GLOSSARY

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A

Abiotic
pertaining to physical and inorganic components. For example, diseases/disorders in plants can be caused by abiotic factors such as extremes of heat, light, moisture, lack of nutrients etc.
Appressorium (pl. Appressoria)
an enlarged fungal filament that adheres to the surface of the host, prior to penetration.
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Avirulent
unable to cause disease; lacking virulence (see virulent); nonpathogenic.

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B

Biotic
pertaining to life and therefore living organisms. For example, plant diseases of a biotic origin are caused by living organisms such as insects, nematodes, etc.
Biotroph
an organism that can live and reproduce only on another living organism. A biotroph is completely dependent on the host organism as a source of nutrients, i.e. it is an obligate parasite. Compare with necrotroph.
Blight
A disease characterised by rapid and widespread death of plant tissue. Blight may take the form of extensive spotting, discoloration, wilting, or destruction of leaves, flowers and stems.
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C

Canker
a sunken necrotic lesion often of a main stem, branch or root.
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Chlorophyll
the green pigment found in chloroplasts of plant cells and algae. Chlorophyll is the receptor of light energy in photosynthesis. Chlorophyll is also found in photosynthetic bacteria.
Chloroplast
a disk-shaped structure in plant and algal cells that contains chlorophyll and is the site of photosynthesis.
Constitutive
constantly present, whether there is demand or not.
Cork
the external protective tissue of a stem or root. It is made up of polygonal-shaped cells that are non-living at maturity. The cell walls of cork tissue contain a fatty material (suberin) that makes them impermeable to water and gasses. Cork cells form the outermost layer of bark.
Cultivar
a cultivated plant variety or cultural selection ; a plant type within a species that has recognizable characteristics as a result of deliberate genetic manipulation. Distinguishable characteristics may include colour, flower shape, fruits, seeds and height.
Cuticle
the waxy or fatty layer covering epidermal cells of plant surfaces exposed to air such as leaves, stems and fruit. The cuticle is water-repellent and aids the plant in conserving water by reducing the amount of water vapour lost to the air from plant surfaces, particularly the upper surface of leaves.In zoology, cuticle also refers to the non-cellular outer layer of an insect or a nematode.

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D

Damping off
the collapse and rot of seedlings near soil level before emergence or soon after emergence. Damping off is caused by fungi: Pythium spp., Phytophthora spp., Fusarium spp., and Rhizoctonia spp.
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Defensins
antimicrobial proteins that inhibit the growth and development of pathogens. Plant defensins are found throughout the plant kingdom and are released upon seed germination, creating an antimicrobial environment around the seed while it germinates.
Dieback
progressive death of shoots, branches and roots, sometimes even leading to complete death of the plant.
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Disease
any malfunctioning of host cells and tissues that results from continuous irritation by a pathogenic agent or environmental factor which leads to the development of symptoms;
- abnormal functioning of physiological processes of an organism.
Disease Incidence
the number of plants affected by a disease within a population.
Disease Severity
the measure of damage done by a disease.
Dominant
a gene is said to be dominant if it expresses its phenotype (physical characteristic) even when contributed by only one of the parents.
Downy Mildew
white or grey 'bloom' on leaves and stems caused by production of sporangiophores and sporangia by members of the Peronosporales (downy mildew fungi).

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E

Ectodesmata
Ectodesmata are pits in the outer wall of epidermal cells, which connect to valleys in the cuticle surface, exposing the plasma membrane. Plant metabolites leak from the ectodesmata and accumulate in the valleys on the leaf surface, providing nutrients for pathogens and saprophytes.
Ectoparasite
a parasite that lives and feeds from the exterior of its host's cells or tissues. Compare with endoparasite.
Elicitor
a molecule produced by the host which induces a response by the pathogen. Conversely, an elicitor can be produced by a pathogen, eliciting a response in the host.
Endoparasite
a parasitic organism that lives and feeds within the cells or tissues of its host. Compare with ectoparasite.
Endobiotic
describes an organism living within the cells or tissues of a host organism. Endobiotic organisms are endoparasites.
Enzyme
a protein produced by living cells that can speed up (catalyse) specific biochemical reactions but which itself remains unaltered in the process.
Epiphytic
living on the surface of a plant, but not as a parasite and without causing infection.
Eradication
the control of plant disease by eliminating the pathogen after it is established or by eliminating all of the plants that carry the pathogen.
Exclusion
a method of disease prevention in which the pathogen or infected plant material is excluded from crop production areas. See quarantine.
Extracellular
outside a cell.

