Illustrated Glossary of Plant Pathology N

Authors: C.J. D'Arcy, D.M. Eastburn, and G. L. Schumann
(with acknowledgement of contributions from M.C. Shurtleff, P.A. Arneson, F.H. Tainter, and T.A. Evans)

D'Arcy, C. J. , D. M. Eastburn, and G. L. Schumann. 2001. Illustrated Glossary of Plant Pathology. The Plant Health Instructor. DOI: 10.1094/PHI-I-2001-0219-01

NOTE: If you have comments on any term, definition, or image or would like another term or image added to this glossary, please contact Anton Baudoin, Editor-in-Chief, at abaudoin@vt.edu. ALSO NOTE: each defined word is bookmarked for specific reference. To find out more about linking to a particular word, click here.

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necrosis (adj. necrotic)
death of cells or tissue, usually accompanied by black or brown darkening

necrotroph
a parasite that typically kills and obtains its energy from dead host cells (see biotroph)

nectary
a nectar-secreting gland in a flower

needle cast (of conifers)
disease symptom caused by fungi resulting in premature drop of needles

negative sense RNA
ribonucleic acid complimentary to the positive or plus sense; not translated into protein (see positive sense RNA)

nematicide
agent, usually a chemical, that kills nematodes

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

nitrogen cycle
the cycling of the element nitrogen from gaseous forms to various inorganic forms, such as ammonium, nitrates and nitrites, and organic forms, such as nucleic acids and proteins and back to elemental nitrogen

nitrogen fixation
the conversion of gaseous N₂ to a form available for plant uptake

nitrogen oxides
primary air pollutants produced from internal combustion engines and other industrial processes

node (adj. nodal)
enlarged portion of a shoot at which leaves or buds arise

nodule (v. nodulation)
small knot or irregular, rounded lump; on leguminous plants, structures on roots that contain nitrogen-fixing bacteria

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

noninfectious disease
a disease that is caused by an abiotic agent that cannot be transmitted from one plant to another

nonpathogenic (syn. avirulent)
unable to cause disease

nonpersistent transmission (syn. stylet-borne 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

nonseptate
without cross-walls; 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 (the old crop is not removed or incorporated into the soil as is common in preparing a plant bed)

nucleic acid
genetic material of all living organisms, including DNA (deoxyribonucleic acid) and RNA (ribonucleic acid)

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

nucleus
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

obligate parasite (syn. biotroph)
organism that can grow only as a parasite in association with its host plant and cannot be grown in artificial culture media

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

oedema (also edema; syn. intumescence)
blisters produced on leaves and other plant parts under conditions of high moisture and restricted transpiration

oligogenic resistance
resistance conferred by a few genes (see monogenic resistance, multigeneic resistance, polygenic resistance, single gene resistance)

oogonium (pl. oogonia)
female gametangium of oomycetes, containing one or more gametes

oomycetes (adj. oomycetous)
group of fungal-like organisms typically with nonseptate mycelium, asexual sporangia and zoospores, and sexual oospores, now comprising the phylum Oomycota in the kingdom Stramenopila (or Straminipila)

oospore
thick-walled, sexually-derived resting spore of oomycetes

ooze
mass of bacterial cells mixed with host fluids

open reading frame
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. mitochondria and chloroplasts

organic
describing a molecule containing carbon atoms; pertaining to living organisms

ostiole (adj. ostiolate)
pore; opening in the papilla or neck of a perithecium, pseudothecium, or pycnidium through which spores are released

outerbark
exterior, nonliving portion of bark in woody plants

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
enclosed structure that, after fertilization, becomes a seed; 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

palisade parenchyma
tissue found beneath the upper epidermis of leaves, composed of elongate, tubular cells arranged upright in the manner of posts in a palisade fortification

PAN (abbr. for peroxyacetyl nitrates)
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 simultaneously in several countries

papilla
nipple-like projection; used to describe the tip of some sporangia and 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 sp. (see 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)
organism that lives in intimate association with another organism on which it depends for its nutrition; not necessarily a pathogen

parenchyma (adj. parenchymatous)
soft tissue of living plant cells with undifferentiated, thin, cellulose walls

parthenogenesis (adj. parthenogenetic)
reproduction by the development of an unfertilized egg

partial resistance
resistance in which disease develops more slowly on the host or to a lesser extent, but the host does become diseased (see complete resistance)

pasteurization
to free a material, usually a liquid, of selected harmful microorganisms using heat

pathogen (adj. pathogenic)
a disease-producing organism or agent

pathogenesis
production and development of disease

pathogenesis-related (PR) proteins
proteins, such as antimicrobial proteins and hydrolytic enzymes, that are synthesized in the early events of the plant defense response

pathogenicity
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; jelly-forming substance found in fruit

