Illustrated Glossary of Plant Pathology S

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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sanitation
destruction or removal of infected and infested plants or plant parts; decontamination of tools, equipment, containers, work space, hands, etc.

sap transmission
transmission, usually of viruses, by rubbing sap from an infected plant onto a healthy plant to cause infection

saprobe (syn. saprophyte)
organism that obtains nourishment from non-living organic matter

saprophyte (adj. saprophytic; syn. saprobe)
organism that obtains nourishment from non-living organic matter

sapwood
physiologically active zone of wood contiguous to cambium (see heartwood)

scab
roughened, crustlike diseased area on the surface of a plant organ

scald
a necrotic condition in which tissue is usually bleached and has the appearance of having been exposed to high temperatures

scion
portion of a shoot used for grafting onto the root stock (see rootstock)

sclerenchyma (adj. sclerenchymatous)
tissue made up of thick-walled plant cells

sclerotium (pl. sclerotia)
a vegetative resting body of a fungus, composed of a compact mass of hyphae with or without host tissue, usually with a darkened rind

scorch
any symptom that suggests the action of flame or fire on the affected part, often seen at the margins of leaves

secondary infection
infection resulting from the spread of infectious material produced after a primary infection or from secondary infections without an intervening inactive period

secondary inoculum
inoculum produced by infections that took place during the same growing season

secondary metabolite
a compound produced in microbes (e.g., mycotoxins, syringomycins) or plants (e.g., caffeine or nicotine) that is not necessary for normal growth and development

secondary organism
organism that multiplies in already diseased tissue but is not the primary pathogen

secondary pollutant
air pollutant that must be chemically produced from other air pollutants, e.g. ozone (O₃) that is a product of a photochemical reaction of exhaust products from combustion engines in the atmosphere

secondary root
branch from a primary root

sedentary
remaining in a fixed location (see migratory)

seed
ripened ovule consisting of an embryo and stored food enclosed by a seed coat

seed treatment
application of a biological agent, chemical substance, or physical treatment to seed, to protect the seed or plant from pathogens or to stimulate germination or plant growth

seedborne
carried on or in a seed

selective medium
a culture medium containing substances that specifically inhibit or prevent the growth of some species of microorganisms

self-fertilization
a process in which sexual reproduction occurs as a result of the fusion of sex cells produced by the same individual (see cross-fertilization)

senesce (adj. senescent, n. senescence)
to decline, as with maturation, age, or disease stress

sepal
one of the modified leaves comprising a calyx

septate
with cross walls; having septa

septum (pl. septa; adj. septate)
dividing wall; in fungi, cross wall

serology (adj. serologic)
a method using the specificity of the antigen-antibody reaction for the detection and identification of antigenic substances and the organisms that carry them

serrate
edges with teeth, like a saw

sessile
used in reference to a leaf, leaflet, flower, floret, fruit, ascocarp, basidiocarp, etc., without a stalk, petiole, pedicel, stipe or stem; (of nematodes) permanently attached; not capable of moving about

seta (pl. setae)
bristle or hair-like structure, usually deep yellow or brown and thick-walled

severity values
a means to quantify accumulating opportunities for pathogen infection (e.g. infection periods) to a pre-determined threshold that requires a disease management activity (e.g. a fungicide application); used in disease prediction or forecasting

sexual reproduction
reproduction involving fusion of two haploid nuclei (karyogamy) to form a diploid nucleus followed by meiosis (reduction division) back to haploid nuclei at some point in the life cycle, resulting in genetic recombination

sexual spore
spore produced during the sexual cycle

sexually compatible
able to be cross-mated or cross-fertile

shot-hole
symptom in which small lesions fall out of leaves, giving the leaf the appearance of being hit by buckshot

sieve element (syn. sieve tube element)
a tube-shaped living cell in the phloem functioning in the transport of dissolved organic substances in the plant

sign
indication of disease from direct observation of a pathogen or its parts (see symptom)

single gene resistance (syn. monogenic resistance)
resistance conferred by a single gene

slime molds (syn. Myxomycetes)
saprophytic organisms that form vegetative amoeboid plasmodia and spores

smut
a disease caused by a smut fungus (Ustilaginales) in the Basidiomycota or the fungus itself; it is characterized by masses of dark brown or black, dusty to greasy masses of teliospores that generally accumulate in black, powdery sori

