Unit 6: Defense mechanism and plant disease management

1. Defense Mechanisms in Plants

Plants interact with pathogens in a complex environment. Defense mechanisms are broadly categorized into two types: Passive (Constitutive) defense present before infection, and Active (Induced) defense activated upon pathogen attack.

A. Pre-existing (Passive) Defense Mechanisms

These are the first line of defense, preventing the entry and establishment of the pathogen.

1. Structural Defense (Morphological Barriers)

• Waxes: The hydrophobic nature of the waxy cuticle prevents the formation of water films necessary for fungal spore germination and bacterial multiplication.
• Cuticle: A thick cuticle physically blocks direct penetration by fungi and acts as a barrier against mechanical injury.
• Epidermis: Tough, lignified epidermal cells resist penetration. E.g., Silicon accumulation in rice epidermis prevents blast disease.
• Stomata Structure: Some varieties possess stomata that are too narrow for bacterial entry or have guard cells that close rapidly in response to pathogen molecular patterns.
• Trichomes (Hairs): Dense hairs on leaves repel insect vectors and prevent water droplets from sitting on the leaf surface.

2. Biochemical Defense (Pre-existing Metabolites)

• Inhibitors released into the environment: Plants exude fungitoxic substances (e.g., phenolic compounds) from roots (root exudates) that inhibit soil-borne pathogens.
• Inhibitors present in cells:
  • Phenolics: Catechol and protocatechuic acid (found in red onion scales) prevent smudge fungus (Colletotrichum circinans).
  • Saponins: Glycosides like tomatine (in tomato) and avenacin (in oats) disrupt fungal cell membranes.
  • Tannins: Bind to proteins and inhibit pathogen enzymes.

B. Induced (Active) Defense Mechanisms

These are triggered only after the pathogen comes into contact with the host or penetrates it.

1. Induced Structural Defense (Histological Defense)

• Cork Layer Formation: The plant produces layers of cork cells (suberin) to wall off the infection site, cutting off nutrient supply to the pathogen (e.g., Potato scab).
• Abscission Layers: A gap is formed between infected and healthy tissue, causing the infected part (e.g., a "shot hole" in stone fruits) to fall off.
• Tyloses: Overgrowths of protoplasts of parenchyma cells into xylem vessels. They block the vessels to prevent the spread of vascular wilt pathogens (e.g., Fusarium wilt).
• Gum Deposition: Gums and resins are secreted to seal wounds and trap pathogens.
• Cell Wall Reinforcement: Deposition of lignin, callose (papillae), and silicon at the site of penetration.

2. Induced Biochemical Defense

• Hypersensitive Response (HR): Rapid, localized cell death at the site of infection. This "scorched earth" policy deprives biotrophic pathogens of living tissue, trapping them in dead cells.
• Phytoalexins: Low molecular weight antimicrobial compounds synthesized de novo in response to infection.
  • Examples: Pisatin (Pea), Phaseollin (Bean), Rishitin (Potato).
• Pathogenesis-Related (PR) Proteins: Enzymes like chitinases and glucanases that degrade fungal cell walls.
• Reactive Oxygen Species (ROS): The "oxidative burst" produces hydrogen peroxide and superoxide radicals that are toxic to pathogens and signal further defense.
• Systemic Acquired Resistance (SAR): A "whole-plant" immunity triggered by salicylic acid, protecting uninfected parts of the plant from subsequent attacks.

2. Principles and Methods of Plant Disease Management

The fundamental goal is to reduce economic and aesthetic damage, not necessarily to kill every pathogen.

A. The Six General Principles

1. Avoidance: Prevents contact between the host and the pathogen.
   • Method: Selecting proper geographical areas, adjusting sowing time to escape spore showers, using pathogen-free seed.
2. Exclusion: Preventing the entry of a pathogen into a new area where it does not currently exist.
   • Method: Plant Quarantine laws, seed certification, inspection.
3. Eradication: Elimination or reduction of the pathogen inoculum that has already established in an area.
   • Method: Crop rotation, sanitation (removal of debris), removal of alternate hosts, soil solarization.
4. Protection: Placing a chemical or physical barrier between the host and pathogen before infection occurs.
   • Method: Foliar fungicidal sprays, seed treatment.
5. Resistance: Improving the genetic constitution of the host to resist infection.
   • Method: Breeding resistant varieties, genetic engineering.
6. Therapy: Curing plants that are already infected (rare in field crops, common in high-value trees).
   • Method: Hot water treatment (for viruses/bacteria in seeds), chemotherapy, tree surgery.

3. Fungicides: Nature and Classification

Definition: A fungicide is a chemical agent that kills (fungicidal) or inhibits the growth (fungistatic) of fungi.

A. Nature and Chemical Combination

• Nature: Most modern fungicides are organic compounds, though historical ones are inorganic. They function by disrupting cell membranes, energy production, or cell division.
• Chemical Combination:
  • Synergism: Mixing two fungicides results in efficacy greater than the sum of individual effects (e.g., Metalaxyl + Mancozeb).
  • Resistance Management: Mixing a site-specific systemic fungicide with a multi-site contact fungicide reduces the risk of the pathogen developing resistance.

B. Classification of Fungicides

1. Based on Mode of Action

• Protectant (Contact/Residual): Remains on the plant surface. Kills the fungus before it enters. Must be applied before infection. (e.g., Mancozeb, Copper oxychloride).
• Systemic: Absorbed by the plant and translocated (moved) within tissues. Can cure established infections.
  • Symplastic: Moves phloem-downward.
  • Apoplastic: Moves xylem-upward (most common).
  • Amphimobile: Moves both ways (e.g., Fosetyl-Al).

