Unit6 - Subjective Questions

1. Pre-existing (Passive) Defense Mechanisms:
These are constitutive defenses present in the plant before pathogen contact.

• Structural:
  • Waxes & Cuticle: Prevent water retention and pathogen penetration.
  • Epidermal Cell Walls: Thickness and lignification act as physical barriers.
  • Trichomes: Hinder fungal spore settlement and insect movement.
• Biochemical:
  • Presence of inhibitors like phenols, tannins, and pre-existing antifungal compounds (Prohibitins).

2. Post-infectional (Active) Defense Mechanisms:
These are induced only after the pathogen attacks the host.

• Structural: Formation of cork layers, abscission layers, tyloses, and gum deposition to seal off the pathogen.
• Biochemical:
  • Phytoalexins: Low molecular weight antimicrobial compounds synthesized de novo.
  • Hypersensitive Response (HR): Rapid, localized cell death to contain the pathogen.
  • PR Proteins: Pathogenesis-related proteins (e.g., chitinases, glucanases) that degrade pathogen cell walls.

Definition: Phytoalexins are low molecular weight antimicrobial compounds that accumulate in plants at sites of infection in response to pathogen attack.

Key Characteristics:

• Induced Resistance: They are not present in healthy plants but are synthesized de novo upon stimulation.
• Elicitors: Their production is triggered by molecules called elicitors (from the pathogen or plant cell wall fragments).
• Mechanism: They inhibit the growth of fungi and bacteria by disrupting cell membranes or metabolic processes.

Examples:

• Pisatin in Peas.
• Phaseollin in Beans.
• Rishitin in Potatoes.

Significance: The speed and magnitude of phytoalexin accumulation often determine whether a plant is resistant (incompatible reaction) or susceptible (compatible reaction) to a specific pathogen.

The six general principles of plant disease management are:

1. Avoidance: Avoiding the contact between the pathogen and the host (e.g., choosing proper planting time, geographic selection).
2. Exclusion: Preventing the entry of a pathogen into a new area where it does not exist (e.g., Plant Quarantine).
3. Eradication: Eliminating or destroying the pathogen after it has established in an area (e.g., removing infected plant parts, crop rotation).
4. Protection: Placing a chemical or physical barrier between the host and pathogen before infection occurs (e.g., fungicide sprays).
5. Resistance: Utilizing the genetic capability of the host to resist infection (e.g., breeding resistant varieties).
6. Therapy: Curing plants that are already infected (e.g., chemotherapy, heat treatment).

Exclusion: This principle aims to prevent the introduction of an inoculum into a pathogen-free area.

Plant Quarantine:

• Definition: Legal restrictions on the movement of agricultural commodities to prevent the entry or spread of pests and pathogens.
• Domestic Quarantine: Restricts movement within a country (e.g., restricting potato movement from Darjeeling to other parts of India to control Wart disease).
• International Quarantine: Regulates movement between countries via airports and seaports.
• Embargo: A total ban on the import of certain risky plant materials.
• Phytosanitary Certificate: A document verifying that the consignment is free from pests/diseases, issued by the exporting country.

Fungicides are broadly classified into two categories based on mobility:

1. Protectant (Contact/Residual) Fungicides:

• Nature: These chemicals remain on the plant surface and do not enter plant tissues.
• Action: They kill the pathogen upon contact or prevent spore germination.
• Requirement: Thorough coverage of the foliage is essential. They cannot cure established infections.
• Examples: Mancozeb, Copper oxychloride, Sulfur.

2. Systemic Fungicides:

• Nature: These are absorbed by the plant (through roots or leaves) and translocated to other parts via the xylem or phloem.
• Action: They can eradicate established infections (curative) and protect new growth.
• Examples: Carbendazim (Benzimidazoles), Metalaxyl, Propiconazole (Triazoles).

History:
Discovered by P.M.A. Millardet in 1885 in France to control Downy mildew of grapes caused by Plasmopara viticola.

