N | O | P | Q | R
N
nanometer
one billionth of a meter; used in measurement of viruses
necrosis (adj. necrotic) 
the death of cells or tissue, usually accompanied by darkening to black or brown
necrotroph
a parasite that typically kills host cells and obtains its energy from them (contrasts with biotroph)
nectary
a nectar-secreting gland in a flower
needlecast (of conifers)
a disease symptom caused by fungi that results in premature drop of needles
negative-sense RNA (-RNA)
the ribonucleic acid sequence complementary to the positive or plus sense sequence; not translated into protein (contrasts with positive-sense RNA)
nematicide
an agent, usually a chemical, that kills nematodes
nematode
a nonsegmented roundworm (animal) that is parasitic on plants or animals, or free living in soil or water
nitrification
the conversion of ammonium (NH4) by bacteria ultimately into nitrate (NO3); both ammonium and nitrate can be absorbed by plants
nitrogen cycle
the cycling of the element nitrogen from gaseous forms to various inorganic forms (e.g., ammonium, nitrate, and nitrite) and organic forms (e.g., nucleic acids, proteins) and back to elemental nitrogen
nitrogen fixation
the conversion of gaseous N2 to a form available for plant uptake
nitrogen oxide (NOx)
a primary air pollutant produced from internal combustion engines and other industrial processes
no observable adverse effect level (acronym NOAEL)
the maximum concentration of a substance that has no negative effects on the test subject
NOAEL (acronym for no observable adverse effect level)
the maximum concentration of a substance that has no negative effects on the test subject
node (adj. nodal)
an enlarged portion of a shoot at which leaves or buds arise
nodule (v. nodulate)
a small knot or irregular, rounded lump; on leguminous plants, a structure on a root that contains nitrogen-fixing bacteria
nomenclature
the scheme by which names are attached to objects, including organisms
noninfectious
pertaining to a disease that is caused by an abiotic agent that cannot be transmitted from one plant to another (see also abiotic) (contrasts with biotic, infectious)
nonpathogenic
unable to cause disease (see also avirulent)
nonpersistent transmission
a type of virus transmission in which the virus is acquired and transmitted by the vector after short feeding times and is retained by the vector for only a short period of time (see also stylet-borne transmission) (contrasts with circulative transmission, persistent transmission, propagative transmission)
nonseptate
without cross walls (see also aseptate, coenocytic)
no-till
a cultural system most often used with annual crops, in which the new crop is seeded or planted directly in a field on which the preceding crop plants were cut down, had the tops harvested, or were destroyed by a nonselective herbicide
nucleic acid
the genetic material of all living organisms, including DNA (deoxyribonucleic acid) and RNA (ribonucleic acid)
nucleoid
a region in prokaryotes, chloroplasts, and mitochondria where the DNA is concentrated. Unlike a nucleus, it is not bound by a membrane.
nucleus
a dense aggregation of proteinaceous matter and nucleic acid in cells, surrounded by a membrane; contains chromosomes and controls heredity
nucleotide
a subunit of a nucleic acid, either DNA or RNA
O
obligate parasite
an organism that can grow only as a parasite in association with its host plant and cannot be grown in artificial culture media (see also biotroph) (contrasts with necrotroph)
occlusion
a block or plug that stops the flow of liquids (as in vessels)
oedema
a swelling or blistering on leaves and other plant parts under conditions of high moisture and restricted transpiration (see also edema, intumescence)
oligogenic resistance
a resistance conferred by a few genes (contrasts with monogenic resistance, multigenic resistance, polygenic resistance, single gene resistance)
oncogen
a compound that induces benign or malignant tumors
oogonium (pl. oogonia)
the female gametangium of members of the Oomycota (the oomycetes), containing one or more gametes
oomycete
an informal term for a member of the Oomycota
Oomycota
a group of funguslike organisms - typically with nonseptate mycelia, asexual sporangia and zoospores, and sexual oospores - in the kingdom Stramenopila (or Straminipila); oomycetes
oospore
a thick-walled, sexually-derived resting spore of the Oomycota (the oomycetes)