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

soil drench
application of a solution or suspension of a chemical to the soil, especially pesticides to control soilborne pathogens

soilborne
carried on or beneath the soil surface

soil inhabitant
an organism that maintains its population in soil over a period of time

soil invader
an organism whose population in soil diminishes in several months to years

soil pasteurization
process used to free soil of selected harmful microorganisms using heat

soil sterilization
process used to free soil of all microorganisms

solarization
disease control practice in which soil is covered with polyethylene sheeting and exposed to sunlight, thereby heating the soil and controlling soilborne plant pathogens

sooty mold
black, nonparasitic, superficial fungal growth on honeydew produced by aphids and other phloem-feeding insects

sorus (pl. sori)
compact fruiting compact fruiting structure, especially the erumpent spore mass in the rust fungi (Uredinales) and smut fungi (Ustilaginales); occasionally a group of fruiting bodies as in Synchytriaceae; a cluster of sporangia on a fern sporophyte

sp. (abbr. for species; pl. spp.)
a genus name followed by sp. means that the particular species is undetermined; spp. after a genus name means that several species are being referred to

species
any one kind of life subordinate to a genus but above a race; a group of closely related individuals of the same ancestry, resembling one another in certain inherited characteristics of structure and behavior and relative stability in nature; the individuals of a species ordinarily interbreed freely and maintain themselves and their characteristics in nature

specific epithet
the second word in a Latin binomial

specific resistance (syn. vertical resistance)
resistance which is effective against some biotypes or races of the pathogen, but not others, usually inherited monogenically and expressed qualitatively. (see general resistance, horizontal resistance, race-nonspecific resistance)

spermagonium (pl. spermagonia; syn. pycnium for rust fungi)
structure in which male reproductive cells are produced; in rust fungi, globose or flask-shaped haploid fruiting body composed of receptive hyphae and spermatia (pycniospores)

spermatium (pl. spermatia; syn. pycniospore for rust fungi)
a male sex cell; a nonmotile male gamete; a haploid male gamete

spicule
copulatory organ of male nematode

spike
botanical term for an unbranched inflorescence in which flowers (or spikelets of grasses) are attached directly, without petioles, to a central stem. Examples: wheat, gladiolus

spikelet
spike-like appendage comprised of one or more reduced flowers and associated bracts; unit of inflorescence in grasses; a small spike

spiroplasma
spiral-shaped plant pathogenic mollicute (prokaryote without cell wall)

spontaneous generation, theory of
the theory, now known to be invalid, that plants, animals and microorganisms arose suddenly from non-living materials under certain environmental conditions

sporangiophore
sporangium-bearing body of a fungus

sporangiospore
non-motile, asexual spore that is borne in a sporangium

sporangium (pl. sporangia)
saclike fungal structure in which the entire contents are converted into an indefinite number of asexual spores

spore
reproductive structure of fungi and some other organisms, containing one or more cells; a bacterial cell modified to survive an adverse environment

sporidium (pl.sporidia)
basidiospore of rust fungi, smut fungi, and other Basidiomycota

sporocarp
spore-bearing fruiting body

sporodochium (pl.sporodochia)
superficial, cushion-shaped asexual fruiting body consisting of a cluster of conidiophores

sporophore
a spore-producing or spore-bearing structure such as a conidiophore, ascocarp or basidiocarp

sporophyte
the diploid stage of a plant (see gametophyte)

sporulate
to produce spores

spot
a symptom of disease characterized by a limited necrotic area, as on leaves, flowers, and stems

springwood
early part of the yearly xylem growth ring in woody plants consisting typically of cells that are larger than those formed later in the season (summerwood)

stabilizing selection
the theorized competitive disadvantage of unnecessary virulence genes; races with excess genes would have decreased fitness relative to races with fewer virulence genes, so a "super-race" would be less likely to appear in multiline crops

staghead
defoliated, dead or dying major branches in the crown of a tree, usually resulting from inadequate water uptake or translocation

stamen (adj. staminal)
male structure of a flower, composed of a pollen-bearing anther and a filament, or stalk

stele
central cylinder of vascular tissue (especially in roots)

