Herbicides and Fungicides
Introduction
Weeds and plant diseases are among the most serious biological constraints in agricultural production. Weeds compete with crop plants for essential resources such as nutrients, water, sunlight, and space, leading to significant yield reduction. Similarly, fungal diseases infect crops and cause severe losses in yield and quality of agricultural produce.
To overcome these problems, farmers rely on chemical control methods, particularly herbicides and fungicides. Herbicides are chemicals used to control weeds, whereas fungicides are chemicals used to prevent or destroy fungal pathogens that cause plant diseases. These agrochemicals play a vital role in modern agriculture by improving crop productivity and ensuring stable food production.
However, their improper or excessive use may lead to environmental pollution, development of resistance in weeds and pathogens, and health hazards. Therefore, understanding their classification, properties, mode of action, and environmental fate is essential for their safe and effective use.
Herbicides
Definition of Herbicides
Herbicides are chemical substances used to destroy or inhibit the growth of unwanted plants known as weeds. These chemicals are applied to soil or foliage to control weeds that compete with crops for resources and reduce agricultural productivity.
Weed infestation can reduce crop yield by 20–80 percent depending on crop type, weed species, and environmental conditions. Herbicides provide an efficient and economical method of weed management compared with manual or mechanical control.
Major Classes of Herbicides
1. Classification Based on Selectivity
Selective Herbicides
Selective herbicides kill certain species of weeds without damaging the crop plants. This selectivity may be due to differences in plant morphology, physiology, or metabolic processes between crop and weed species.
Examples include:
- 2,4-D – controls broad-leaved weeds in cereal crops
- Atrazine – controls weeds in maize and sorghum
- Pendimethalin – controls grasses and broad-leaved weeds
Non-selective Herbicides
Non-selective herbicides destroy all types of vegetation regardless of species. They are mainly used in non-crop areas such as roadsides, railway tracks, industrial sites, and before planting crops.
Examples include:
- Glyphosate
- Paraquat
2. Classification Based on Time of Application
Pre-plant Herbicides
These herbicides are applied before the crop is planted. They are usually incorporated into the soil to control weeds before crop emergence.
Pre-emergence Herbicides
These herbicides are applied after sowing but before the emergence of weeds and crop seedlings. They prevent weed seeds from germinating or emerging.
Examples: Atrazine, Pendimethalin.
Post-emergence Herbicides
These herbicides are applied after weeds have emerged above the soil surface.
Examples: 2,4-D and Glyphosate.
3. Classification Based on Mode of Action
Contact Herbicides
Contact herbicides kill only the plant tissues that come into direct contact with the chemical. They are fast acting but may not kill underground plant parts.
Example: Paraquat.
Systemic Herbicides
Systemic herbicides are absorbed by plants and transported through vascular tissues to different plant parts, including roots and shoots. They provide more effective weed control.
Example: Glyphosate.
Properties of Herbicides
An ideal herbicide should possess several desirable characteristics to ensure effectiveness and safety.
- High toxicity to weeds but low toxicity to crops
- High selectivity
- Effective at low concentrations
- Low persistence in the environment
- Minimal harmful effects on soil microorganisms
- Non-toxic to humans and animals
- Economical and easy to apply
Important Herbicides
2,4-D (2,4-Dichlorophenoxyacetic Acid)
2,4-D is one of the most widely used selective herbicides for controlling broad-leaved weeds in cereal crops such as wheat and rice. It acts as a synthetic auxin, causing uncontrolled growth in weeds which eventually leads to plant death.
Atrazine
Atrazine is a selective herbicide used mainly in maize and sorghum crops. It inhibits photosynthesis in susceptible plants by blocking electron transport in chloroplasts.
Glyphosate
Glyphosate is a non-selective systemic herbicide widely used for controlling perennial and annual weeds. It inhibits the synthesis of aromatic amino acids by blocking the shikimic acid pathway.
Pendimethalin
Pendimethalin is a pre-emergence herbicide that inhibits cell division and root growth in germinating weed seeds.
Fate of Herbicides in the Environment
After application, herbicides undergo various processes in the environment. These processes determine their persistence, movement, and impact on soil and water.
Absorption
Herbicides may be absorbed by plant tissues, soil particles, and soil microorganisms.
Adsorption
Herbicide molecules may attach to soil particles such as clay and organic matter. Adsorption reduces herbicide mobility.
Leaching
Some herbicides move downward through the soil with water and may contaminate groundwater.
Volatilization
Certain herbicides evaporate into the atmosphere after application.
Runoff
Herbicides may be transported by surface runoff into nearby water bodies.