Composition:
It is a mixture of Copper Sulfate (), Lime ( or ), and Water.

Standard Preparation (1% or 100 gallons):

• Formula: (5 lbs Copper Sulfate : 5 lbs Lime : 50 gallons Water).
• Method:
  1. Dissolve copper sulfate in half the volume of water in a non-metallic vessel (copper is corrosive).
  2. Slake the lime and mix in the other half of the water.
  3. Slowly pour the copper sulfate solution into the lime solution (not vice versa) while stirring constanty.
• Testing: Use a clean iron knife; if copper deposits on it, the mixture is acidic and needs more lime.

Mode of Action:
The copper ions () are slowly released and accumulate in fungal spores, displacing essential metals in enzymes and disrupting cellular proteins.

Classification:
Dithiocarbamates are organic sulfur fungicides derived from dithiocarbamic acid. They are divided into:

1. Mono-alkyldithiocarbamates: Examples: Ziram, Ferbam, Thiram.
2. Bis-alkyldithiocarbamates (EBDCs): Examples: Nabam, Zineb, Maneb, Mancozeb.

Mode of Action:

• Multi-site Inhibitors: They act on multiple enzymatic sites within the fungal cell.
• They interact with sulfhydryl () groups of amino acids and enzymes, disrupting respiration and other metabolic activities.
• Because they affect multiple sites, the risk of pathogens developing resistance to dithiocarbamates is very low compared to systemic fungicides.

Usage: Widely used as broad-spectrum protectants for leaf spots, blights, and rusts.

Examples:

• Benomyl (Benlate)
• Carbendazim (Bavistin)
• Thiophanate-methyl

Mechanism of Action:

• Systemic Action: Benzimidazoles are absorbed by the plant and translocated acropetally (upward) through the xylem.
• Microtubule Interference: They bind to -tubulin protein subunits.
• Inhibition of Mitosis: By binding to tubulin, they prevent the assembly of microtubules, which are essential for spindle fiber formation during nuclear division. This effectively stops fungal cell division (mitosis).
• Spectrum: Effective against a wide range of Ascomycetes and Deuteromycetes (e.g., Powdery mildews, wilts) but generally ineffective against Oomycetes (Downy mildews) and bacteria.

Definition: Antibiotics are chemical substances produced by microorganisms (mostly Actinomycetes) that, in low concentrations, inhibit the growth of or kill other microorganisms.

1. Streptomycin:

• Source: Streptomyces griseus.
• Formulation: Often sold as Streptocycline (Streptomycin sulfate + Tetracycline).
• Mode of Action: Inhibits protein synthesis by binding to the 30S ribosomal subunit in bacteria causing misreading of mRNA.
• Use: Controls bacterial diseases like Fire blight of apple, Citrus canker, and Bacterial leaf blight of rice.

2. Tetracycline (Oxytetracycline):

• Mode of Action: Inhibits protein synthesis by preventing the attachment of aminoacyl-tRNA to the ribosomal acceptor site.
• Use: Specifically effective against Phytoplasmas (e.g., Little leaf of brinjal, Sandal spike) and some bacterial diseases. Usually applied via trunk injection in trees.

Fungicides are formulated to improve handling, storage, and efficacy.

1. Wettable Powders (WP): Finely ground solid particles mixed with a wetting agent. They form a suspension in water (require agitation). Example: Mancozeb 75 WP.
2. Dusts (D): Fine powder mixed with an inert carrier (talc/clay). Used dry without water. Example: Sulfur dust.
3. Emulsifiable Concentrates (EC): Liquid formulation containing the active ingredient dissolved in a solvent with an emulsifier. Forms a milky emulsion in water. Example: Propiconazole 25 EC.
4. Granules (G): Active ingredient impregnated onto small pellets/granules. Applied to soil. Example: Carbofuran (insecticide/nematicide).
5. Suspension Concentrates (SC) / Flowables: Finely ground solid particles suspended in a liquid base. Easier to measure than WP.
6. Slurry / Seed Treatment formulations (DS/WS): Designed specifically to coat seeds.