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

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

penetration
initial invasion of a host by a pathogen

penetration peg (syn. infection peg)
the specialized, narrow, hyphal strand on the underside of an appressorium that penetrates host cells

penicillin
antibiotic produced by certain species of Penicillium and effective against Gram-positive bacteria

perennial
something that occurs year after year; plant that survives for several to many years (see annual, biennial)

perfect
sexual; capable of sexual reproduction (see teleomorph)

perfect flower
flower possessing both stamens and pistils

perfect state (syn. teleomorph)
the sexual form in the life cycle of a fungus (see anamorph)

pericarp
fruit wall

pericycle
layer or layers of cells between the phloem and the endodermis of roots, giving rise to branch roots

perithecium (pl. perithecia)
flask-shaped or subglobose, thin-walled fungus fruiting body (ascocarp) containing 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

peroxyacetal nitrates (PAN)
a secondary air pollutant formed when the exhaust of automobiles and other internal combustion engines is in the presence of sunlight

persistent transmission (syn. circulative transmission)
a type of virus transmission in which the virus is acquired and transmitted by the vector after relatively long feeding times and remains transmissible for a prolonged period while in association with its vector

pest
any organism that damages plants or plant products

pesticide
a chemical used to control pests

petiole
stalk portion of a leaf (see lamina)

pH
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)

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

phellogen
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 so that parasites can successfully infect their hosts, such as 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
external, visible physical characteristics of an organism determined by the interaction of its genotype with the environment (see genotype)

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

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

phloem
food-conducting, food-storing tissue in the vascular system of roots, stems, and leaves

phloem necrosis
death of phloem cells, often visible, caused by infection by systemic phloem pathogens such as phytoplasmas, spiroplasmas, and viruses

photochemical
chemical reactions that require sunlight

photosynthate
product of photosynthesis

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

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

phyllody
change of floral organs to leaflike structures

phylloplane
leaf surface

phylloplane-competent
used to describe epiphytic microorganisms adapted to living on the leaf surface

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

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

phytopathogenic
able to cause disease in plants

phytopathology (syn. plant pathology)
the study of plant diseases

phytoplasma (syn. mycoplasmalike organism, MLO)
plant-parasitic pleomorphic mollicute (prokaryote with no cell wall) found in phloem tissue; cannot yet be grown on artificial nutrient media

phytosanitary certificate
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
parenchymatous tissue occupying the center of the stem

plant pathology (syn. phytopathology)
the study of plant diseases

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)
cytoplasmic strands that connect living cells

plasmodiophoromycetes
a fungus-like group of organisms, sometimes called the Plasmodiophoromycota or the endoparasitic slime molds, characterized by the production of zoospores and plasmodia that are restricted to the cells of their host.

plasmodium (pl. plasmodia)
naked multinucleate mass of protoplasm moving and feeding in amoeboid fashion

plasmogamy
the fusion of two sex cells

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

pleomorphic
able to assume various shapes (and perhaps sizes); occurrence of more than one distinct form in the life cycle of an organism

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

pollen
male sex cells produced by anthers of flowering plants or the cones of seed plants

pollination
transfer of pollen from anther to stigma or from a staminate cone to an ovulate cone

polyclonal antibody
a preparation containing antibodies against more than one epitope of an antigen (see monoclonal antibody)

polycyclic
having several to many disease cycles in a growing season (see monocyclic)

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

polygenic
pertaining to, or governed by, many genes (see monogenic)

polygenic resistance (syn. multigenic resistance)
resistance conferred by several genes (see monogenic resistance, oligogenic resistance, single gene resistance)

polymerase chain reaction (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 uch as identification and cloning

polyploid
having three or more complete sets of chromosomes (see 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 (syn. pome fruit)
simple, fleshy, indehiscent fruit derived from several carpels, e.g. apple, pear, quince

positive sense RNA
RNA that can serve directly as messenger RNA (see negative sense RNA)

posterior
toward the back or rear; (see anterior)

powdery mildew
a white, powdery, superficial ascomycetous fungus that is an obligate parasite and generally is found only on one or a few closely related species of plants

