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3 Biological management


Welcome back we are going to talk about biological management as a component of plant disease management in this particular talk today. Last time we have talked about cultural practices and cultural practices along with biological practices they are very very essential for management of plant diseases in a sustainable manner. So biological management can be talked or biological management can be described as the use of an antagonist or a living microorganism to suppress another microbial pathogens that cause disease in a plant is known as biological control. So biological control is mostly the term that is used in plant pathology and entomology. The agent that suppress the pathogenic ones is known as bio control agent the types of interaction contributing to biological control include it help to appreciate different ways that organisms interact and that in order to interact organisms must have some form of direct or indirect contact the types of interactions that are referred to as competition, parasitism, antagonism and induce systemic resistance. So whether the pathogen come in direct contact with the bio control agent or the antagonist may reduce or suppress the pathogen without coming directly in contact with it is the basic method of biological control, but the interaction may be defined in terms of competition, parasitism, antagonism and induce systemic resistance. Let us see what are these different terms. So when we talk about antagonism we normally apply the biocontrol agent as a seed treatment along with the sowing time and once the seed germinates and it starts developing then the biocontrol agent they colonize the plant root and the other root pathogens that they come across to the root zone then the biocontrol agents which have the ability to produce some antibiotic compounds or antimicrobial compounds these compounds are released in the root zone and they suppress the pathogenic ones through this antimicrobial compounds. This is what we call it as Antagonism. Second one is Induced Systemic Resistance. Here again the delivery method is same when we apply the biocontrol agent as seed treatment once the splash sheet starts germinating and plants start growing then the biocontrol agent they come across the plant in root system and then plant root then the sense certain components of the biocontrol agent, mostly they are surface components like polysaccharides or you can say certain proteins on the plant surface. When plants sense then there is a signal that is generated at the point of interaction between the biocontrol agent and the host and there is a systemic signal that generates at this interaction point and it migrates to all part of the plant right to the tip of the leaf to the tip of the root. So this particular interaction then stimulates certain defense genes in the plant and this enhancement of expression of defense genes in the host plant leads to suppression of the pathogens that may come and attack in a later stage. So we can say that the plant is already enhanced these defense Arsenal’s and when the pathogen arrived the plant has already the defense strategy to suppress the pathogen attack. The third one is competition for nutrients and niches. That means if a bio control agent is able to compete well with the pathogenic microbes in terms of nutrient acquisition and space that particularly in the root surface then if the biocontrol reason is a better competitive ability then it can suppress the pathogenic ones. Say for example certain biocontrol agents can acquire nutrients under limiting condition and thereby once it acquires that minimum available of these nutrients then they make the pathogens deprived of this particular nutrient. So once the pathogen is deprived of this particular nutrients their growth is suppressed. So indirectly without coming directly in contact with the pathogen now microbial biocontrol agents can suppress the disease by competing well with nutrients and space for their establishment. Let us see the mechanisms of bio control agent into great detail. So there is direct antagonism which we call it as hyper parasitism. Here the first case is an example of Trichoderma harzianum which is hyperparasitism on the mycelium of Rhizoctonia solani. Here you can see this is the Trichoderma mycelium and the blue one is the Rhizoctonia mycelium. So the Rhizoctonia mycelium is surrounded by Trichoderma growth and Trichoderma releases certain cell wall degrading enzymes like chitinase and thereby it degrades the mycelium of Rhizoctonia and utilizes its nutrient by Trichoderma for its own growth and development. Next one is Arthrobptrys oligospora this is a nematode trapping fungus, it forms a net-like structure and when the nematode come in contact with this net like structures they get trapped in this particular network. So Arthrobptrys thus trap the nematode and they parasitize on it. So this is the direct interaction of the biocontrol agent and its pathogens that is mostly through hyperparasitism.

Mixed pathogen antagonism – here you can see that this is a bacterial antagonist and it is releasing certain antibiotics, lytic enzymes, unregulated waste products and these compounds ultimately have a effect on the growth of the pathogenic fungus and here you can see that the growth is restricted at this level. So this is a Mixed pathogen antagonism where the pathogen or biocontrol agent antagonizes the pathogen by application or by secreting different types of antibiotics, lytic enzymes and other related products. Indirect antagonism that is – competition we can take an example of fluorescent pseudomonas mostly pseudomonas fluorescens where some of them produce Siderophore and this Siderophore are basically have a capacity to absorb the ferric form of iron. So under iron limiting condition Siderophore can trap the minimum available form of iron in the soil and when they migrate to the plant system they carry this iron to the plants indirectly and at the same time they compete with the pathogenic microbes for this particular iron and they deprived the parasitic microbes for this iron and indirectly they suppress their growth and reduce the disease development potential of the pathogenic microbes. Next one is Induction of host resistance. This is another mechanism of biological control where there are two concepts are available for induction of host resistance. First one we call it as Systemic acquired resistance and the other one is known as Induced systemic resistance. Although the outcome is same that is the plant is defense is mechanism is enhanced and the disease causing ability of the pathogen is reduced but the mechanism of induction of both this type of resistance is different. In the case of systemic acquired resistance normally when a mild strain of a pathogenic fungus causes a necrotic lesion in a particular leaf then a systemic signal generate at this particular point and this is mostly salicylic acid and this salicylic acid then migrate to other parts of the plant and then go and cause induction of certain defense genes that are normally responsible for broad host resistance against a different types of pathogens. At the same time when induced systemic resistance takes place it is mostly through non-pathogenic microbes maybe certain fungi or bacteria. When plant sends some of the components of these microbes then a signal is generated in the interacting zone and these signals are either just monic acid or ethylene and this also migrate to other parts of the plant and then the whole plant get resistant towards a broad range of microbial pathogens and the same way the defense mechanism is activated in the plant and then this enhanced defense expression of the defense genes lead to suppression or it can avoid the plant from getting infected by the invading pathogen in a later stage. So these are two different concepts but the ultimatle outcome is the same and the mechanisms are different and the inducers by losing microbes are also different. So whatever the mechanisms it may be monic acid or ethylene induction are normally taken place through Pseudomonas fluorescens components Pseudomonas P.putida components Bacillus amyloliquefaciens components and Bacillus cereus and there are a number of bacterial microbe has been identified which are non pathogenic to plants and they have the potentiality to induce systemic resistance and thereby leading to enhancement in defense mechanisms. So these microbial organisms can be formulated as a product and then that they can be applied to agriculture for enhancement of defense mechanisms in the plants and that is how we can achieve biological control by application of different types of microbes against a variety of pathogens and this is a very very environmentally friendly approach along with cultural practices. These two can be combined to have a good control over different plant pathogens for management of several severe diseases.

Thank you.



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