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F

Fungicide
a substance (chemical or physical) that kills or inhibits the growth of fungi.

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G

Gall
an abnormal growth or swelling produced as a result of pathogenic invasion.
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Gene Expression
conversion of the information encoded in a gene via transcription and translation, resulting in the production of a protein and the appearance of the phenotype determined by that gene.
Genotype
the genetic makeup of an individual or group.
Gum
gelatinous, sugary substance that is synthesized and secreted by plant tissues.

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H

Haustorium (pl. Haustoria)
a specialized branch of a fungal hypha formed inside a living cell of the host plant in order to obtain nutrients.
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Hemibiotroph
a parasite that initially forms an association with living cells of the host, much like a biotroph, and then in the later stages of infection it becomes necrotrophic, actively killing host cells.
Host
an organism harbouring a parasite or pathogen.
Host Range
the range of plants on which an organism feeds, particularly a parasite ; the range of plants in which a pathogen is capable of causing disease.
Hydathode
a specialized epidermal leaf structure with one or more openings through which water is discharged from the leaf interior to its surface.
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Hypersensitive
the state of being extremely or excessively sensitive. It often refers to an extreme reaction by a plant to an invading pathogen in which the plant tissue around infected sites dies in order to prevent further spread of the infection.
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Hypha (pl. Hyphae)
a single tubular thread-like filament of a fungal mycelium. The hypha is the basic structural unit of a fungus.

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I

Inoculate
to introduce a microorganism or virus into an environment ( i.e. an organism or culture medium) suitable for its growth ; to insert a pathogen into healthy tissue.
Inoculum (pl. Inocula)
a pathogen or its parts which can cause infection when transferred to a favourable location ; the population of microorganisms introduced in an inoculation.

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J

K

L

Latent Infection
where the host is infected with a pathogen but does not show any symptoms.
Lignin
a complex polymer deposited in some cell walls of vascular plants. Lignin is one of the main constituents of secondary walls and wood. It gives compressive strength and rigidity to the cell wall as well as making it impermeable to water.
Lignituber
a structure formed by the deposition of lignin around the tip of a fungal hypha penetrating a host plant cell. The formation of a lignituber is an active defense mechanism and its function is to inhibit the growth of the hypha into other cells.
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Lumen
central cavity of a cell, vessel or other structure.

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M

Mesophyll
the photosynthetic tissue of a leaf, located between the upper and lower epidermis. Mesophyll is commonly differentiated into palisade parenchyma and spongy parenchyma.
messenger RNA
a form of RNA that carries information to direct the synthesis of protein.
Mosaic
patchy variation of normal green colour in leaves, usually light and dark green mosaic, symptomatic of many viral diseases.
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Mycelium (pl. Mycelia)
a mass of hyphae that forms the body (thallus) of a fungus.
Multigene Family
a set of genes that, due to their high degree of sequence similarity, are believed to have evolved from a single ancestral gene.

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N

Necrosis
the death of cells, often accompanied by black or brown darkening of the tissue.
Necrotroph
an organism (parasite) that causes the death of host tissues as it grows through them, obtaining its energy from the dead cells. Compare with biotroph.

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O

Obligate Parasite
an organism that is only capable of living as a parasite in association with its host plant. The term is synonymous with biotroph.

