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.

E  |  F   |  G  |  H

E

echinulate  
having small spines projecting from cell walls

ecology
the study of the interactions that occur between individual organisms, groups of organisms and organisms and their environment

economic threshold
the pathogen density at or above which the value of crop losses (in the absence of management efforts) would exceed the cost of management practices

ecosystem
community of living things and their environment

ectomycorrhiza (pl. ectomycorrhizae)  
symbiotic association between a nonpathogenic or weakly pathogenic fungus and the roots of plants with fungal hyphae between and external to root cells (see arbuscular mycorrhiza)

ectoparasite  
parasite that feeds from the exterior of its host (see endoparasite)

ectotrophic  
fungal development primarily over the root surface

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

effector
a pathogen molecule, usually a protein, that is translocated into host cells where it may act to directly manipulate host innate immunity

egg mass  
group of eggs held together by a gelatinous matrix

electron microscope
microscope that uses a focused beam of electrons to produce a greatly enlarged image of minute objects, such as a virus, in the same way that light is used in a compound microscope

elicitor
a molecule produced by the host (or pathogen) that induces a response by the pathogen (or host)

ELISA (acronym for Enzyme-Linked ImmunoSorbent Assay)  
a serological test in which the sensitivity of the reaction is increased by attaching an enzyme that produces a colored product to one of the reactants

embryo
an organism in the early stages of development, such as a young plant in the seed, or a nematode before hatching from the egg

emergence
growth of the seedling shoot through the surface of the soil

enation  
abnormal outgrowth from the surface of a stem or leaf

encapsidate
to cover virus nucleic acid with a protein coat

encyst
to form a cyst or protective covering

endemic
native to a particular place; pertaining to a low and steady level of natural disease occurrence

endocarp
inner layer of a fruit wall

endoconidium (pl. endoconidia)  
a conidium produced inside a hypha or conidiophore

endodermis  
layer of cells within the root, between the vascular tissue and the cortex

endogenous
arising from the inside (see exogenous)

endomycorrhiza (pl. endomycorrhizae; syn. arbuscular mycorrhiza) 
symbiotic association between a nonpathogenic or weakly pathogenic fungus and the roots of plants in which fungal hyphae invade cortical cells of the root (see ectomycorrhiza)

endoparasite  
parasitic organism that lives and feeds from inside its host (see ectoparasite)

endophyte  
plant developing inside another organism; also used for endoparasitic fungi found in grass species

endoplasmic reticulum
system of interconnected cytoplasmic membranes that transports materials within the cell

endosperm
nutritive tissue formed within the embryo sac of seed plants

endospore
resistant, thick-walled asexual spore formed within a bacterial or fungal cell

enzyme
protein that catalyzes a specific biochemical reaction

enzyme-linked immunosorbent assay (acronym ELISA) 
a serological test in which the sensitivity of the reaction is increased by attaching an enzyme that produces a colored product to one of the reactants

EPA (acronym for Environmental Protection Agency) 
the federal agency responsible for controlling the various aspects of air, water, and soil pollution, including pesticide regulations

epicotyl  
portion of the stem of a plant embryo or seedling above the node where the cotyledons are attached (see hypocotyl)

epidemic
an increase of disease in a population; a general and serious outbreak of disease (see epiphytotic)

epidemiology (adj. epidemiologic) 
the study of factors influencing the initiation, development, and spread of infectious disease; the study of disease in populations of plants

epidermis (adj. epidermal) 
surface layer of cells of leaves and other plant parts

epinasty  
abnormal, downward curling of a leaf, leaf part, or stem

epiphytic
living on the surface of plants, but not as a parasite

epiphytotic
an epidemic in a plant population

epiphyllous
on the upper surface of a leaf

epiphyte
organism growing on a plant surface, but not as a parasite

epitope
an amino acid (or other) sequence that effects formation of an antibody

EPS (extracellular polysaccharides)  
sugar polymers that contribute to the slimy appearance of bacteria probably promote colonization of plant tissues and disease development in bacteria

eradication 
control of plant disease by eliminating the pathogen after it is established or by eliminating the plants that carry the pathogen

ergot  
a disease of certain grasses and cereals, especially rye, caused by Claviceps spp.;
a sclerotium, or resting structure, produced by Claviceps species and other closely related fungi in infected flowers of parasitized grain plants

ergotism 
a disease caused by ingestion of grain contaminated with alkaloids of ergot fungi, often called the holy fire or St. Anthony's fire in the Middle Ages

erumpent 
bursting or erupting through the substrate surface

escape
failure of inherently susceptible plants to become diseased, even though disease is prevalent

