Principles of Plant Pathology

Comprehensive practice flashcards covering terms, definitions, concepts, and anatomical defense structures from the Principles of Plant Pathology lecture course.

Plant Disease

A malfunctioning process that is caused by continuous irritation by a pathogenic agent or environmental factor, resulting in suffering and the development of symptoms.

Plant Disorder

A non-infectious condition caused by abiotic factors, such as the lack or excess of environmental or nutritional elements, occurring in the absence of infectious pathogens.

Pathogenicity

The ability of a pathogen to cause disease in a host organism.

Pathogenesis

The sequence of progress in disease development from initial contact between the pathogen and its host through to the completion of the disease syndrome.

Sign

The physical structures or parts of the pathogen, or its products, seen directly on an infected host plant.

Symptom

The external or internal reactions or alterations shown by a plant as a result of disease.

Syndrome

The entire complex or set of varying symptoms that collectively characterize a particular disease.

Biotroph

An obligate parasite that can live and multiply only on living host tissues to complete its life cycle (e.g., rusts, smuts, and powdery mildews).

Hemibiotroph (Facultative Saprophyte)

A parasite that attacks living tissues in the same manner as a biotroph, but continues to grow and reproduce saprophytically after host tissues die.

Perthotroph (Necrotroph)

A parasite that kills host cells and tissues in advance of hyphal penetration using enzymes or toxins and then feeds saprophytically (e.g., Sclerotium rolfsii).

Inoculum Potential

The energy of growth of a parasite available for host infection at the organ surface, determined as a function of inoculum density, pathogen vigour, host susceptibility, and environment.

Incubation Period

The period of time elapsed between the penetration of a host by a pathogen and the first visual appearance of symptoms.

Predisposition

The action of environmental conditions prior to penetration and infection that renders a host plant vulnerable to pathogen attack.

Endemic Disease

A disease that is constantly present in moderate to severe form and confined to a particular locality, country, or district (e.g., potato wart in Darjeeling).

Epidemic (Epiphytotic) Disease

A disease that occurs widely and periodically in a highly destructive form over large plant populations (e.g., late blight of potato during the 1845 Irish famine).

Sporadic Disease

A disease that breaks out at very irregular intervals and locations in relatively few instances (e.g., udbatta disease of rice).

Collateral Host

A wild plant of the same family that is susceptible to a crop pathogen, providing nutrition and reproduction for the pathogen during the off-season without completing a sexual cycle.

Alternate Host

A host plant belonging to a distinct family that is essential for the completion of the life cycle and sexual stages of a heteroecious rust fungus (e.g., Berberis vulgaris for Puccinia graminis tritici).

Soil Inhabitants

Unspecialized fungal saprophytes with a wide host range that can survive indefinitely in the soil in the complete absence of a host plant (e.g., Pythium, Rhizoctonia, Sclerotium).

Soil Invaders (Root Inhabitants)

Specialized parasitic pathogens that survive in soil in close vital association with host residues, whose active saprophytic phase declines once host tissues fully decompose (e.g., vascular wilt Fusarium, Verticillium).

Oospore

A thick-walled sexual resting spore formed by Oomycetes within an oogonium following fertilization by an antheridium.

Rhizomorph

A thick, dark, root-like cord composed of aggregated hyphal strands that aids in long-term survival, translocation, and host penetration (e.g., Armillaria mellea).

Anemochory

The dispersal and dissemination of plant pathogen propagules (such as fungal spores, bacteria, and nematode cysts) through wind and air currents.

Hydrochory

The dissemination of plant pathogens via flowing surface water, canal irrigation, or rain splashing.

Active Invaders

Pathogens (such as phytopathogenic fungi and phanerogamic parasites) that make an aggressive effort to enter intact host cells without requiring assistance from external vectors.

Passive Invaders

Pathogens (such as plant viruses and bacteria) that enter plants passively through existing wounds, natural openings, or vectors without exerting direct mechanical force.

Appressorium

A specialized, flattened, swollen structure formed at the tip of a germ tube or hypha that facilitates adhesion and direct cuticle penetration via an infection peg.

Cutin

An insoluble structural polyester made of unbranched derivatives of $\text{C}_{16}$ and $\text{C}_{18}$ hydroxy fatty acids present in the plant cuticle.

Pectin Methyl Esterase (PME)

A pectolytic enzyme that splits ester bonds and hydrolyzes methyl groups from pectin, yielding pectic acid and methanol ($\text{CH}_3\text{OH}$).

Polygalacturonase (PG)

A pectolytic enzyme that hydrolyzes $\alpha\text{-1,4-glycosidic}$ bonds in pectic chains by introducing a molecule of water across the bond.