stem pitting
a viral disease symptom characterized by depressions on the stem

sterigma (pl. sterigmata)
small, usually pointed projection that supports a spore

sterile
unable to reproduce sexually;
to be free of living microorganisms

sterile fungus
a fungus that is not known to produce any kind of spores

sterilization (adj. sterilized)
the total destruction of living organisms by various means, including heat, chemicals or irradiation

stigma
portion of a flower that receives pollen and on which the pollen germinates

stipe
stalk

stippling
series of small dots or speckles in which chlorophyll is absent

stipule
small leaflike appendage at the base of a leaf petiole, usually occurring in pairs

stolon (syn. runner)
a slender, horizontal stem that grows close to the soil surface; in fungi, a hypha that grows horizontally along the surface

stoma (pl. stomata; adj. stomatal; also stomate)
structure composed of two guard cells and the opening between them in the epidermis of a leaf or stem, functioning in gas exchange

stone fruit
fruit with a stony endocarp, e.g. cherry, peach, plum

strain
a distinct form of an organism or virus within a species, differing from other forms of the species biologically, physically, or chemically

Stramenopila (Straminipila):
a kingdom including the Oomycota or oomycetes, along with brown and golden algae and diatoms. "Stramenopila" is the originally proposed spelling, whereas "Straminipila" has been proposed as more correct in form on the basis of the Latin root of the term

striate (n. striations)
marked with delicate lines, grooves, or ridges

stroma (pl. stromata)
compact mass of mycelium (with or without host tissue) that supports fruiting bodies or in which fruiting bodies are embedded

stunting
reduction in height of a vertical axis resulting from a progressive reduction in the length of successive internodes or a decrease in their number

style
slender part of many pistils located between the stigma and the ovary and through which the pollen tube grows

stylet
stiff, slender, hollow feeding organ of plant-parasitic nematodes or sap-sucking insects, such as aphids or leafhoppers

stylet knob (syn. basal knob)
structure at the base of a nematode stylet

stylet-borne transmission (syn. 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

suberize
to convert into cork tissue

subgenomic RNA
a piece of viral RNA, shorter than the entire genome of the virus, found in cells infected by the virus and sometimes encapsidated

subspecies
a subpopulation of a species, defined on the basis of more than one character (morphologic for many organisms) that distinguishes the members of the subpopulation from other members of that species

substrate
the substance on which an organism lives or from which it obtains nutrients; chemical substance acted upon, often by an enzyme

sulfur dioxide (SO₂)
a primary air pollutant produced in industrial processes and coal burning that causes interveinal necrosis on broadleaf plants and tip necrosis on conifers

summerwood
part of the yearly xylem growth ring in woody plants formed late in the growing season and consisting of cells smaller than those of springwood

sunscald or sunburn
injury of plant tissues burned or scorched by direct sun or or sunburn

suppressive soil
soil in which various diseases are naturally at lower levels than expected due to biological factors in the soil; an example of natural biological control

suscept
an abbreviated term for a susceptible plant

susceptible (n. susceptibility)
prone to develop disease when infected by a pathogen (see resistance)

symbiosis (adj. symbiotic; n. symbiont)
mutually beneficial association of two different kinds of organisms

sympodial
pertaining to proliferation of axes, in which each successive spore or branch develops behind and to one side of the previous apex where growth has ceased

symptom
indication of disease by reaction of the host, e.g. canker, leaf spot, wilt (see sign)

symptomless carrier
a plant that, although infected with a pathogen (usually a virus), produces no obvious symptoms

syncytium (pl. syncytia)
a multinucleate structure in root tissue formed by dissolution of common cell walls induced by secretions of certain sedentary plant-parasitic nematodes, e.g. cyst nematodes

synergism (adj. synergistic)
greater than additive effect of interacting factors

synnema (pl. synnemata; syn. coremium)
compact or fused, generally upright conidiophores, with branches and spores forming a headlike cluster

systematics
the study of the kinds of organisms and the relationships between them

systemic
pertaining to a disease in which the pathogen (or a single infection) spreads generally throughout the plant; pertaining to chemicals that spread internally through the plant

systemic acquired resistance (SAR)
reduced disease symptoms on a portion of a plant distant from the area where a hypersensitive response occurred or other stimulus was applied; a rapid and coordinated defense response against a variety of pathogens as a signal travels throughout the plant (see induced systemic resistance)

systemic fungicide
a fungicide that is absorbed into plant tissue and may offer some curative or after-infection activity; includes fungicides that are locally systemic, xylem-mobile (upward moving), and amphimobile (move in phloem upward as well as downward in the plant) (see contact or protectant fungicide)

taproot
primary root that grows vertically downward and from which smaller lateral roots branch

taxonomy (adj. taxonomic)
the science dealing with naming and classifying organisms