Microbial Degradation
Soil microorganisms can break down herbicides into simpler compounds.
Chemical Decomposition
Herbicides may also degrade through chemical reactions such as hydrolysis and oxidation.
Fungicides
Definition
Fungicides are chemical substances used to prevent, destroy, or control fungi that cause plant diseases. These chemicals help protect crops from fungal infections that can severely reduce yield and quality.
Classification of Fungicides
1. Based on Chemical Nature
- Inorganic fungicides
- Organic fungicides
2. Based on Mode of Action
- Protectant (contact) fungicides
- Systemic fungicides
3. Based on Application Method
- Seed treatment fungicides
- Foliar fungicides
- Soil fungicides
Inorganic Fungicides
Inorganic fungicides are among the earliest chemicals used for plant disease control. These fungicides mainly consist of sulfur and copper compounds.
Characteristics of Inorganic Fungicides
- Broad-spectrum activity against many fungal pathogens
- Low cost and easy availability
- Mainly act as protective fungicides
- Long history of safe use
- Low risk of resistance development
Sulfur Fungicides
Characteristics
Sulfur is one of the oldest fungicides used in agriculture. It is highly effective against powdery mildew diseases and certain mites.
- Low toxicity to plants and animals
- Effective against powdery mildew
- Acts mainly as a contact fungicide
Preparation
Sulfur fungicides are prepared by grinding elemental sulfur into fine powder or by producing wettable sulfur formulations.
Uses
- Control of powdery mildew in grapes
- Control of fungal diseases in cucurbits
- Protection of fruits and vegetables
Copper Fungicides
Copper compounds are widely used fungicides for controlling both fungal and bacterial diseases.
Common Copper Fungicides
- Bordeaux mixture
- Copper oxychloride
- Copper hydroxide
Bordeaux Mixture
Composition
Bordeaux mixture is composed of copper sulfate, lime, and water.
Preparation
It is prepared by dissolving copper sulfate in water and mixing it with lime suspension. The lime neutralizes the acidity of copper sulfate and prevents plant injury.
Mode of Action
Bordeaux mixture acts as a protective fungicide. It releases copper ions which interfere with fungal enzyme systems and inhibit spore germination.
Uses
- Control of downy mildew
- Control of leaf spot diseases
- Protection of fruit crops such as grapes and mango
Copper Oxychloride
Characteristics
Copper oxychloride is a widely used copper-based fungicide with strong protective action.
Mode of Action
It releases copper ions which disrupt enzyme activity in fungal cells, preventing fungal growth and spore germination.
Uses
- Control of blight diseases
- Control of leaf spot diseases
- Control of bacterial diseases in crops.
Organic Fungicides:
Introduction
Fungal diseases are among the most destructive constraints in crop production, causing severe yield losses and deterioration in quality of agricultural produce. Pathogens such as fungi infect leaves, stems, roots, fruits, and seeds, leading to diseases like blight, rust, smut, mildew, and leaf spots.
To manage these diseases effectively, fungicides are widely used. Among them, organic fungicides represent a major advancement in plant protection chemistry. These compounds are carbon-based and are generally more efficient, target-specific, and safer compared to traditional inorganic fungicides.
Organic fungicides are widely used in modern agriculture due to their improved efficacy, reduced phytotoxicity, and compatibility with integrated disease management practices.
Definition of Organic Fungicides
Organic fungicides are chemical compounds containing carbon that are used to prevent, inhibit, or destroy fungal pathogens affecting crops. These fungicides may act as protective (preventive), curative, or systemic agents depending on their nature.
Classification of Organic Fungicides
Organic fungicides are classified based on their chemical structure and mode of action:
- Dithiocarbamates (Zineb, Maneb, Mancozeb)
- Phthalimides (Captan)
- Benzimidazoles (Carbendazim)
- Triazoles (Propiconazole)
- Strobilurins (Azoxystrobin)
Among these, dithiocarbamates are the most widely used group of contact fungicides.
Mode of Action of Organic Fungicides
The mode of action of fungicides refers to the biochemical and physiological processes through which they inhibit fungal growth or kill fungal cells.
General Mechanisms
- Inhibition of enzyme activity
- Disruption of cell membrane permeability
- Inhibition of respiration
- Blocking of spore germination
- Interference with nucleic acid and protein synthesis
Flowchart: Mode of Action of Fungicides
Application of Fungicide
↓
Deposition on Plant Surface
↓
Contact with Fungal Spores / Mycelium
↓
Penetration into Fungal Cell (in some cases)
↓
Biochemical Interference:
→ Inhibition of enzymes (–SH group blockage)
→ Disruption of respiration
→ Damage to cell membrane
→ Inhibition of nucleic acid synthesis
↓
Inhibition of Spore Germination
↓
Arrest of Mycelial Growth
↓
Death or Suppression of Fungus
Dithiocarbamates
Introduction
Dithiocarbamates are an important group of organic fungicides derived from dithiocarbamic acid. They are widely used as protectant fungicides in agriculture and horticulture.