Pectin Lyase (PL)

A pectolytic enzyme that cleaves $\alpha\text{-1,4-glycosidic}$ bonds by trans-elimination of water, releasing pectic fragments with unsaturated double bonds.

Pathotoxin

A microbial metabolite produced during pathogenesis that plays a primary role in disease production and reproduces all or most typical disease symptoms in a susceptible host (e.g., victorin).

Phytotoxin

A parasite product that induces non-specific symptoms in plants, where a causal relationship between toxin output and pathogenicity is merely suspected rather than established (e.g., alternaric acid).

Vivotoxin

A toxic substance produced in the infected host by the pathogen and/or the host that functions in disease development but is not itself the initial inciting causal agent (e.g., fusaric acid).

Host-Specific Toxin

A pathogen metabolite selectively toxic only to susceptible host genotypes of that pathogen, serving as a primary determinant of pathogenicity (e.g., T-toxin, victorin).

Cork Layer

An induced histological barrier consisting of multiple layers of suberized cells formed beyond an infection locus to stop pathogen advance, cut off nutrients, and block toxin diffusion.

Tyloses

Protoplasmic outgrowths from adjacent living xylem parenchyma cells that protrude into xylem vessels through pits, physically blocking the vascular movement of wilt pathogens.

Abscission Layer

A gap created by the dissolution of middle lamellae between healthy and infected parenchymatous leaf tissue, causing the diseased locus to fall out (shot-hole symptom).

Hyphal Sheathing

A cellular structural defense mechanism wherein an invading fungal hypha penetrating the cell wall is enveloped by a callose-rich, phenolic-infused sheath extending from the host wall.

Phytoalexin

A low-molecular-weight antimicrobial compound synthesized and accumulated in host tissues in response to infection or injury, which inhibits fungal and bacterial growth (e.g., rishitin in potato, pisatin in pea).

Hypersensitive Response (HR)

A rapid, localized induced host cell death at the point of pathogen entry that halts the spread of obligate and biotrophic parasites.

Plantibodies

Antibodies encoded by animal (such as mouse) genes that are genetically engineered into and expressed by transgenic plants to provide resistance against pathogens.

Disease Triangle

A visual model illustrating that an infectious disease requires the simultaneous interaction of three components: a susceptible host, a virulent pathogen, and a favourable environment.

Disease Pyramid

A tetrahedral conceptual model that incorporates the fourth dimension of time (and human activity) into the host-pathogen-environment disease triangle.

Compound Interest (Polycyclic) Disease

A disease whose rate of increase is mathematically analogous to compound interest, characterized by rapid spore production, airborne dissemination, short incubation periods, and multiple cycles per season (e.g., cereal rusts).

Simple Interest (Monocyclic) Disease

A disease whose rate of increase is mathematically analogous to simple interest, characterized by slow sporulation, soil or seed transmission, long incubation periods, and only one generation per crop cycle (e.g., cereal smuts).

Plant Quarantine

Legal restrictions enforced on the transport and movement of agricultural commodities to exclude, prevent, or delay the introduction and spread of plant pests and pathogens into uninfested areas.

Soil Solarization

A hydrothermal soil disinfestation method accomplished by covering moist soil with transparent polyethylene sheets for 4 to 6 weeks during hot summer months.

Siderophores

Low-molecular-weight iron-chelating agents ($500\text{--}1000\,\text{daltons}$) secreted by rhizobacteria (e.g., Pseudomonas fluorescens) that sequester $\text{Fe}^{3+}$ to starve competing plant pathogens.

Hyperparasitism

The direct parasitism, lysis, and killing of a plant pathogen by another microorganism while the pathogen is active (e.g., Trichoderma harzianum parasitizing Rhizoctonia mycelium).

Bordeaux Mixture

A preparatory copper fungicide prepared by suspending $5\,\text{kg}$ of copper sulphate and $5\,\text{kg}$ of lime in $500\,\text{liters}$ of water ($1\%$ concentration).

Vertical Resistance

Host resistance that is effective against only specific physiological races of a pathogen, typically controlled by a single gene (monogenic) and prone to breakdown.

Horizontal Resistance

Host resistance that operates uniformly against all races of a pathogen, governed by multiple genes (polygenic) and offering durable, stable protection in the field.

Cross Protection

The phenomenon in which systemically infecting a plant with a mild strain of a virus protects that plant from subsequent infection by severe strains of the same virus.

Totipotency

The innate capacity of a differentiated plant cell to regenerate into an entire, complete plant through division and differentiation.

Ti Plasmid

A large tumor-inducing plasmid ($\sim 200\,\text{kb}$) of Agrobacterium tumefaciens containing a T-DNA region transferred into host cells to drive auxin, cytokinin, and opine synthesis.