ethylene
a plant hormone influencing various aspects of vegetative growth, fruit ripening, abscission of plant parts, and the senescence of flowers

etiolation
elongation of stems caused by reduced light intensities

etiology
the study of the causes of diseases

eukaryote
an organism containing a membrane-bound nucleus and other organelles, i.e. all higher plants, animals, fungi, and protists (see prokaryote)

exclusion  
control of disease by excluding the pathogen or infected plant material from crop production areas (e.g. by quarantines and embargoes)

exogenous
originating from the outside (see endogenous)

extracellular
outside a cell

extracellular polysaccharides (EPS)  
sugar polymers that aid in movement, help prevent desiccation, and contribute to the slimy appearance of bacteria, fungi, and nematodes; and probably promote colonization of plant tissues and disease development in bacteria

extrachromosomal inheritance(also cytoplasmic or maternal inheritance) 
inheritance of genes not located in the nucleus, i.e. those in mitochondria and chloroplasts

exudate  
liquid excreted or discharged from diseased tissues, from roots and leaves, or by fungi

F

f. sp. (abbr. for forma specialis) 
a taxonomic group within a pathogenic species defined in terms of host range, i.e. members of different formae speciales infect different groups of plants

facultative
capable of changing life-style, e.g. from saprophytic to parasitic or the reverse

facultative parasite
organism that is normally saprophytic but is capable of being parasitic

facultative saprophyte
organism that is normally parasitic but is capable of being saprophytic

fallow
cultivated land kept free from a crop or weeds during the normal growing season

fasciation  
malformation in shoots or floral organs manifested as enlargements and flattening as if several parts were fused

fascicle (adj. fasciculate) 
small group, bundle, or cluster

fastidious  
in reference to prokaryotic organisms, having special growth and nutritional requirements

feeder root
fine root that absorbs water and dissolved nutrients

fermentation
oxidation of certain organic substances in the absence of molecular oxygen

FFDCA (acronym for Federal Food, Drug, and Cosmetic Act) 
the federal law which sets legal tolerances of pesticides on food products at harvest

fiber
elongated plant cell with thick walls (in xylem and phloem), dead at maturity

FIFRA (acronym for Federal Insecticide, Fungicide, and Rodenticide Act) 
the federal law which legislates the conditions for use of pesticides

filamentous(syn. filiform)  
threadlike

fission
form of cell division in prokaryotes

fitness
the ability of an organism to survive and reproduce;
the ability of an organism to pass its genes to the next generation

flaccid  
wilted, lacking turgor

flag  
isolated, wilted or necrotic branch with dead leaves attached

flagellum (pl. flagella) 
hairlike, whiplike, or tinsel-like appendage of a motile cell, bacterium or zoospore that provides locomotion

fleck  
minute, discolored spot in green tissue

flexuous  
having turns or windings; capable of bending

flower break  
break or stripe in flower color (see breaking)

focus (pl. foci) 
small area of diseased plants within a population

foliar
pertaining to leaves

forest decline  
combination of biotic factors (e.g. insect pests, pathogens) and abiotic factors (e.g. drought, air pollution) that contributes to poor growth and death of populations of trees

form genus
a genus based on morphology and not on evolutionary relationships, such as used for imperfect fungi

forma specialis (abbr. f.sp.; pl. formae speciales) 
a taxonomic group within a pathogenic species defined in terms of host range, i.e. members of different formae speciales infect different groups of plants

FQPA (acronym for Food Quality Protection Act) 
the federal law which sets a health-based standard governing pesticide tolerance levels in food

free water
unbound water; often used to describe a film of water on a plant surface

fructification  
general term for spore-bearing organs in both macro- and microfungi

fruiting body  
any of various complex, spore-bearing fungal structures

fumigant (v. fumigate) 
a gas or volatile substance used to kill or inhibit the growth of microorganisms or other pests