ooze
a mass of bacterial cells mixed with host fluids
open reading frame
a portion of a nucleic acid molecule that is translated into a protein
organelle
a membrane-bound structure within a cell having a specialized function, e.g., mitochondrion and chloroplast
organic
describing a molecule containing carbon atoms; pertaining to living organisms
osmosis
the diffusion of water from an area of low solute concentration to one of higher concentration
ostiole (adj. ostiolate)
a pore; opening in the papilla or neck of a perithecium, pseudothecium, or pycnidium through which spores are released
outer bark
the exterior, nonliving portion of bark in woody plants (contrasts with inner bark)
ovary
the female reproductive structure of organisms; in plants, enlarged basal portion of a pistil, containing the ovules and developing into the fruit
overseason
to survive or persist from one planting season to the next
overwinter
to survive or persist through the winter period
oviposit
to deposit or lay eggs with an ovipositor
ovule
an enclosed structure that, after fertilization, becomes a seed; an egg contained within an ovary
ozone
a secondary air pollutant that is a highly reactive form of oxygen (O3 ) formed when the exhaust of automobiles and other internal combustion engines is in the presence of sunlight
ozone layer
a protective layer of ozone in the upper atmosphere that reduces ultraviolet radiation
P
palisade parenchyma
the tissue found beneath the upper epidermis of a leaf, composed of elongate, tubular cells arranged upright, in the manner of posts in a palisade fortification
PAMPS (acronym for pathogen-associated molecular patterns)
the molecular signals or elicitors from pathogens that are recognized by plant or animal receptors and can influence innate immunity of the host (see also with microbe-associated molecular patterns, acronym MAMPS)
PAN (acronym for peroxyacetyl nitrate)
a secondary air pollutant formed when the exhaust of automobiles and other internal combustion engines is in the presence of sunlight
pandemic
a widespread and destructive outbreak of disease occuring simultaneously in several countries
papilla (pl. papillae, adj. papillate)
a nipplelike projection; a structure formed at the tip of some sporangia; also, the localized wall thickenings on the inner surface of plant cell walls at sites penetrated by fungi
paragynous
having the antheridium contact the oogonium on the side, as in many Pythium spp. (contrasts with amphigynous)
parasexualism
a type of recombination, found in certain heterokaryotic fungi, that is based on mitosis rather than meiosis. Genetically distinct haploid nuclei fuse in the heterokaryon. The resulting diploid nuclei multiply by mitotic division, with some crossing over, and a diploid homokaryon develops. During repeated mitotic divisions, chromosomes may be shed by the diploid nucleus to produce a haploid nucleus.
parasite (adj. parasitic)
an organism that lives in intimate association with another organism on which it depends for its nutrition; not necessarily a pathogen (contrasts with saprophyte)
parenchyma (adj. parenchymatous)
the soft tissue of living plant cells with undifferentiated, thin, cellulose walls
parthenogenesis (adj. parthenogenetic; sometimes parthenogenic)
reproduction by the development of an unfertilized egg
partial resistance
a resistance in which disease develops more slowly, or to a lesser extent, on the host, but the host does become diseased (contrasts with complete resistance)
pasteurization
a process to free a material, usually a liquid, of selected harmful microorganisms using heat
pathogen (adj. pathogenic)
a disease-producing organism or biotic agent
pathogen-associated molecular patterns (acronym PAMPS)
the molecular signals or elicitors from pathogens that are recognized by plant or animal receptors and can influence innate immunity of the host (see also microbe-associated molecular patterns, acronym MAMPS)
pathogenesis
the production and development of disease
pathogenesis-related (PR) proteins
the proteins, such as antimicrobial proteins and hydrolytic enzymes, that are synthesized in the early events of the plant defense response
pathogenicity
the ability to cause disease
pathology
the study of diseases
pathotype
a subdivision of a pathogen species characterized by its pattern of virulence or avirulence to a series of differential host varieties
pathovar (abbr. pv.)
