovering new classes of chemical control agents, and biologically active molecules produced by BCAs are an initiating point for natural product chemistry.
Similarly, it may be possible to produce transgenic plants, which may express novel defence compounds originating from biological control agents. Success in obtaining an insecticidal toxin from Bacillus thuringiensis is an encouraging result. It is now emerging on the screen that disease management devices based separately on chemical, biological, and genetic approaches are converging, and integration of these approaches will be practically used to manage diseases of plants in near future.
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