Fungi Imperfecti (syn. Deuteromycetes) 
group of fungi without a sexual stage; also, the asexual stage of ascomycetes and basidiomycetes

fungicide (adj. fungicidal)
chemical or physical agent that kills or inhibits the growth of fungi

fungistasis  
inhibition of fungal growth, sporulation, or spore germination but not death; used to describe the nonspecific phenomenon in natural soils where spore germination is inhibited and often overcome by rhizosphere nutrients

fungistat (adj. fungistatic) 
a chemical or physical agent that inhibits fungal growth, sporulation, or spore geminiation, but does not cause death

fungus (pl. fungi)
a eukaryotic organism that is usually filamentous (forming a mycelium) and heterotrophic, has cell walls composed of chitin, and reproduces by sexual and/or asexual spores

fusiform  
spindle-shaped; tapering at each end

G

gall (syn. tumor) 
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

gallery  
insect tunnel in bark and wood

gametangium (pl. gametangia) 
cell containing gametes or nuclei that act as gametes

gamete
sex cell

gametophyte  
the haploid stage of a plant (see sporophyte)

gene
unit within an organism controlling heritable characteristics; genes are organized on chromosomes

gene gun (see biolistic transformation)  
a device used to bombard plant cells with metal particles coated with foreign genes to accomplish genetic engineering

gene silencing
the switching off of a gene by a process other than genetic modification; one example is the disabling of a gene by small interfering RNAs (siRNAs) that target messenger RNA for destruction

gene-for-gene hypothesis
the hypothesis that corresponding genes for resistance and virulence exist in the host and pathogen, respectively

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

genetic
relating to heredity; referring to heritable characteristics

genetic code
the system of triplet codons composed of nucleotides of DNA or RNA that determine the amino acid sequence of a protein

genetic engineering
the transfer of specific genes between organisms using enzymes and laboratory techniques rather than biological hybridization

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

genome
the complete genetic information of an organism or virus

genotype
genetic constitution of an individual or group; class or group of individuals sharing a specific genetic makeup (see phenotype)

genus (pl. genera) 
a taxonomic category that includes a group of closely related (structurally or phylogenetically) species; the genus or generic name is the first name in a Latin binomial

geocarp
fruit that ripens beneath the ground (e.g. peanut pod)

germ theory
the theory that infectious or contagious diseases are caused by microorganisms (germs)

germ tube  
hypha resulting from an outgrowth of the spore wall and cytoplasm after germination

germinate (n. germination) 
begin growth (as of a seed, spore, sclerotium, or other reproductive body)

germplasm
bearer of heredity material, often loosely applied to cultivars and breeding lines

giant cell  
enlarged, multinucleate cell formed in roots by repeated nuclear division without cell division induced by secretions of certain sedentary plant-parasitic nematodes

gibberellins
plant hormones (growth regulators) that affect stem elongation

gill  
thin, radial membrane producing basidiospores in the cap of certain basidiomycetous fungi (e.g. mushrooms)

girdle  
to circle and cut through a stem or the bark and outer few rings of wood, disrupting the phloem and xylem

glabrous
smooth; without hairs

glucoside
a substance that, on decomposition, yields glucose and certain other compounds; some glucosides are defense compounds produced by plants, e.g. cyanogenic or phenolic glucosides

gonad
sexual structure in animals that produces gametes

graft  
transfer of aerial parts of one plant (e.g. buds or twigs - the scion) into close cambial contact with the root or trunk (the rootstock) of a different plant; a method of plant propagation; the joining of cut surfaces or growing roots of two plants to form a living union

graft transmission
transmission of a pathogen from one host plant to another through fusion of living tissue from the diseased host with living tissue of a healthy host

Gram-negative 
bacteria staining red or pink in the Gram staining procedure after treatment with Gram's stain

Gram-positive  
bacteria staining violet or purple in the Gram staining procedure after treatment with Gram's stain

Gram stain
procedure used for identification of bacteria in which crystal violet stain, Gram's iodine, ethyl alcohol and safranin stain are applied in succession to cells of the bacteria

gravid
containing an egg or eggs; capable of depositing eggs

green manure
a crop plowed under while still green and growing to improve the soil; sometimes used to enhance populations of antagonistic microorganisms for biological control

growth regulator (syn. hormone) 
a chemical substance produced in one part of an organism and transported in minute quantities to induce a growth response in another part, e.g. in plants, auxins, cytokinins, and gibberellins

guard cells  
paired, specialized, epidermal cells that contain chloroplasts and surround a stoma

gum  
gelatinous, sugary aggregate that is synthesized and exuded by plant tissues

gummosis (pl. gummoses) 
pathologic condition characterized by excessive formation of gums; the products of cell degeneration

guttation  
exudation of watery, sticky liquid from hydathodes, especially along leaf margins