2. Based on Chemical Nature

• Inorganic:
  • Sulfur: Elemental sulfur (oldest fungicide, effective against powdery mildews).
  • Copper: Bordeaux mixture (Copper sulfate + Lime), Burgundy mixture.
  • Mercury: Previously used for seed treatment (now largely banned due to toxicity).
• Organic:
  • Dithiocarbamates: Zineb, Maneb, Mancozeb (Broad spectrum protectants).
  • Benzimidazoles: Carbendazim, Benomyl (Systemic).
  • Triazoles: Propiconazole, Tebuconazole (Sterol biosynthesis inhibitors).
  • Phenylamides: Metalaxyl (Specific for Oomycetes like Phytophthora).
  • Strobilurins: Azoxystrobin (Derived from wood-rotting mushrooms).

3. Based on Spectrum of Activity

• Narrow Spectrum: Effective against a specific group (e.g., Metalaxyl for Downy mildews).
• Broad Spectrum: Effective against a wide range of fungi (e.g., Chlorothalonil).

4. Mode of Action and Formulations

A. Mode of Action (MOA)

The specific biochemical pathway disrupted by the chemical.

1. Multi-site Action (Contact Fungicides):
   • Affects multiple enzymatic processes simultaneously.
   • Example: Copper ions denature proteins; Sulfur interferes with electron transport.
   • Resistance Risk: Low (hard for fungi to mutate multiple sites at once).
2. Specific-site Action (Systemic Fungicides):
   • Biosynthesis of Ergosterol: Triazoles inhibit demethylation in sterol synthesis, compromising cell membrane integrity.
   • Nuclear Division (Mitosis): Benzimidazoles (Carbendazim) bind to tubulin protein, preventing microtubule formation and cell division.
   • Respiration: Strobilurins inhibit mitochondrial respiration at the Qo site (Cytochrome bc1 complex).
   • RNA Synthesis: Phenylamides inhibit RNA polymerase.
3. Antibiotics MOA:
   • Streptomycin: Inhibits bacterial protein synthesis by binding to the 30S ribosomal subunit (effective against Bacterial Leaf Blight).
   • Tetracycline: Interferes with tRNA attachment (effective against Phytoplasmas).
   • Cycloheximide: Antifungal, inhibits protein synthesis in eukaryotes.

B. Formulations

Pure chemicals (technical grade) are rarely applied directly. They are formulated to improve handling, storage, and efficacy.

• Wettable Powders (WP/W): Finely ground solid particles + wetting agent. Mixed with water to form a suspension. (e.g., Carbendazim 50 WP). Note: Requires agitation.
• Emulsifiable Concentrates (EC): Liquid active ingredient dissolved in organic solvent + emulsifier. Mixed with water to form a milky emulsion. (e.g., Propiconazole 25 EC).
• Dusts (D): Active ingredient mixed with inert carrier (talc, clay). Applied dry. Low concentration (e.g., Sulfur dust).
• Granules (G): Pellets applied to soil. Systemic release upon watering. (e.g., Carbofuran).
• Suspension Concentrates (SC): Solid particles dispersed in water. Flowable.
• Seed Dressings (DS/WS): Specifically designed to adhere to seeds.

5. Methods of Application of Chemicals

Selecting the right application method ensures the chemical reaches the target site efficiently.

1. Seed Treatment

• Dry Dressing: Mixing fungicide powder with seeds (e.g., Thiram).
• Wet Dressing / Slurry: Dipping seeds in liquid suspension or mixing with a slurry.
• Significance: Most economical and eco-friendly method. Protects against seed-borne and early soil-borne pathogens.

2. Soil Application

• Soil Drenching: Applying diluted fungicide near the root zone (e.g., for Root rot or Wilts).
• Broadcasting: Spreading granules over the field.
• Fumigation: Injecting volatile chemicals (fumigants like Methyl bromide or Formalin) into soil under plastic covers to sterilize it (used in nurseries/greenhouses).

3. Foliar Application

• Spraying: Most common method. Using hydraulic or pneumatic sprayers (High Volume, Low Volume, or Ultra Low Volume) to cover leaves.
• Dusting: Applying dry powders during early morning when dew is present to help adhesion.

4. Post-Harvest Application

• Dipping fruits or vegetables in fungicide solutions or wax emulsions to prevent storage rots (e.g., dipping mangoes in hot water + fungicide).

5. Special Methods

• Trunk Injection: Injecting systemic chemicals directly into the tree trunk (used for Coconut root wilt or Citrus greening).
• Root Feeding: Immersing an active root in a chemical bottle.

6. Safe Handling of Chemicals

Pesticide safety is critical to prevent toxicity to humans, animals, and the environment.

A. Before Application

• Label Literacy: Always read the label for dosage, expiry, compatibility, and toxicity level (color codes: Red=Extreme, Yellow=High, Blue=Moderate, Green=Slight).
• Storage: Store in a locked, cool, dry place away from food and children.
• Equipment Check: Ensure sprayers are not leaking and nozzles are clean.

B. During Application

• PPE (Personal Protective Equipment): Wear gloves, masks, goggles, long-sleeved shirts, and boots.
• Wind Direction: Never spray against the wind to avoid inhalation or drift.
• Timing: Apply during cool hours (morning/evening) to prevent photodecomposition and leaf scorch.
• Hygiene: Do not eat, drink, or smoke while spraying.

C. After Application

• Disposal: Triple-rinse empty containers and crush/puncture them. Do not reuse containers for domestic purposes. Bury them in designated landfills.
• Cleaning: Wash the sprayer thoroughly. Bathe with soap and water immediately after work.
• Re-entry Interval: Observe the "waiting period" before entering the field or harvesting the crop to avoid residue ingestion.