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P

Papilla
a hump or swelling. In plant pathology, papilla refers to the accumulation of material between the host plant's cell wall and cell membrane at the point of fungal penetration. A papilla is usually composed of silicon, lignin and proteins.
Parasite
an organism or virus living in or on another organism (host) from which it obtains its nutrient supply. A parasite is not necessarily a pathogen.
Parenchyma
the soft tissue comprised of living, thin-walled cells of variable size and form. Parenchyma cells are the most abundant cell type in plants.
In leaves, parenchyma is differentiated into two forms:
- Palisade parenchyma: the tissue found beneath the upper epidermis of leaves, composed of elongate, tubular cells aligned perpendicular to the leaf surface. Palisade parenchyma cells generally contain an abundance of chloroplasts.
- Spongy parenchyma: the tissue typically found between the palisade parenchyma and the lower epidermis of leaves. Spongy parenchyma consists of loosely arranged, irregularly shaped cells containing chloroplasts and is interspersed with intercellular spaces.
Pathogen
a disease causing organism or agent.
Pathogenesis
the sequence of processes in disease development that describes a pathogen's association with its host. The sequence begins with initial contact between the pathogen and host and ends when the pathogen is no longer associated with that host (i.e. when the host/pathogen dies or the pathogen moves to another host).
Pathogenicity
the ability to cause disease.
Penetration
initial invasion of a host by a pathogen.
Penetration Peg
a structure found in some plant parasitic fungi. The penetration peg is a specialised, narrow, hyphal strand located on the underside of an appressorium that penetrates the epidermal cell wall.
Phenotype
the visible physical characteristics of an organism determined by the interaction of its genotype with the environment.
Phloem
the food-conducting and food-storing tissue in the roots, stems, and leaves of vascular plants.
Photosynthesis
the production of carbohydrates from carbon dioxide and water in the presence of chlorophyll(s), using light energy and releasing oxygen as a by product.
Phytoalexin
a substance produced in higher plants in response to a number of stimuli (chemical, physical or biological) that inhibits the development of a microorganism.
Phytoanticipin
these are constitutive phytoalexins i.e. phytoanticipins are synthesised by the plant at a constant rate and therefore always present in the tissues of the plant, whereas phytoalexins are only produced in response to a stimulus such as a pathogenic invasion. The distinction between phytoalexins and phytoanticipins is not always clear as some compounds may be phytoalexins in one species and phytoanticipins in another species. In general, the distinction between the two compounds depends on when they are produced (either before or after infection) and the extent to which the compound is antimicrobial (phytoalexins are antimicrobial).
Phytoplasmas
phloem-dwelling prokaryotic microorganisms, transmitted by phloem-feeding insects.
Powdery Mildew
White powdery 'bloom' on the plant surface caused by the production of fungal mycelium, conidiophores and conidia by members of the Erysiphales (powdery mildew fungi).
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Primary Inoculum
inoculum that initiates disease in the field following a dormant stage in its life cycle (called overwintering or oversummering). Compare with secondary inoculum.
Propagules
any part of an organism capable of independent growth (e.g., a spore, a mycelial fragment, etc.).
Protectant
any chemical agent that interacts with a pathogen on the plant surface to prevent infection.
- Protectant fungicide: a protectant that kills or inhibits the growth of fungi. See fungicide.
Pustule
A blister-like spore mass breaking through a plant epidermis.
Pleiotropic
having multiple effects.
Proteinaceous
of or related to protein.

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Q

Quarantine
legal restriction of the transport of plants and/or plant parts in order to prevent the spread of pests and pathogens. In order to accomplish this plants may be held in isolation (i.e. quarantine) for an extended period of time to ensure that they are free of pests and diseases.

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R

Race
a subgroup of pathogens within a species that infect a given set of plant varieties. Races may be distinguished from each other by virulence or symptom expression but not by morphology.
Resistant
possessing qualities that prevent or impede the development of a disease.
Resistance
the power of an organism to exclude or overcome, completely or partially, the effects of a pathogen or some other damaging factor.
Respiration
an intracellular process which consists of a series of chemical reactions that make energy available through the oxidation of carbohydrates, fats and proteins. This process produces carbon dioxide and water as byproducts and is termed aerobic respiration because it uses oxygen (although the initial stages of the process are anaerobic, i.e. do not require oxygen).
Resting Spore
a thick-walled spore, usually formed by a sexual process, that germinates after remaining in a dormant state for an extended period of time.
Rot
the disintegration of tissue, often caused by enzyme or toxins produced by pathogens.
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Rust
rust-coloured pustules formed by members of the Uredinales (rust fungi).
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S