H

habitat
a place with a particular kind of environment suitable for the growth of an organism

haploid
having a single complete set of chromosomes (see diploid, polyploid)

hardiness
ability to withstand stress

haulm
stem or stalks collectively

haustorium (pl. haustoria) 
specialized branch of a parasite formed inside host cells to absorb nutrients

head
in plants, a type of inflorescence in which numerous small flowers are densely crowded on a disk or around a central stalk

heartwood  
central cylinder of nonfunctional xylem in a woody stem (see sapwood)

hemiparasite  
parasitic flowering plant that contains chlorophyll when mature (photosynthetic) and obtains water with dissolved nutrients by connecting to the host xylem via the haustorium, e.g. true mistletoe (Phoradendron spp. or Viscum spp.) or witchweed (Striga spp.)

herbaceous
describing primary, soft, nonwoody tissue, as a plant or plant part; having the characteristics of an herb

herbicide  
a chemical used for killing plants or inhibiting plant growth, e.g. a weed or grass killer

heritability
a measure of the ability of a characteristic to be passed from one generation to the next

hermaphrodite (adj. hermaphroditic) 
having both male and female reproductive organs

heteroecious
pertaining to a rust fungus requiring two unrelated host plants for completion of its life cycle (see autoecious)

heterokaryon (adj. heterokaryotic) 
cell with genetically different nuclei

heterosis (syn. hybrid vigor) 
increased vigor of hybrid offspring when compared to either parent

heterothallism (adj. heterothallic) 
condition in which sexual reproduction can occur only in the presence of genetically different mycelia (see homothallism)

heterotroph
an organism that obtains nourishment from outside sources and must obtain its carbon from organic carbon compounds (see autotroph)

heterozygous
having alternate forms (allele) of a gene on homologous chromosomes (see homozygous)

holomorph
the whole fungus in all its states (see anamorph, teleomorph)

holoparasite  
parasitic flowering plant that lacks chlorophyll (nonphotosynthetic) and must rely totally on the contents of the xylem and the phloem of the host plant, e.g. broomrape (Orobanche spp.)

homothallism (adj. homothallic) 
condition in which sexual reproduction occurs with a single thallus; self-fertile (see heterothallism)

homozygous
having the same form (allele) of a gene on homologous chromosomes (see heterozygous)

honeydew 
sugary ooze or exudate, often from aphids, and a characteristic symptom of ergot

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

hormone (syn. growth regulator) 
a chemical substance produced in one part of a an organism and transported in minute quantities to induce a growth response in another part, e.g. in plants, auxins, cytokinins, and gibberellins

host plant
living plant attacked by or harboring a parasite or pathogen and from which the invader obtains part or all of its nourishment

host range
the range of plants on which an organism, particularly a parasite, feeds

hyaline  
transparent or nearly so; translucent; often used in the sense of colorless

hybrid (v. hybridize) 
offspring of two individuals of different genotypes

hybrid vigor (syn. heterosis) 
increased vigor of hybrid offspring when compared to either parent

hybridoma
a cell produced by the fusion of an antibody-producing cell and a lymphoma (cancer) cell for production of monoclonal antibodies

hydathode 
epidermal leaf structure specialized for secretion or exudation of water; leaf opening at terminus of vein

hydrogen fluoride (HF)  
a common pollutant from volcanoes and several industries such as brick factories, aluminum smelters, and phosphate fertilizer factories.

hymenium  
continuous, spore-bearing layer of a fungus fruiting body

hyperparasite  
organism that is parasitic upon another parasite

hyperplasia (adj. hyperplastic) 
abnormal increase in the number of cells, often resulting in the formation of galls or tumors

hypersensitive  
extremely or excessively sensitive; often refers to an extreme reaction to a pathogen

hypersensitive reaction and pathogenicity (hrp) gene
gene required for elicitation of the hypersensitive (HR) response in resistant plants and causation of disease in susceptible plants; a type of avirulence gene

hypersensitive response (HR)  
rapid and localized cell death at the site of infection in resistant interactions between plants and pathogens

hypertrophy (adj. hypertrophic)  
abnormal increase in the size of cells in a tissue or organ, often resulting in the formation of galls or tumors

hypha (pl. hyphae; adj. hyphal)  
single, tubular filament of a fungal thallus or mycelium; the basic structural unit of a fungus

hyphal sheath (syn. mantle)  
dense hyphal mass of ectomycorrhizal fungus enclosing the short feeder roots of plants

hyphomycetes
imperfect fungi that produce conidia freely or on a sporodochium or synnema (see coelomycetes)

hyphopodium  
stalked, thick-walled, lobed cells that stick to plant surfaces; sometimes used to describe the infection structures produced by ectotrophic hyphae of certain root-infecting fungi such as Gaeumannomyces (take-all pathogen)

hypocotyl  
portion of the stem below the cotyledons and above the root (see epicotyl)

hypovirulence
reduced ability to cause disease

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