These fungicides are non-systemic and act on the surface of plant tissues, preventing fungal spores from germinating and infecting the plant.
Chemical Nature
Dithiocarbamates are formed by the reaction of amines with carbon disulfide (CS₂), followed by combination with metal ions such as zinc or manganese.
General Characteristics
- Broad-spectrum activity against many fungi
- Protective (preventive) fungicides
- Non-systemic in action
- Multi-site mode of action
- Low risk of resistance development
- Low phytotoxicity
- Require repeated applications
Mode of Action of Dithiocarbamates
Dithiocarbamates act primarily by reacting with sulfhydryl (-SH) groups of fungal enzymes. These enzymes are essential for metabolic processes such as respiration and energy production.
By inactivating these enzymes, the fungicides disrupt multiple biochemical pathways, leading to inhibition of spore germination and fungal growth.
Since they act on multiple sites, fungi are less likely to develop resistance against them.
Zineb
Chemical Nature
Zineb is a zinc-containing dithiocarbamate fungicide, chemically known as zinc ethylene bis-dithiocarbamate.
Characteristics
- Non-systemic protectant fungicide
- Broad-spectrum activity
- Low phytotoxicity
- Effective against foliar diseases
- Acts on multiple biochemical sites
Preparation
Zineb is prepared by the reaction of ethylene diamine with carbon disulfide to form dithiocarbamate, which is then combined with zinc salts to produce the final compound.
Uses
- Control of leaf spot diseases
- Control of downy mildew
- Control of blight diseases
- Used in vegetables, fruits, and cereals
Maneb
Chemical Nature
Maneb is a manganese-containing dithiocarbamate fungicide, chemically known as manganese ethylene bis-dithiocarbamate.
Characteristics
- Protective fungicide
- Broad-spectrum activity
- Non-systemic in nature
- Low phytotoxicity
- Widely used in crop protection
Preparation
Maneb is prepared similarly to Zineb, but manganese salts are used instead of zinc salts during the synthesis process.
Uses
- Control of early and late blight of potato
- Control of leaf spot diseases
- Used in vegetables, fruits, and field crops
Comparison: Zineb vs Maneb
| Feature | Zineb | Maneb |
|---|---|---|
| Metal Component | Zinc | Manganese |
| Chemical Name | Zinc ethylene bis-dithiocarbamate | Manganese ethylene bis-dithiocarbamate |
| Nature | Protectant | Protectant |
| Systemicity | Non-systemic | Non-systemic |
| Uses | Leaf spot, mildew | Blight, leaf spot |
Advantages of Dithiocarbamates
- Wide spectrum of activity
- Low cost
- Low resistance development
- Safe to crops when used properly
Limitations
- Non-systemic (only preventive)
- Require repeated application
- Easily washed off by rain
- Less effective after infection has occurred
Systemic Fungicides and Insecticides
Introduction
Plant protection is an essential component of modern agriculture. Crop losses due to diseases and insect pests can range from 20% to 50% under severe conditions. To minimize these losses, chemical control using fungicides and insecticides has become a vital tool.
Among fungicides, systemic fungicides represent a major advancement over traditional contact fungicides. Similarly, insecticides have evolved from simple inorganic compounds to highly sophisticated organic and biorational pesticides.
Systemic Fungicides
Definition
Systemic fungicides are chemical substances that are absorbed by plants and translocated through vascular tissues (xylem and/or phloem) to protect plants from fungal pathogens. These fungicides can act both as preventive and curative agents.
Characteristics of Systemic Fungicides
- Absorbed by plant tissues
- Translocated within plant system
- Provide internal protection
- Effective at low concentrations
- Have curative and eradicative action
- Target-specific (single-site action)
- Higher risk of resistance development
Mode of Action
- Inhibition of cell division (mitosis)
- Inhibition of nucleic acid synthesis
- Inhibition of sterol biosynthesis
- Interference with respiration
Important Systemic Fungicides
1. Benomyl
Chemical Nature
Benomyl belongs to the benzimidazole group of fungicides.
Mode of Action
It inhibits fungal cell division by binding to tubulin and preventing spindle formation during mitosis.