Saprophyte
an organism that obtains nourishment from non-living organic matter (usually dead and decaying plant or animal matter) by absorbing soluble organic compounds.
Scab
a discrete, superficial roughened lesion.
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Sclerotium (pl. Sclerotia)
a hard, resistant vegetative resting body of a fungus composed of a compact mass of hyphae and capable of surviving under unfavourable environmental conditions. Under favourable conditions the sclerotium can produces sexual or asexual fruiting bodies.
Secondary Inoculum
inoculum produced by infections that took place during the same growing season. Compare with primary inoculum.
Secondary Metabolite
a compound that is not necessary for growth or maintenance of cellular functions but is, in general, synthesised for the protection of a cell or microorganism during the stationary phase of its growth cycle. In plants, secondary metabolites are believed to be important in the attraction of pollinators and to have a function in defense, as some metabolites have antimicrobial properties. Also known as secondary products.
Smut
a disease characterised by black spore masses on leaves, stems or inflorescences, caused by members of the Ustilaginales (smut fungi).
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Sporangiophore
sporangium-bearing body of a fungus.
Sporangium (pl. Sporangia)
a unicellular or multicellular sac-like structure in fungi that produces asexual spores.
Spore
a specialised reproductive body in fungi (and some other organisms), containing one or more cells, capable of developing into an adult.
Stoma (pl. Stomata)
a minute opening in the epidermis of leaves and stems through which gasses pass. The stoma is bordered by two guard cells which open and close depending on their turgidity. The term stoma is often used to refer to the entire stomatal apparatus i.e. the guard cells as well as the pore they surround.
Systemic
(1) pertaining to a disease in which the pathogen spreads generally throughout the plant ; (2) pertaining to a chemical absorbed into the plant through root or foliage and transported internally throughout the plant.
- Systemic fungicide: a chemical agent that spreads internally through the plant and eradicates established fungal infections. See fungicide.

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T

Tillage
the process of turning or mixing the soil.
Toxin
in general, a poisonous substance of biological origin. Specifically, a compound produced by a microorganism which is toxic to other organisms.
Transcription
the process whereby a base sequence of messenger RNA (mRNA) is synthesized on a complementary segment of DNA.
Translation
the process that occurs at the ribosome whereby the information in messenger RNA (mRNA) is used to assemble amino acids into a protein.
Translocation
the long-distance transport of water, nutrients, chemicals, or food materials within a plant.
Transpiration
the loss of water vapour from leaf surfaces and other parts of the plant exposed to the air. Most transpiration occurs through stomata.
Tylose, Tylosis (pl. Tyloses)
a balloon-like outgrowth from a parenchyma cell that expands through a pit in a xylem vessel wall and into the lumen of the vessel, either blocking it completely or partially.

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V

Vascular
pertaining to any plant tissue or region consisting of fluid-conducting tissue; for example, xylem and phloem. The term is also sometimes applied to a pathogen that grows primarily in the conducting tissues of the plant.
Vascular Wilt
a disease in which the pathogen is confined to the vascular system of the host and in which wilting is a characteristic symptom; plants lose their turgidity and become flaccid, leaves collapse.
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Vector
any living organism (e.g., insect, mite, bird, nematode, parasitic plant, human, etc.) that transmits a pathogen from an infected organism to an uninfected one.
Virulence
the degree or measure of pathogenicity of a given pathogen; relative capacity to cause disease.
Virulent
strongly pathogenic; capable of cause severe disease (see avirulent).
Virus
a submicroscopic, non-cellular structure consisting of a core of infectious nucleic acid (either RNA or DNA) within a protein coat.

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W

X

Xylem
a complex vascular tissue through which water and minerals are conducted through the plant. Xylem also aids in the structural support of a plant as the primary component of its cell walls is lignin.

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Y

Z

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