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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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Image.
- 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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Image.
- 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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Image.
- 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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Image.
- 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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U
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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