a subdivision of a plant-pathogenic bacterial species defined by host range; pathovar for bacteria is equivalent to forma specialis for fungi
PCR (acronym for polymerase chain reaction)
a technique used to amplify the number of copies of a specific region of DNA in order to produce enough of the DNA for use in various applications such as identification and cloning
pectin
a methylated polymer of galacturonic acid found in the middle lamella and the primary cell wall of plants; the jelly-forming substance found in fruit
pedicel
a small slender stalk; stalk bearing an individual flower, inflorescence, or spore
peduncle
a stalk or main stem of an inflorescence; part of an inflorescence, or a fructification
penetration
the initial invasion of a host by a pathogen
penetration peg
the specialized, narrow hyphal strand on the underside of an appressorium that penetrates host cells (see also infection peg)
penicillin
an antibiotic produced by certain species of Penicillium and effective against gram-positive bacteria
perennial
something that occurs year after year; a plant that survives for several to many years (contrasts with annual, biennial)
perfect
sexual, capable of sexual reproduction
perfect flower
a flower possessing both stamens and pistils
perfect state
the sexual form in the life cycle of a fungus (see also teleomorph) (contrasts with anamorph, holomorph, imperfect state)
pericarp
a fruit wall
pericycle
the layer or layers of cells between the phloem and the endodermis of roots, giving rise to branch roots
perithecium (pl. perithecia)
a flask-shaped or subglobose, thin-walled fungus fruiting body (ascocarp) containing unitunicate asci and ascospores; spores are expelled or released through a pore (ostiole) at the apex
peritrichous
having hairs or flagella distributed over the whole surface (contrasts with polar)
peroxyacetyl nitrate (acronym PAN)
a secondary air pollutant formed when the exhaust of automobiles and other internal combustion engines is in the presence of sunlight
persistent transmission
a type of virus transmission in which the virus is acquired and transmitted by the vector after a relatively long feeding time and remains transmissible for a prolonged period while in association with its vector (see also circulative transmission, propagative transmission) (contrasts with nonpersistent transmission, stylet-borne transmission)
pest
any organism that damages plants or plant products
pesticide
a chemical used to kill pests
pesticide label
a legal document approved by the U.S. Environmental Protection Agency that defines uses and provides other safety information for a commercial pesticide
petiole
the stalk portion of a leaf ((contrasts with lamina)
pH
the negative logarithm of the effective hydrogen ion concentration; a measure of acidity (pH 7 is neutral; values less than pH 7, acidic; greater than pH 7, alkaline or basic)
phellem
cork; a protective tissue composed of nonliving cells with suberized walls produced by the phellogen and accumulating on the surface of stems and roots
phelloderm
the tissue formed by and internal to the phellogen; resembles the cortical parenchyma in morphology
phellogen
the cork cambium; lateral meristem forming the periderm, a protective tissue in stems and roots; phellem (cork) is produced toward the surface, phelloderm toward the inside
phenological synchrony
the coordinated development of parasites and their hosts based on degree-days and other environmental factors such that parasites can successfully infect their hosts (e.g., the production and release of ascospores of Venturia inaequalis, the apple scab pathogen, from fallen leaves from the previous season, when new apple leaves are expanding)
phenotype
the expressed characteristics of an organism determined by the interaction of its genotype with the environment (contrasts with genotype)
pheromone
a chemical substance that attracts members of the same species or one sex of that species, esp. insects and nematodes
phialide
an end cell of a conidiophore with one or more open ends through which a basipetal succession of conidia develops
phloem
the food-conducting, food-storing tissue in the vascular system of roots, stems, and leaves
phloem necrosis
the death of phloem cells, often visible, caused by infection by systemic phloem pathogens such as phytoplasmas, spiroplasmas, and viruses
photochemical reaction
a chemical reaction that requires sunlight
photosynthate
a product of photosynthesis
photosynthesis
the manufacture of carbohydrates from carbon dioxide and water in the presence of chlorophyll(s), using light energy and releasing oxygen
phycomycetes
an archaic term for a group of fungi without cross walls (septa) in their mycelium
phyllody
the change of floral organs to leaflike structures
phylloplane
the leaf surface
phylloplane-competent
pertaining to epiphytic microorganisms adapted to living on the leaf surface
phyllotaxy
the arrangement of leaves on a stem in relation to one another
phytoalexin
a substance produced in higher plants in response to a number of chemical, physical, and biological stimuli that inhibits the growth of certain microorganisms
phytopathogenic
able to cause disease in plants
phytopathology
the study of plant diseases (see also plant pathology)
phytoplasma
a plant-parasitic pleomorphic mollicute (prokaryote with no cell wall) found in phloem tissue; cannot yet be grown on artificial nutrient media (contrasts with mycoplasmalike organism, acronym MLO)
phytosanitary certificate
an official document that indicates that plant material has been inspected and found to be free of certain pathogens
phytotoxic
harmful to plants (usually used to describe chemicals)
pigment
a colored compound, such as chlorophyll, in the cells of plants or fungi
pistil
the ovule-bearing organ of the plant consisting of the ovary and its appendages (e.g., style, stigma)
pith
the parenchymatous tissue occupying the center of the stem
plant pathology