Characteristics
- Systemic fungicide
- Curative and protective action
- Highly effective against many fungi
- Single-site mode of action
Uses
- Control of powdery mildew
- Control of leaf spots
- Control of fruit rots
2. Carbendazim
Chemical Nature
Carbendazim is also a benzimidazole fungicide and a metabolite of benomyl.
Mode of Action
It inhibits mitosis by interfering with microtubule formation.
Characteristics
- Systemic and curative
- Highly effective at low doses
- Long residual action
Uses
- Control of leaf spots and blights
- Seed treatment fungicide
- Control of wilts and rots
3. Carboxin
Chemical Nature
Carboxin is an oxathiin fungicide.
Mode of Action
It inhibits mitochondrial respiration by blocking succinate dehydrogenase enzyme.
Characteristics
- Systemic fungicide
- Effective as seed treatment
- Specific against smut and rust diseases
Uses
- Control of loose smut of wheat
- Control of bunt diseases
4. Oxycarboxin
Chemical Nature
Oxycarboxin is a derivative of carboxin with similar properties.
Mode of Action
It inhibits fungal respiration.
Characteristics
- Systemic fungicide
- Effective against rust diseases
- Long-lasting effect
Uses
- Control of rust diseases in crops
- Seed and foliar treatment
5. Metalaxyl
Chemical Nature
Metalaxyl belongs to the phenylamide group.
Mode of Action
It inhibits RNA synthesis in fungal cells, especially in oomycetes.
Characteristics
- Highly systemic
- Curative and protective
- Specific against downy mildew and late blight
Uses
- Control of late blight of potato
- Control of downy mildew in crops
Insecticides
Introduction
Insect pests cause significant damage to crops by feeding on plant tissues and transmitting diseases. Insecticides are chemical substances used to kill or control insect pests and protect crops.
Classification of Insecticides
1. Inorganic Insecticides
These are mineral-based insecticides used in earlier agriculture.
- Arsenic compounds
- Sulfur compounds
- Fluorine compounds
These are now rarely used due to high toxicity.
2. Organic Insecticides
These are carbon-based compounds widely used in modern agriculture.
Major Classes of Organic Insecticides
1. Organochlorine Insecticides
Examples
- DDT
- BHC
Characteristics
- Persistent in environment
- Bioaccumulate in food chain
- Broad-spectrum
Mode of Action
Affect nervous system by altering sodium ion transport.
2. Organophosphates
Examples
- Malathion
- Chlorpyrifos
Characteristics
- Highly toxic
- Less persistent than organochlorines
- Widely used
Mode of Action
Inhibit acetylcholinesterase enzyme, causing nerve impulse disruption.
3. Carbamates
Examples
- Carbaryl
- Carbofuran
Characteristics
- Moderate toxicity
- Short residual effect
Mode of Action
Inhibit acetylcholinesterase enzyme (reversible inhibition).
4. Synthetic Pyrethroids
Examples
- Cypermethrin
- Deltamethrin
Characteristics
- Highly effective at low doses
- Low mammalian toxicity
- Fast knockdown effect
Mode of Action
Affect sodium channels in nerve cells causing paralysis.
5. Neonicotinoids
Examples
- Imidacloprid
- Thiamethoxam
Characteristics
- Systemic insecticides
- Effective against sucking pests
- Long residual effect
Mode of Action
Act on nicotinic acetylcholine receptors causing paralysis and death.
Biorational Insecticides
Definition
Biorational insecticides are eco-friendly pesticides that are less harmful to non-target organisms and environment.
Examples
- Neem-based insecticides (Azadirachtin)
- Bacillus thuringiensis (Bt)
- Insect growth regulators (IGRs)
- Pheromones
Characteristics
- Target-specific
- Biodegradable
- Safe to beneficial organisms
- Compatible with IPM
Insecticides
Introduction
Insect pests cause significant losses in agricultural production by damaging crops directly and indirectly through transmission of diseases. To manage these pests effectively, insecticides are widely used. However, improper use of insecticides can lead to environmental pollution, pest resistance, and health hazards. Therefore, strict regulatory frameworks and sustainable approaches are essential for their safe use.
This chapter covers the legal aspects of insecticide use, their environmental fate, and modern developments such as insect growth regulators, biopesticides, reduced-risk insecticides, and botanicals.
Insecticides Act and Rules
Insecticides Act, 1968
The Insecticides Act, 1968 is an important legislation enacted by the Government of India to regulate the import, manufacture, sale, transport, distribution, and use of insecticides in order to prevent risks to human beings and animals.
