Basic Terms

• Pathogen: An organism that causes disease in plants (e.g., fungi, bacteria, viruses, nematodes).
• Host: A plant that harbors the pathogen and shows symptoms of disease.
• Disease: A deviation from normal plant growth caused by pathogens or environmental factors.
• Symptom: The external or internal reaction of a plant to a disease (e.g., wilting, spots, blight).
• Sign: The physical presence of a pathogen on a plant (e.g., mold, spores).

Types of Diseases

• Biotic Diseases: Caused by living organisms like fungi, bacteria, viruses, nematodes, and parasitic plants.
• Abiotic Diseases: Caused by non-living factors like nutrient deficiencies, pollutants, temperature, and water stress.
• Systemic Diseases: Diseases that affect the entire plant.
• Localized Diseases: Diseases confined to specific plant parts.

Causal Organisms

• Fungi: The largest group of plant pathogens, reproducing through spores.
• Bacteria: Microscopic, single-celled organisms causing diseases like blight and rot.
• Viruses: Microscopic pathogens causing mosaic and stunting symptoms.
• Nematodes: Microscopic worms that feed on roots, causing galls and stunted growth.
• Phytoplasmas: Bacteria-like organisms causing yellowing and phyllody.
• Parasitic Plants: Plants like Cuscuta (dodder) that derive nutrients from host plants.

Infection and Disease Development

• Inoculum: The part of a pathogen that comes in contact with a host to cause infection.
• Pathogenicity: The ability of a pathogen to cause disease.
• Virulence: The degree of damage a pathogen causes to a host.
• Infection: The process by which a pathogen invades a host plant.
• Incubation Period: The time between infection and appearance of symptoms.
• Epidemiology: The study of disease outbreaks and spread.

Host-Pathogen Interaction

• Resistance: The ability of a plant to suppress or avoid infection.
• Susceptibility: The inability of a plant to resist a pathogen.
• Hypersensitive Response (HR): Localized plant cell death to restrict pathogen spread.
• Systemic Acquired Resistance (SAR): Long-term, broad-spectrum resistance developed after exposure to a pathogen.
• Disease Triangle: Interaction between the host, pathogen, and environment required for disease development.

Symptoms

• Blight: Sudden, widespread death of plant tissues.
• Canker: Sunken, necrotic lesions on stems or branches.
• Chlorosis: Yellowing of leaves due to lack of chlorophyll.
• Dieback: Progressive death of twigs and branches.
• Gall: Abnormal growth caused by pathogens like nematodes or bacteria.
• Leaf Spot: Localized necrotic areas on leaves.
• Wilt: Loss of rigidity due to pathogen-induced vascular blockage.
• Mosaic: Patchy discoloration of leaves, often caused by viruses.
• Powdery Mildew: White, powdery fungal growth on leaves.
• Rust: Orange, brown, or yellow pustules on leaves caused by rust fungi.

Management Practices

• Cultural Control: Practices like crop rotation, sanitation, and proper spacing.
• Chemical Control: Use of fungicides, bactericides, and nematicides.
• Biological Control: Using natural enemies (e.g., Trichoderma fungi) to suppress pathogens.
• Resistance Breeding: Developing plant varieties resistant to specific diseases.
• Integrated Disease Management (IDM): Combining multiple strategies for sustainable control.

Laboratory and Diagnostic Tools

• Koch’s Postulates: Steps to establish a pathogen as the cause of a disease.
• Microscopy: Used to identify pathogens like fungi and bacteria.
• ELISA: A test to detect viral pathogens.
• PCR: Molecular technique for identifying pathogens based on DNA.