the study of plant diseases (se also phytopathology)
plasmid
a circular, self-replicating hereditary element that is not part of a chromosome; plasmids are used in recombinant DNA experiments as acceptors and vectors of foreign DNA
plasmodesma (pl. plasmodesmata)
a cytoplasmic strand that connects living plant cells
plasmodiophoromycete
an informal term for a member of the Plasmodiophoromycota
Plasmodiophoromycota
a funguslike group of organisms, sometimes called the endoparasitic slime molds, characterized by the production of zoospores and plasmodia that are restricted to the cells of their host; plasmodiophoromycetes
plasmodium (pl. plasmodia)
a naked, multinucleate mass of protoplasm moving and feeding in amoeboid fashion
plasmogamy
the fusion of two sex cells
plasmolysis
the shrinking of a protoplast due to water loss from the vacuole of a plant cell
pleomorphic
able to assume various shapes (and perhaps sizes); pertaining to a life cycle in which an organism has more than one distinct form
ploidy
the number of (complete) sets of chromosomes in a cell, e.g., haploid, diploid, polyploid
polar
at one end or pole of the cell, as for flagella (contrasts with peritrichous)
pollen
the male sex cells produced by anthers of flowering plants or within the male cones of seed plants that produce them
pollination
the transfer of pollen from anther to stigma or from a staminate cone to an ovulate cone
polyclonal antibodies
a mixture of antibodies produced by different antibody-producing cells against more than one epitope of an antigen (contrasts with monoclonal antibodies)
polyculture
production of multiple crops in the same space, in imitation of the diversity of natural ecosystems (contrasts with monoculture)
polycyclic
having several to many disease cycles in a growing season (contrasts with monocyclic)
polyetic
requiring more than one year to complete one life or disease cycle
polygenic
pertaining to, or governed by, many genes (contrasts with monogenic)
polygenic resistance
a resistance conferred by several genes (see also multigenic resistance) (contrasts with monogenic resistance, oligogenic resistance, single gene resistance)
polymerase chain reaction (acronym PCR)
a technique used to amplify the number of copies of a specific region of DNA in order to produce enough of the DNA for use in various applications such as identification and cloning
polyploid
having three or more complete sets of chromosomes (contrasts with haploid, diploid)
polyprotein
a protein translated from an entire viral genome, which is then cleaved by proteases (protein-degrading enzymes) into the active protein products
pome
a simple, fleshy, indehiscent fruit derived from several carpels, e.g., apple, pear, quince
positive sense RNA (+RNA)
the RNA that can serve directly as messenger RNA (contrasts with negative sense RNA)
post-transcriptional gene silencing (acronym PTGS)
a mechanism for sequence-specific RNA degradation in plants, resulting in the turning off of a gene; used as a host plant defense against viruses by degrading viral RNAs created during replication
posterior
toward the back or rear (contrasts with anterior)
powdery mildew
a common name for a disease caused by a white, powdery, superficial ascomycetous fungus that is an obligate parasite
predispose (n. predisposition)
to make prone to infection and disease
primary host
sometimes used to refer to the telial host of a heteroecious rust fungus, and sometimes used to refer to the most economically important host (contrasts with alternate host)
primary inoculum
the inoculum, usually from an overwintering source, that initiates disease in the field, as opposed to inoculum that spreads disease during the season (see also initial inoculum) (contrasts with secondary inoculum)
primary leaf
the first true leaf that emerges on a plant following the cotyledons
primary pollutant
an air pollutant that is released directly into the atmosphere and is harmful to plants, e.g., SO2 or NOx (contrasts with secodary pollutant)
primary root
a root that develops directly from the radicle of an embryo rather than from a crown or node
primer
a small fragment of nucleic acid with a free 3'-hydroxyl group necessary for initiation of DNA and, sometimes, RNA synthesis; often specific fragments chosen for use in polymerase chain reaction (PCR) for rapid identification of pathogens
proinhibitin
a plant defense compound that is already present before invasion by a pathogen
prokaryote
an organism without internal membrane-bound organelles, lacking a distinct nucleus, such as bacteria and mollicutes (contrasts with eukaryote)
promycelium (pl. promycelia)
in rust and smut fungi, a germ tube issuing from the teliospore and bearing the basidiospores
propagative transmission
a type of pathogen transmission characterized by a long period of acquisition of the pathogen (usually a mollicute, e.g., phytoplasma or spiroplasma, and sometimes a virus) by a vector (typically an insect), a latent period before the vector is able to transmit the pathogen, and retention of the pathogen by the vector for a long period because the pathogen reproduces or replicates in the vector (see also circulative transmission, persistent transmission) (contrasts with nonpersistent transmission, stylet-borne transmission)
propagule
any part of an organism capable of independent growth
protectant
an agent, usually a chemical, applied to a plant surface in advance of a pathogen to prevent infection
protectant fungicide
a fungicide that remains on the surface where it is applied and prevents infection often through the inhibition of spore germination; no after-infection activity (see also contact fungicide) (contrasts with systemic fungicide)
protection
various methods of plant disease management, including cultural practices that create barriers or reduce the chance of infection, chemical protection, methods of biological control that protect plants, and genetic resistance.
