General Concepts

• Host: A plant that is invaded by a parasite and from which the parasite obtains its nutrition.
• Resistance: The ability of the host plant to restrict or prevent infection or disease development.
• Susceptibility: Inability of the plant to prevent disease or pathogen establishment.

Types of Pathogens

• Obligate Parasite (Biotroph): Survives only on living host. Ex: Rusts, Powdery mildew.
• Facultative Saprophyte: Primarily parasitic but can grow as saprophyte. Ex: Alternaria, Colletotrichum.
• Facultative Parasite: Primarily saprophyte but can become parasitic under favorable conditions. Ex: Fusarium.
• Necrotroph: Kills host cells and lives on dead tissues. Ex: Sclerotinia, Botrytis.

Infection & Disease Development

• Infection: Successful establishment of a pathogen in/on host tissue.
• Invasion: Spread of the pathogen within the host tissue.
• Colonization: Multiplication of pathogen in host tissue.
• Dissemination: Spread of pathogen from one plant to another.

Infection Related Terms

• Latent infection: Infection without immediate symptom expression. Ex: Colletotrichum in mango fruits.
• Primary infection: First infection in a growing season.
• Secondary infection: Spread from primary infection during the same season.

Pathogen Structures

• Appressorium: A swollen tip of a germ tube or hypha used to penetrate host surface.
• Haustorium: Specialized structure in biotrophs for nutrient absorption from host cells.
• Sclerotium: A compact mass of hardened fungal mycelium; survival structure. Ex: Sclerotium rolfsii.
• Chlamydospore: Thick-walled resting spore for survival in harsh conditions.

Disease Diagnosis Terms

• Koch’s Postulates: Steps to prove pathogenicity of an organism.
• Enzyme Linked Immunosorbent Assay (ELISA): Technique used to detect viruses and bacteria.
• PCR (Polymerase Chain Reaction): Detects DNA of pathogens – very sensitive diagnostic tool.

Host Responses

• Hypersensitive reaction (HR): Localized cell death at infection site to restrict pathogen.
• Phytoalexins: Antimicrobial compounds produced by plant in response to infection.
• Quarantine: Legal restriction on movement of plant material to prevent disease spread.

Disease Measurement

• Incidence: % of plants infected in a population.
• Severity: Intensity or extent of disease on a plant.
• Disease triangle: Host + Pathogen + Environment (all three necessary for disease).
• Disease cycle: Sequence of events from infection to new inoculum production.

Transmission Methods

• Airborne: Ex: Rust, Powdery mildew.
• Soil-borne: Ex: Fusarium, Sclerotium.
• Seed-borne: Ex: Loose smut of wheat.
• Vector-borne: Ex: Mycoplasma by leafhoppers, viruses by aphids.
• Mechanical transmission: Ex: TMV by touch/contact.

Disorders & Symptoms

Reproduction of Pathogens

• Sporangiospore: Formed inside sporangia; Ex: Phytophthora.
• Conidia: Asexual spores; Ex: Fusarium, Alternaria.
• Zoospore: Motile spore with flagella; Ex: Pythium, Phytophthora.
• Oospore: Thick-walled sexual spore; Ex: Oomycetes.
• Ascospore: Formed in ascus; Ex: Ascomycetes.
• Basidiospore: Formed on basidium; Ex: Basidiomycetes.

Famous Diseases & Pathogens