predispose (n. predisposition)
to make prone to infection and disease

primary inoculum (syn. initial inoculum)
inoculum, usually from an overwintering source, that initiates disease in the field, as opposed to inoculum that spreads disease during the season

primary leaf
first true leaf that emerges on a plant following the cotyledons

primary pollutant
air pollutant that is released directly into the atmosphere and is harmful to plants, e.g. SO₂ or NO_x

primary root
root that develops directly from the radicle of an embryo rather than from a crown or node

primer
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 (see eukaryote)

promycelium (pl. promycelia)
in rust and smut fungi, a germ tube issuing from the teliospore and bearing the basidiospores

propagative transmission (syn. circulative propagative transmission)
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

propagule
any part of an organism capable of independent growth

protectant
agent, usually a chemical, applied to a plant surface in advance of a pathogen to prevent infection

protectant fungicide (syn. contact fungicide)
a fungicide that remains on the surface where it is applied; no after-infection activity (see systemic fungicide)

protection
a principle of plant disease control in which a barrier is placed between the susceptible plant and pathogen (e.g. the use of protective chemical dusts or sprays)

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

protoplasm
living contents of a cell

protoplast
living cell exclusive of a wall

proximal
nearest to the point of attachment (see distal)

pseudothecium (pl. pseudothecia)
perithecium-like fruiting body containing asci and ascospores dispersed rather than in an organized hymenium; an ascostroma with a single locule or cavity and containing bitunicate asci

Puccinia pathway
the region through which rust urediniospores move from southern areas through all grain-producing areas of the United States to Canada each season

pupa (pl. pupae; v. pupate)
quiescent stage between the larva and the adult of certain insects

pustule
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
spore (conidium) produced in a pycnidium

pycnidium (pl. pycnidia)
asexual, globose or flask-shaped fruiting body of certain imperfect fungi producing conidia

pycniospore (syn. spermatium)
haploid, sexually derived spore formed in a pycnium of rust fungi

pycnium (pl. pycnia; syn. spermagonium)
globose or flask-shaped haploid fruiting body of rust fungi bearing receptive hyphae and pycniospores

pyramiding
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. (see 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. (see qualitative resistance)

quarantine
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

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

rachis
elongated main axis of an inflorescence

radicle
part of the plant embryo that develops into the primary root

race non-specific resistance (syn. general resistance, horizontal resistance)
resistance which is effective against all biotypes of the pathogen. (see durable resistance, specific resistance, vertical resistance)

ray
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 (see dominant)

recombinant DNA
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 DNA or RNA molecule makes an exact copy of itself;
repetition of an experiment or procedure at the same time and place (one of several identical experiments, procedures, or samples);
the process by which a virus particle induces the host cell to reproduce the virus

resin
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 (see susceptible)

respiration
series of chemical reactions that make energy available through oxidation of carbohydrates and fat

resting spore
spore, often thick-walled, that can remain alive in a dormant state for some time, later germinating and capable of initiating infection

restriction endonuclease
enzyme that cleaves DNA at a particular base sequence; sometimes informally referred to as a restriction enzyme

reticulate
having netlike markings

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

Rhizobium
genus of bacteria that lives symbiotically with roots of leguminous plants; during the symbiosis, atmospheric nitrogen gas is converted into a form useable by the plant

Rhizobium nodules
galls on roots caused by Rhizobium spp.

rhizomorph
macroscopic rope-like strand of compacted tissue formed by certain fungi

rhizosphere
microenvironment in the soil, immediately around plant roots

rhizosphere-competent
used to describe microorganisms adapted to living in the rhizosphere of a plant

ribonucleic acid (abbr. RNA)
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

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

ribosomal RNA (abbr. 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
disease symptom characterized by yellowish or necrotic rings enclosing green tissue, as in some plant diseases caused by viruses

RNA (abbr. for ribonucleic acid)
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

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 exudates
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
threadlike, single-celled outgrowths from a root epidermal cell

rootstock
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 (see scion)

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

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

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

roundworm
nematode

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

rugose
wrinkled, roughened

runner (syn. stolon)
a slender horizontal stem that grows close to the soil surface

runner plants
new plants produced asexually on a runner or stolon

russet
brownish, roughened areas resulting from cork formation

rust
a disease caused by a specialized group of basidiomycetes that often produces spores of a rusty color

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