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Illustrated Glossary of Plant Pathology

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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

a parasite that typically kills host cells and obtains its energy from them (contrasts with biotroph)

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)

an agent, usually a chemical, that kills nematodes

a nonsegmented roundworm (animal) that is parasitic on plants or animals, or free living in soil or water

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

the scheme by which names are attached to objects, including organisms

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)

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)

without cross walls (see also aseptate, coenocytic)

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)

a region in prokaryotes, chloroplasts, and mitochondria where the DNA is concentrated. Unlike a nucleus, it is not bound by a membrane.

a dense aggregation of proteinaceous matter and nucleic acid in cells, surrounded by a membrane; contains chromosomes and controls heredity

a subunit of a nucleic acid, either DNA or RNA


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)

a block or plug that stops the flow of liquids (as in vessels)

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)

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

an informal term for a member of the Oomycota

a group of funguslike organisms - typically with nonseptate mycelia, asexual sporangia and zoospores, and sexual oospores - in the kingdom Stramenopila (or Straminipila); oomycetes

a thick-walled, sexually-derived resting spore of the Oomycota (the oomycetes)

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

a membrane-bound structure within a cell having a specialized function, e.g., mitochondrion and chloroplast

describing a molecule containing carbon atoms; pertaining to living organisms

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)

the female reproductive structure of organisms; in plants, enlarged basal portion of a pistil, containing the ovules and developing into the fruit

to survive or persist from one planting season to the next

to survive or persist through the winter period

to deposit or lay eggs with an ovipositor

an enclosed structure that, after fertilization, becomes a seed; an egg contained within an ovary

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


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

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

having the antheridium contact the oogonium on the side, as in many Pythium spp. (contrasts with amphigynous)

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)

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)

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

the ability to cause disease

the study of diseases

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

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

a small slender stalk; stalk bearing an individual flower, inflorescence, or spore

a stalk or main stem of an inflorescence; part of an inflorescence, or a fructification

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)

an antibiotic produced by certain species of Penicillium and effective against gram-positive bacteria

something that occurs year after year; a plant that survives for several to many years (contrasts with annual, biennial)

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)

a fruit wall

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

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)

any organism that damages plants or plant products

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

the stalk portion of a leaf ((contrasts with lamina)

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)

cork; a protective tissue composed of nonliving cells with suberized walls produced by the phellogen and accumulating on the surface of stems and roots

the tissue formed by and internal to the phellogen; resembles the cortical parenchyma in morphology

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)

the expressed characteristics of an organism determined by the interaction of its genotype with the environment (contrasts with genotype)

a chemical substance that attracts members of the same species or one sex of that species, esp. insects and nematodes

an end cell of a conidiophore with one or more open ends through which a basipetal succession of conidia develops

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

a product of photosynthesis

the manufacture of carbohydrates from carbon dioxide and water in the presence of chlorophyll(s), using light energy and releasing oxygen

an archaic term for a group of fungi without cross walls (septa) in their mycelium

the change of floral organs to leaflike structures

the leaf surface

pertaining to epiphytic microorganisms adapted to living on the leaf surface

the arrangement of leaves on a stem in relation to one another

a substance produced in higher plants in response to a number of chemical, physical, and biological stimuli that inhibits the growth of certain microorganisms

able to cause disease in plants

the study of plant diseases (see also plant pathology)

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

harmful to plants (usually used to describe chemicals)

a colored compound, such as chlorophyll, in the cells of plants or fungi

the ovule-bearing organ of the plant consisting of the ovary and its appendages (e.g., style, stigma)

the parenchymatous tissue occupying the center of the stem

plant pathology  
the study of plant diseases (se also phytopathology)

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

an informal term for a member of the 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

the fusion of two sex cells

the shrinking of a protoplast due to water loss from the vacuole of a plant cell

able to assume various shapes (and perhaps sizes); pertaining to a life cycle in which an organism has more than one distinct form

the number of (complete) sets of chromosomes in a cell, e.g., haploid, diploid, polyploid

at one end or pole of the cell, as for flagella (contrasts with peritrichous)

the male sex cells produced by anthers of flowering plants or within the male cones of seed plants that produce them

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)

production of multiple crops in the same space, in imitation of the diversity of natural ecosystems (contrasts with monoculture)

having several to many disease cycles in a growing season (contrasts with monocyclic)

requiring more than one year to complete one life or disease cycle

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

having three or more complete sets of chromosomes (contrasts with haploid, diploid)

a protein translated from an entire viral genome, which is then cleaved by proteases (protein-degrading enzymes) into the active protein products

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

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

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

a plant defense compound that is already present before invasion by a pathogen

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)

any part of an organism capable of independent growth

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)

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.

a nitrogen-containing organic compound composed of units called amino acids

the living contents of a cell

a living cell exclusive of a wall

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

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

a spore (conidium) produced in a pycnidium

pycnidium   (pl. pycnidia)    
an asexual, globose or flask-shaped fruiting body containing conidia

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)

the addition, through plant breeding or genetic engineering, of several resistance genes into a single plant cultivar


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)

the legislative control of the transport of plants or plant parts to prevent the spread of pests or pathogens

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


a subgroup or biotype within a species or variety, distinguished from other races by virulence, symptom expression, or host range, but not by morphology

the elongated main axis of an inflorescence

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)

the tissue that extends radially in the secondary xylem and phloem of a woody plant

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)

a site that recognizes and binds an elicitor; any organ or molecular site that is sensitive to a distinct (specific) signal molecule

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


an enzyme involved in the replication (copying) of nucleic acid

the process by which a virus particle induces the host cell to reproduce the virus; copying or multiplication of DNA

a sticky to brittle plant product derived from essential oils; often a defense compound of conifers

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)

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

having netlike markings

reverse transcriptase
an enzyme used to make complementary DNA from a piece of RNA, such as a plant virus genome

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.

a mostly horizontal, jointed, fleshy, often elongated, usually underground stem

a macroscopic ropelike strand of compacted tissue formed by certain fungi

the microenvironment in the soil, immediately around roots

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

a subcellular protoplasmic particle, made up of one or more RNA molecules and several proteins, involved in protein synthesis

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

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

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)

a disease symptom characterized by short, bunchy growth habit due to shortened internodes and no comparable reduction in leaf size

the softening, discoloration, and often disintegration of plant tissue as a result of fungal or bacterial infection

the growth of different kinds of crops in succession in the same field

a nematode

rRNA (acronym for ribosomal RNA)
RNA molecules forming part of the ribosomal structure

wrinkled, roughened

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

brownish, roughened areas resulting from cork formation

a disease caused by a specialized group of the Basidiomycota (the basidiomycetes) that often produces spores of a rusty color