Seed Treatment: The application of fungicides/antibiotics to seeds to control seed-borne or soil-borne pathogens.

Methods:

1. Dry Seed Dressing:
   • The fungicide (usually dust or WP) is mixed with seeds in a rotating drum.
   • Provides a protective coating.
   • Example: Treating wheat seeds with Carboxin for loose smut.
2. Wet Seed Dressing / Slurry:
   • The chemical is mixed with a small amount of water to form a soup-like slurry, then mixed with seeds.
   • Ensures better adherence than dry dressing.
3. Seed Dipping / Soaking:
   • Seeds or seedlings are dipped in a chemical solution for a specific time.
   • Example: Dipping potato tubers or sugarcane setts.
4. Seed Pelleting:
   • Seeds are coated with the chemical along with an adhesive and a filler (like lime) to alter the shape/size for mechanical planting and provide protection.

Soil application targets soil-borne pathogens (e.g., Fusarium, Rhizoctonia, Pythium).

Methods:

1. Soil Drenching:
   • The fungicide solution is poured around the base of the plant or over the soil surface.
   • Used in nurseries (e.g., to control Damping-off).
   • Example: Drenching with Copper Oxychloride.
2. Broadcasting:
   • Granular fungicides are scattered by hand or machine over the field.
   • Example: Application of bio-control agents or granular nematicides.
3. Furrow Application:
   • Fungicides are applied directly into the planting furrow at the time of sowing.
4. Fumigation:
   • Volatile chemicals (fumigants) are injected into the soil covered with plastic tarps to sterilize it.
   • Example: Methyl bromide (phased out), Formaldehyde.

When a pathogen breaches the initial barriers, the plant may respond by creating histological barriers to limit spread.

1. Cork Layers:
   • The plant induces cork cambium (phellogen) nearby the infection site.
   • Produces suberized cork cells that block the flow of nutrients and water to the pathogen and block toxin flow to the plant.
   • Common in potato scab.
2. Abscission Layers:
   • A gap is formed between infected and healthy tissue by the dissolution of the middle lamella.
   • The infected tissue shrivels and falls off (Shot-hole symptom).
   • Common in stone fruits (Prunus spp).
3. Tyloses:
   • Balloon-like outgrowths of parenchyma cells into xylem vessels.
   • They block the vessel, preventing the vertical spread of vascular wilt pathogens (e.g., in Fusarium wilt).
4. Gum Deposition:
   • Gums are deposited in intercellular spaces to form an impenetrable barrier.

1. Storage:

• Store chemicals in a locked, dry, well-ventilated room away from food, fodder, and children.
• Keep in original containers with labels intact.

2. Preparation:

• Read the Label: Always check dosage, compatibility, and expiry.
• PPE: Wear Personal Protective Equipment (gloves, masks, goggles, apron).
• Never mix chemicals with bare hands.

3. Application:

• Spray during the cool hours of the day (morning/evening).
• Wind Direction: Never spray against the wind to avoid inhalation.
• Do not eat, drink, or smoke while spraying.
• Ensure spray equipment is leak-proof.

4. Post-Application:

• Disposal: Triple rinse empty containers and crush/bury them; never re-use for household purposes.
• Hygiene: Wash hands and body thoroughly with soap and water.
• Re-entry: Observe the specific re-entry interval for the field.

Definition: Fungicide resistance is the stable, inheritable adjustment by a fungus to a fungicide, resulting in a significantly reduced sensitivity to that chemical. It often occurs with systemic, single-site inhibitors.

Management Strategies:

1. Rotation: Do not use the same mode of action continuously. Rotate between systemic and contact fungicides.
2. Tank Mixing: Mix a systemic fungicide (high resistance risk) with a contact fungicide (low resistance risk, e.g., Mancozeb). This kills mutants that might survive the systemic one.
3. Correct Dosage: Avoid using sublethal doses which allow the selection of resistant strains.
4. Limit Applications: Restrict the number of systemic sprays per season.
5. IDM: Integrate chemical control with cultural and biological methods to reduce reliance on chemicals.