Objectives of the Act
- To ensure the availability of quality insecticides
- To prevent misuse and overuse of toxic chemicals
- To regulate manufacture, sale, and distribution
- To protect human and animal health
Key Provisions
- Registration of insecticides before sale
- Licensing of manufacturers, dealers, and distributors
- Quality control through inspection and sampling
- Labeling and packaging requirements
- Penalties for violation
Central Insecticides Board (CIB)
The Central Insecticides Board advises the government on technical matters related to insecticides, including safety, toxicity, and usage guidelines.
Registration Committee (RC)
The Registration Committee registers insecticides after evaluating their efficacy and safety. No insecticide can be sold without registration.
Insecticides Rules, 1971
The Insecticides Rules, 1971 were framed under the Act to provide detailed procedures for implementation.
Important Provisions
- Licensing procedures
- Qualifications for dealers
- Storage and transport guidelines
- Labeling specifications (toxicity color codes)
- Safety precautions and handling guidelines
Insecticides: Banned, Withdrawn and Restricted Use
Banned Insecticides
These are insecticides that are completely prohibited due to their high toxicity, persistence, and environmental hazards.
- DDT (restricted mainly for public health use)
- BHC (Lindane)
- Aldrin
- Endrin
- Chlordane
Withdrawn Insecticides
These insecticides are temporarily removed from use due to safety concerns or lack of data.
- Some formulations of Carbaryl
- Other compounds withdrawn pending review
Restricted Use Insecticides
These insecticides are allowed but under specific conditions such as limited crops, trained applicators, or specific doses.
- Monocrotophos (restricted due to toxicity)
- Methyl parathion
- Aluminum phosphide
Fate of Insecticides in Soil and Plant
Introduction
After application, insecticides undergo various physical, chemical, and biological processes in soil and plants. These processes determine their persistence, movement, and impact on the environment.
Fate in Soil
1. Adsorption
Insecticides bind to soil particles such as clay and organic matter, reducing their mobility.
2. Leaching
Water-soluble insecticides may move downward through soil and contaminate groundwater.
3. Volatilization
Some insecticides evaporate into the atmosphere.
4. Chemical Degradation
Insecticides break down through hydrolysis, oxidation, and photodegradation.
5. Microbial Degradation
Soil microorganisms degrade insecticides into simpler compounds.
6. Runoff
Insecticides may be carried to water bodies through surface runoff.
Fate in Plants
1. Absorption
Systemic insecticides are absorbed through roots or leaves.
2. Translocation
Movement occurs through xylem and phloem to different plant parts.
3. Metabolism
Plants convert insecticides into less toxic or non-toxic compounds.
4. Residue Accumulation
Some insecticides remain as residues in plant tissues, posing food safety concerns.
Insect Growth Regulators (IGRs)
Definition
IGRs are chemicals that interfere with the normal growth and development of insects rather than killing them directly.
Types of IGRs
- Juvenile hormone analogues
- Chitin synthesis inhibitors
- Ecdysone inhibitors
Mode of Action
IGRs disrupt molting, metamorphosis, and reproduction in insects.
Examples
- Methoprene
- Diflubenzuron
Biopesticides
Definition
Biopesticides are pest control agents derived from natural materials such as plants, microorganisms, and animals.
Types
- Microbial pesticides (Bacillus thuringiensis)
- Botanical pesticides (Neem)
- Biochemical pesticides
Advantages
- Eco-friendly
- Specific to target pests
- Safe to humans and animals
Reduced Risk Insecticides
Definition
Reduced-risk insecticides are chemicals designed to minimize risks to human health, non-target organisms, and the environment.
Characteristics
- Low toxicity
- Short persistence
- Target specificity
- Compatibility with IPM
Examples
- Spinosad
- Imidacloprid
Botanicals
Definition
Botanicals are insecticides derived from plant sources.
Examples
- Neem (Azadirachtin)
- Pyrethrum
- Nicotine
Characteristics
- Biodegradable
- Low toxicity
- Environment-friendly
Systemic Insecticides
Definition
Systemic insecticides are chemicals absorbed by plants and transported to different parts, making the plant toxic to insects.
Types
Plant Systemic Insecticides
- Absorbed and translocated within plants
- Examples: Imidacloprid, Dimethoate
Animal Systemic Insecticides
These are used in veterinary science where the chemical is administered to animals to kill parasites feeding on them.
- Examples: Ivermectin
Characteristics
- Effective against sucking pests
- Provide long-lasting protection
- Less affected by environmental conditions
Uses
- Control of aphids, whiteflies, and jassids
- Protection of crops from internal feeders