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

teliospore (sometimes called teleutospore, teleutosporodesm)
thick-walled resting or overwintering spore produced by the rust fungi (Uredinales) and smut fungi (Ustilaginales) in which karyogamy occurs; it germinates to form a promycelium (basidium) in which meiosis occurs

telium (pl. telia)
fruiting body (sorus) of a rust fungus that produces teliospores

temporary wilt
wilt due to insufficient soil water from which a plant can recover when water is supplied

teratogen
a chemical that causes malformations in the fetus

testa (pl.testae)
seed coat

thallus
vegetative body of a fungus

thatch
tightly intertwined layer of plant litter from accumulations of undecomposed or partially decomposed plant residues

thermotherapy
use of heat to reduce or eliminate pathogens in plant tissue; often used on plants prior to meristem culture to produce pathogen-free plants

thorax
insect body part between the head and abdomen

tillage
the process of turning or stirring the soil

tiller
a lateral shoot, culm, or stalk arising from a crown bud; common in grasses

tissue
group of cells, usually of similar structure, that perform the same or related functions

tissue culture
in vitro method of propagating healthy cells from plant tissues

titer
concentration of a virus

tolerance (adj. tolerant)
ability of a plant to endure an infectious or noninfectious disease, adverse conditions, or chemical injury without serious damage or yield loss; (of pesticides) the amount of chemical reside legally permitted on an agricultural product entering commercial channels and usually measured in parts per million (ppm)

totipotency
the concept that even specialized cells contain all of the genetic information for an organism and, therefore, any cell should be able to regenerate into any tissue or into an entire plant

toxicity
capacity of a substance to interfere with the vital processes of an organism

toxin
poisonous substance of biological origin

tracheid
elongated conducting cell of the xylem, with tapering or oblique end walls and pitted walls

transcription
the production of a complementary strand of RNA from a segment of DNA

transduction
the transfer of genes from one organism to another by viruses, especially in bacteria

transfer RNA (abbr. tRNA)
the RNA that moves amino acids to the ribosome to be placed in the order prescribed by the messenger RNA

transformation
the transfer of genetic materials from one organism to another by humans (genetic engineering); a means of genetic variation in bacteria by absorption and incorporation of DNA from another bacterial cell

transgenic (syn. genetically modified organism; GMO)
possessing a gene from another species; used to describe the organisms that have been the subject of genetic engineering

translation
the assembling of amino acids into a protein using messenger RNA, ribosomes and transfer RNA

translocation
movement of water, nutrients, chemicals, or food materials within a plant

translucent
so clear that light may pass through

transmit (n. transmission)
to spread or transfer, as in spreading an infectious pathogen from plant to plant or from one plant generation to another

transpiration
water loss by evaporation from leaf surfaces and through stomata

trap crop
crop planted around a field to protect the inner crop from diseases transmitted by aerial vectors; host crop of a parasitic plant, such as witchweed (Striga spp.), that is planted to stimulate seed germination, and later sacrified by plowing under before the parasitic plant produces new seeds

trenching
physical separation of soil in a vertical plane to sever grafted roots between trees

trichogyne
a female receptive hypha

trichome
a plant epidermal hair, of which several types exist

triplet codon
a set of three nucleotide bases in DNA or RNA that code for an amino acid

tRNA (abbr. for transfer RNA)
the RNA that moves amino acids to the ribosome to be placed in the order prescribed by the messenger RNA

tuber
an underground stem adapted for storage, typically produced at the end of a stolon

turgidity
state of being rigid or swollen as a result of internal water pressure

tumor (syn. gall)
abnormal swelling or localized outgrowth, often roughly spherical, produced by a plant as a result of attack by a fungus, bacterium, nematode, insect or other organism

