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5 Chemical management


I welcome you to the last topic of this week and that is Chemical Management of Plant Diseases. In the past lectures we have seen the methods of cultural management, biological management and host resistance through which we are able to suppress plant pathogens. But here we will see how chemicals are applied for management of plant diseases and what are the different types of chemicals that are applied from plant disease management. So a variety of chemicals that are applied in terms of their role to protect the plants from invading pathogens. Among them the first one is a Eradicants, the other one is Protectants and the last one is Therapeutic chemicals.

Eradicants are those chemicals which are used either to treat the soil or they are used to treat the seeds, bulbs, corms as a planting material because by treating them we can eradicate the already present inoculum of the pathogen either on its surface or internally. Protectants basically they create a toxic layer between the plant surface and the microbial pathogen. So they protect the plant by creating a chemical barrier between the arriving pathogen inoculum.

Therapeutic chemicals are used to treat the already infected plants or plant parts and thereby cured of plants from the further progressing of the diseases. Soil treatments are designed basically to kill soil innovating nematodes, fungi and bacteria. This eradication can be accomplished by using steam or chemical fumigants. So it is a tactic to eradicate pathogenic microbial species from the soil either through steam or chemical fumigants.

Fungicides are broadly grouped into two groups they are Protective and Curative. Protective fungicides may or may not get absorbed into the plant system. At the same time they may or may not get translocated. So they may be only remain in the surface of the plant part where it is applied. But they prevent infection from sporulation and they are used before infection. So Strobilurin group of fungicides along with Chlorothalonil these are known as protective fungicides and they are able to prevent infection or sporulation of a pathogenic fungus. Then Curative fungicides are mostly absorbed by the plant system and they are also able to translocate within the plant tissues. They kill fungal tissues and use normally after infection. So once the infection takes place then we use this type of chemicals to protect or to cure the disease plants. So Triazoles are the fungicides group of fungicides that I use as a protectant or the curative fungicide for plant disease management.

Eradicants as we mentioned earlier are treated for the seeds, bulbs, corms and tubers to eradicate the surface present microbial pathogens present on the surface of these plant parts so that they can eradicate this microbial pathogens from causing further disease. Say for example this damping-off is a pathogen disease which is caused by several species of fungal pathogens and the eradicants help them to lower down the disease like damping off. So Protectants fungicides here you can see that it creates a chemical layer between the plant and the pathogen and this layer is in between the plant and the pathogen and that is how they are able to check the pathogen from invading the plant tissues. So these protectants are also referred to as contact fungicides which help the plants to protect itself from invading pathogens.

At the same time Systemic fungicides are those fungicides which are able to absorb, which are absorbed by the plant system along with they are translocated within the plant tissues and they are able to check the growth or development of the pathogen at various stages right at the surface level or at various level of infection that is progressed by the development of the microbial pathogen. So system if fungicide is basically a very very good fungicide to cure plant from already established infections. So Antibiotics are nothing but these are chemical compounds mostly microbial origin used to suppress bacterial infection, say for example Streptomycin. It is very effective against fruit pathogens such as blights and cankers and it is used to cure plants from this particular bacterial diseases. And at the same time cyclohexamine can also be used to control certain fungal pathogens of crops particularly powdery mildews and rust. So these are certain antibiotics of microbial origin and they are used to control either bacterial pathogens or certain fungal pathogens in different crop species.

At the same time the Nematicides are those chemicals which are used to suppress the or which are used to kill the nematodes that are parasitic to plants. So nematicides they are basically applied as granules or as fumigants in the soil to suppress the nematode population or even to kill the nematodes that are parasitic to certain crop species.

So there are different Methods of application of fungicides or agricultural pesticides. One is Soil fumigation where this method can be used either in poly houses or under natural field condition where you can see that the soil is covered with a polythene sheet and that is how the once the chemical is applied to the soil and then covered with a polythene sheet then the soil volatile compounds that are released from the chemical is not getting released to the atmosphere immediately rather it is confined to that environment through the cover of the polythene sheet. So this is very very effective in management of certain soil borne pathogens. Then the next method is Soil drenching where the chemical is mixed with the water or mostly it is water and the soil where the pathogen is present is drenched by pouring this fungicide mostly it is a practice that is done in fruit orchards where the big fruit trees are under threat of certain pathogens then this drenching method could be developed or could be applied into those plants where the root zone is drenched with this chemical to protect them from the soil borne pathogens.

Drip application is another method where we can regulate the application level of the fungicide, where water scarcity is there. So sometimes some of the pathogens they don’t require good amount of moisture leads, more disease development in those pathogens where that is where we don’t drench the soil with chemicals rather we don’t fully irrigate those plants, rather what we use is that drip method of application of fungicides where regulated release of water along with the fungicide will be there to treat the desired pathogen. So here you can see that this is a ring method of drip application and these are pipes that are laid down along with the crops where the regulated or required amount of fungicide is released at a regulated manner. Next one is seed pressing or seed treatment. So when the seeds have to be treat with certain powder form of fungicides or dust fungicides which are present in dust form. So we mix both them the fungicide and the seeds at a desired ratio in a drum and then rotate the drum by using the handle and thereby the seeds are coated with the powdered fungicides and the seeds get protected thereafter. The same may be done in a small scale if it is in a small scale by mixing the fungicide with the seeds through hand. The same soil dressing can be done by putting the fungicide in a slurry or suspension and then application of seeds are done into that tub and the seeds get coated by this slurry or suspension and thereby the seed surface is protected from invading pathogens.

There are different Spray techniques that are also used to control several pathogens depending on the severity or the requirement The spray may be a hand sprayer a small that may be used to manage the pathogen and if the field is small then a single man can have the spray on the desired areas, if the fields are large then we can have mechanized spring and this is a method of spraying on fruit orchards where the application of the fungicide is not in the soil rather on the foliage and if the fields are very large then Aerial method of application of fungicide may also be adopted. Then another method is Seedling dip method where the seedling roots are dipped in fungicidal solution before they are transplanted to the main field from the nursery beds. So here again the seedlings were first they are dipped in the solution of fungicides and then ready for transportation for in the main field. So this is another method that is known as seedling deep method and then another method is known as Seed bed treatment where the only the seed beds were treated with the fungicides and it is protected because the pathogen may be present only in limited areas and that is how this particular method helps us to reduce the disease of nurseries by application of the fungicide on the seed beds. This also help us to reduce application cost of the expensive chemicals by not spraying to the other areas where crop is not taken.

So altogether there are different types of chemicals mostly they are eradicants, protectants and therapeutic and they may be either protective or curative and these chemicals have different mechanisms of inhibiting or suppressing the plant pathogens at the same time there are different methods of application of these fungicidal compounds depending on their requirement, the type of crops that are going to be taken and the type of pathogens whether it survives in soil or it comes through air or wind or it is released through rain splashes. So considering all these things we have to adopt a suitable fungicide and a method of application for successful management of plant diseases.

Thank you.



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