Overview: Triazoles are the largest group of systemic fungicides belonging to the Sterol Biosynthesis Inhibitors (SBIs) or Demethylation Inhibitors (DMIs).

Mode of Action:

• They inhibit the C-14 demethylation step in the biosynthesis of ergosterol.
• Ergosterol is a vital component of fungal cell membranes.
• Depletion of ergosterol disrupts membrane structure and function, stopping fungal growth.

Key Features:

• Broad-spectrum systemic action with both protective and curative properties.
• Often have a "green effect" on plants (delay senescence).

Examples:

• Propiconazole (Tilt) - Effective against Rusts and Leaf spots.
• Tebuconazole (Folicur).
• Hexaconazole.

Cultural Methods:

• Definition: Modification of farming practices to make the environment less favorable for the pathogen.
• Mechanism: Avoidance or Eradication.
• Examples:
  • Crop Rotation: Starves host-specific soil pathogens.
  • Sanitation: Removing infected debris/weeds.
  • Summer Ploughing: Exposes soil pathogens to heat (Solarization).
  • Sowing Time: Adjusting planting dates to escape peak spore load.

Biological Methods:

• Definition: Use of living organisms (antagonists) to suppress plant pathogens.
• Mechanism: Antibiosis, Competition, Hyperparasitism, or Induced Resistance.
• Examples:
  • Trichoderma viride (fungus) against Rhizoctonia root rot.
  • Pseudomonas fluorescens (bacteria) against wilt pathogens.

Definition: The Hypersensitive Response (HR) is a rapid, localized death of plant cells at the site of infection.

Mechanism:

1. Recognition: The plant recognizes specific pathogen effectors via R-genes (Gene-for-Gene hypothesis).
2. Oxidative Burst: This triggers a rapid production of Reactive Oxygen Species (ROS) like and superoxide.
3. Cell Death: The accumulation of ROS leads to programmed cell death (apoptosis) of the infected cells and immediate neighbors.

Function:

• It deprives the pathogen (especially biotrophs like rusts and viruses) of living tissue required for nutrition.
• It traps the pathogen in dead tissue.
• It releases signals to trigger Systemic Acquired Resistance (SAR) in the rest of the plant.

Definition: IDM is a holistic approach that selects and applies a combination of compatible control measures (Cultural, Physical, Biological, Chemical, and Legal) to maintain disease levels below the Economic Threshold Level (ETL).

Why it is the best approach:

1. Sustainability: Reduces reliance on chemicals, preserving soil health and beneficial microbes.
2. Resistance Management: Alternating methods prevents pathogens from developing resistance to fungicides.
3. Cost-Effective: Often reduces the cost of inputs by optimizing chemical use.
4. Environmental Safety: Minimizes residue in food and pollution of water sources.
5. Reliability: Since it attacks the pathogen at multiple stages of its life cycle, control is more stable than relying on a single method.

Apart from Bordeaux mixture, several other copper compounds are used as inorganic, contact fungicides:

1. Burgundy Mixture:
   • Composition: Copper Sulfate + Sodium Carbonate (Washing soda) + Water.
   • Use: Similar to Bordeaux but leaves no stain; generally less persistent.
2. Copper Oxychloride (COC):
   • Usually formulated as 50% WP (e.g., Blitox-50).
   • Benefits: Ready-to-use powder (unlike Bordeaux which requires fresh preparation), less phytotoxic, easy to handle.
   • Use: widely used for fruit rots, leaf spots, and blights.
3. Copper Hydroxide:
   • Example: Kocide.
   • High copper content and efficacy.
4. Cheshunt Compound:
   • Composition: Copper sulfate + Ammonium carbonate.
   • Use: Specifically for soil drenching to control Damping-off in nurseries.