tylosis (pl. tyloses)
balloonlike extrusions of parenchyma cells into lumina of contiguous vessels that partially or completely block them

type
the example on which the description of a scientific name is based, and which fixes the application of the name (the type genus of a family, or the type species of a genus)

ultrastructure
submicroscopic structure of a macromolecule, cell, or tissue

unicellular
one-celled (see multicellular)

uniflagellate
having one flagellum

unitunicate
having a single ascus wall (see bitunicate)

uninucleate
having one nucleus (see multinucleate)

urediniospore (also urediospore, uredospore)
the asexual, dikaryotic, often rusty-colored spore of a rust fungus, produced in a structure called a uredinium; the "repeating stage" of a heteroecious rust fungus, i.e. capable of infecting the host plant on which it is produced

uredinium (also uredium; pl. uredinia)
fruiting body (sorus) of rust fungi that produces urediniospores

vacuole
generally spherical organelle within a plant cell bound by a membrane and containing dissolved materials such as metabolic precursors, storage materials, or waste products

variegation
pattern of two or more colors in a plant part, as in a green and white leaf

variety (syn. cultivar; adj. varietal)
a plant type within a species, resulting from deliberate manipulation, which has recognizable characteristics (color, shape of flowers, fruits, seeds, height and form)

vascular
pertaining to fluid-conducting (xylem and phloem) tissues in plants

vascular bundle
strand of conductive tissue, usually composed of xylem and phloem (in leaves, small bundles are called veins)

vascular cylinder (syn. stele)
the cylinder of vascular tissue in stems or roots

vascular wilt disease
a xylem disease that disrupts normal uptake of water and minerals, resulting in wilting and yellowing of foliage

vector
a living organism (e.g., insect, mite, bird, higher animal, nematode, parasitic plant, human) able to carry and transmit a pathogen and disseminate disease;
(in genetic engineering) a vector or cloning vehicle is a self-replicating DNA molecule, such as a plasmid or virus, used to introduce a fragment of foreign DNA into a host cell

vegetative
referring to somatic or asexual parts of a plant, which are not involved in sexual reproduction

vegetative propagation
asexual reproduction; in plants, the use of cuttings, bulbs, tubers, and other vegetative plant parts to grow new plants

vein
small vascular bundle in a leaf

vein banding
symptom of virus disease in which regions along veins are either darker green or distinctly more yellow than tissue between veins

vein clearing
disappearance of green color in or around leaf veins

vermiform
worm-shaped

vertical resistance (syn. specific resistance)
resistance which is effective against some biotypes or races of the pathogen, but not others, usually inherited monogenically and expressed qualitatively (see general resistance, horizontal resistance, race non-specific resistance)

Vertifolia effect
a term used to describe the loss of general (horizontal, minor gene, multigenic, polygenic) resistance in a cultivar after several generations of selection during which a major gene confers resistance to the dominant race or biotype of the pathogen; first observed in the potato cultivar Vertifolia with late blight resistance

vesicle
thin sac in which zoospores are differentiated and released;
the bulbous head terminating the conidiophores of Aspergillus;
structure formed by endomycorrhizal fungi within living cells of the root

vessel
water-conducting structure of xylem tissue with pit openings in end walls

viable (n. viability)
the state of being alive; able to germinate, as seeds, fungus spores, sclerotia, etc.; capable of growth

virescence
state or condition in which normally white or colored tissues (e.g. flower petals) become green

virion
complete virus particle

viroid
an infectious, nonencapsidated (naked) circular, single-stranded RNA

viroplasm
cellular inclusions that are sites of synthesis of viral components and the assembly of virus particles

virulence
degree or measure of pathogenicity; relative capacity to cause disease

virulent
highly pathogenic; having the capacity to cause severe disease (see avirulent)

viruliferous
virus-laden, usually applied to insects or nematodes as vectors

virus
a submicroscopic,intracellular, obligate parasite consisting of a core of infectious nucleic acid (either RNA or DNA) usually surrounded by a protein coat

viscin
sticky substance produced on the seed coat of parasitic flowering plants in the Viscaceae (mistletoes) that helps attach the seed to the host plant branch

volunteer
self-set plant; plant seeded by chance

vulva
exterior opening of a mature female nematode's reproductive system

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