protein
a nitrogen-containing organic compound composed of units called amino acids
protoplasm
the living contents of a cell
protoplast
a living cell exclusive of a wall
proximal
nearest to the point of attachment (contrast with distal)
pseudothecium (pl. pseudothecia)
a peritheciumlike fruiting body containing bitunicate asci in unwalled locales or cavities
PTGS (acronym for post-transcriptional gene silencing)
a mechanism for sequence-specific RNA degradation in plants, resulting in the turning off of a gene; used as a host plant defense against viruses by degrading viral RNAs created during replication
Puccinia pathway
the region through which rust urediniospores move each season from southern areas through all grain-producing areas of the United States to Canada
pupa (pl. pupae; v. pupate)
the quiescent stage between the larva and the adult of certain insects
pustule
a small, blisterlike elevation of epidermis formed as spores emerge
pv. (abbr. for pathovar)
a subdivision of a plant pathogenic bacterial species defined by host range; pathovar for bacteria is equivalent to forma specialis for fungi
pycnidiospore
a spore (conidium) produced in a pycnidium
pycnidium (pl. pycnidia)
an asexual, globose or flask-shaped fruiting body containing conidia
pycniospore
a haploid, sexually derived spore formed in a pycnium of a rust fungus (see also spermatium)
pycnium (pl. pycnia)
a globose or flask-shaped haploid fruiting body of a rust fungus bearing receptive hyphae and pycniospores (see also spermagonium)
pyramiding
the addition, through plant breeding or genetic engineering, of several resistance genes into a single plant cultivar
Q
qualitative resistance
resistance reactions that can be placed in distinct categories, usually conferred by one or a few genes (contrasts with quantitative resistance)
quantitative resistance
resistance reactions that have no distinct classes but vary continuously from resistant to susceptible, the result of few to many genes, the individual effects of which may be small and difficult to detect (contrasts with qualitative resistance)
quarantine
the legislative control of the transport of plants or plant parts to prevent the spread of pests or pathogens
quiescent
dormant or inactive
quorum sensing
the ability of bacteria to interact with each other through a variety of mechanisms; allows a population of bacteria to behave more like a multicellular organism
R
race
a subgroup or biotype within a species or variety, distinguished from other races by virulence, symptom expression, or host range, but not by morphology
rachis
the elongated main axis of an inflorescence
radicle
the part of the plant embryo that develops into the primary root
race-nonspecific resistance
a resistance that is effective against all biotypes of the pathogen. (see also durable resistance, general resistance, horizontal resistance) (contrasts with specific resistance, vertical resistance)
ray
the tissue that extends radially in the secondary xylem and phloem of a woody plant
receptacle
the structure of a flower that bears the reproductive organs
receptive hypha
the part of a rust fungus pycnium (spermogonium) that receives the nucleus of a pycniospore (spermatium)
receptor
a site that recognizes and binds an elicitor; any organ or molecular site that is sensitive to a distinct (specific) signal molecule
recessive
describes a phenotypic trait that is expressed in diploid organisms only if both parents contribute the trait to the progeny (contrasts with dominant)
recombinant DNA (acronym rDNA)
DNA molecules in which sequences, not normally contiguous, have been placed next to each other by in vitro methods
reniform
kidney-shaped
replicase
an enzyme involved in the replication (copying) of nucleic acid
replication
the process by which a virus particle induces the host cell to reproduce the virus; copying or multiplication of DNA
resin
a sticky to brittle plant product derived from essential oils; often a defense compound of conifers
resinosis
a disease symptom in which there is a copious flow of resin from a wound or infection site of a conifer
resistant (n. resistance)
possessing properties that prevent or impede disease development (contrasts with susceptible)
respiration
a series of chemical reactions that make energy available through oxidation of carbohydrates and fat
resting spore
a spore, often thick-walled, that can remain alive in a dormant state for some time, later germinating and capable of initiating infection
restriction endonuclease
an enzyme that cleaves DNA at a particular base sequence; sometimes informally referred to as a restriction enzyme
reticulate
having netlike markings
reverse transcriptase
an enzyme used to make complementary DNA from a piece of RNA, such as a plant virus genome
Rhizobium
a genus of bacteria that live symbiotically with roots of leguminous plants; during the symbiosis, atmospheric nitrogen gas is converted into a form useable by the bacterium and the plant
Rhizobium nodules
the galls on roots caused by Rhizobium spp.
rhizome
a mostly horizontal, jointed, fleshy, often elongated, usually underground stem
rhizomorph
a macroscopic ropelike strand of compacted tissue formed by certain fungi
rhizosphere
the microenvironment in the soil, immediately around roots
rhizosphere-competent
used to describe microorganisms adapted to living in the rhizosphere of a plant
ribonucleic acid (acronym RNA)
any of several nucleic acids composed of repeating units of ribose (a sugar), a phosphate group, and a purine (adenine or guanine) or a pyrimidine (uracil or cytosine) base; transcribed from DNA and involved in translation to proteins
ribosomal RNA (acronym. rRNA)
RNA molecules forming part of the ribosomal structure
ribosome
a subcellular protoplasmic particle, made up of one or more RNA molecules and several proteins, involved in protein synthesis
ringspot
a disease symptom characterized by yellowish or necrotic rings enclosing green tissue, as in some plant diseases caused by viruses
RNA (acronym for ribonucleic acid)
any of several nucleic acids composed of repeating units of ribose (a sugar), a phosphate group, and a purine (adenine or guanine) or a pyrimidine (uracil or cytosine) base; transcribed from DNA and involved in translation to proteins
RNA interference (acronym RNAi)
a process within living cells in which a double-stranded complementary RNA targets a specific messenger RNA for destruction, blocking the function of (silencing) the gene from which the mRNA was transcribed
RNAi (acronym for RNA interference)
a process within living cells in which a double-stranded complementary RNA targets a specific messenger RNA for destruction, blocking the function of (silencing) the gene from which the mRNA was transcribed
rogue
to remove and destroy individual plants that are diseased, infested by insects, or otherwise undesirable
root cap
a group of cells on a root that protects the growing tip
root exudate
one of the various compounds that leak from growing and expanding sections of roots as well as from broken cells at exit points of lateral roots
root graft
the fusion of roots of two adjacent plants so that their water- and food-conducting (vascular) systems become joined
root hair
a threadlike, single-celled outgrowth from a root epidermal cell
rootstock
the portion of the stem (trunk) and associated root system into which a bud or scion is inserted in grafting; fleshy overwintering part of a herbaceous perennial plant with buds and eyes (contrasts with scion)
rosette
a disease symptom characterized by short, bunchy growth habit due to shortened internodes and no comparable reduction in leaf size
rot
the softening, discoloration, and often disintegration of plant tissue as a result of fungal or bacterial infection
rotation
the growth of different kinds of crops in succession in the same field
roundworm
a nematode
rRNA (acronym for ribosomal RNA)
RNA molecules forming part of the ribosomal structure
rugose
wrinkled, roughened
runner
a slender, horizontal stem that grows close to the soil surface (see also stolon)
runner plant
a new plant produced asexually on a runner or stolon
russet
brownish, roughened areas resulting from cork formation
rust
a disease caused by a specialized group of the Basidiomycota (the basidiomycetes) that often produces spores of a rusty color
