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.

I  |   J  |   K  |   L  |   M

I

icosahedral (n. icosahedron)  
having 20 faces, as a polyhedral-shaped virus particle (see isometric)

immune
cannot be infected by a given pathogen

immunoassay
a detection method based on antibodies specifically selected to react with the substance to be detected (the antigen)

immunosupressant
chemical or activity that suppresses the natural immune responses in animals, including humans

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

imperfect state (syn. anamorph)
the asexual form in the life cycle of a fungus, when asexual spores (such as conidia) or no spores are produced

in planta
in a plant

in situ
in its original place or environment

in vitro
in glass, on artificial media, or in an artificial environment; outside the host

in vivo
within a living organism

inclusion body  
structure developed within a plant cell as a result of infection by a virus, often useful in identifying the virus

incomplete dominance
the interaction of alleles of a gene that produces an intermediate phenotype, as in the production of pink flowers when red and white alleles are present in a heterozygous individual

incubation period
the time between penetration of a host by a pathogen and the first appearance of disease symptoms; the time during which microorganisms inoculated onto a medium are allowed to grow

indehiscent
pertaining to fruit that does not split open at maturity

indeterminate  
continuing to grow vegetatively while producing flowers or reproductive structures

indexing  
testing of a plant for infection, often by mechanical transmission or by grafting tissue from it to an indicator plant

indicator plant  
plant that reacts to a pathogen or an environmental factor with specific symptoms, used to detect or identify the pathogen or determine the effects of the environmental factor

indirect penetration  
penetration of plant tissues by a pathogen through natural openings (e.g. stomata) or wounds

induced
produced in response to a stimulus (see constitutive)

induced systemic resistance (ISR)
reduced disease symptoms on a portion of a plant distant from the area where the inducing agent is active, caused by the triggering of active plant defenses against a variety of pathogens; used to describe increased resistance in plants induced by certain rhizobacteria (see systemic acquired resistance- SAR)

infect
process in which an organism enters, invades, or penetrates and establishes a parasitic relationship with a host plant

infection court
site in or on a host plant where infection can occur

infection cushion
organized mass of hyphae formed on the surface of a plant from which numerous infective hyphae develop

infection focus  
initial site of infection, generally with reference to a population of plants

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

infection period
the time required for infection to occur under conducive environmental conditions, usually hours of leaf wetness and temperature

infectious
pertaining to disease, capable of spreading from plant to plant

infective
referring to an organism able to attack a host and cause infection; referring to a vector carrying or containing a pathogen and able to transfer it to a host plant

infest (n. infestation)  
to attack as a pest (used especially of insects and nematodes); to contaminate, as with microorganisms; to be present in large numbers

inflorescence
flower or flower cluster

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

injury  
damage caused by transitory interaction with an agent such as an insect, chemical, or unfavorable environmental condition

inner bark
interior, living portion of the bark; secondary phloem

inoculate (n. inoculation)
to place inoculum in an infection court; to insert a pathogen into healthy tissue

inoculum (pl. inocula)
pathogen or its parts, capable of causing infection when transferred to a favorable location

inoculum density
a measure of the number of propagules of a pathogenic organism per unit area or volume

insect
member of the class Hexapoda (phylum Arthropoda) possessing three sets of limbs attached to a central body segment

instar
one of the stages of growth between molts, in the life history of certain insects

integrated pest management (abbr. IPM) 
a combination of strategies to reduce losses due to pests and pathogens based on environmental and economic considerations

intercellular
between or among cells

intercrop
to grow two or more crops simultaneously on the same area of land

internode (adj. internodal) 
the portion of a stem between two successive nodes

internal transcribed spacers
regions of a DNA sequence that separate genes for certain ribosomal RNAs (rRNAs). These genes appear to be nearly identical among a wide variety of species, but the DNA spacers between them are quite variable and may be species-specific. This makes them good potential targets for "primers" for polymerase chain reaction (PCR) amplification that is used to identify a wide variety of organisms.

interveinal  
between (leaf) veins

intracellular
through or within cells

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

IPM (abbr. for integrated pest management) 
a combination of strategies to reduce losses due to pests and pathogens based on environmental and economic considerations

isolate
(n.) a culture or subpopulation of a microorganism separated from its parent population and maintained in some sort of controlled circumstance;
(v.) to remove from soil or host material and grow in pure culture

isometric  
usually used for virus particles to describe those that are icosahedral in structure and appear approximately round

ITS (acronym for internal transcribed spacers)
regions of a DNA sequence that separate genes for certain ribosomal RNAs (rRNAs). These genes appear to be nearly identical among a wide variety of species, but the DNA spacers between them are quite variable and may be species-specific. This makes them good potential targets for "primers" for polymerase chain reaction (PCR) amplification that is used to identify a wide variety of organisms

J

juvenile  
an immature form that appears similar to but usually smaller than the adult and is not sexually mature (e.g. insects with gradual metamorphosis, nematodes)

K

karyogamy
the fusion of nuclei

Koch's postulates
the procedure used to prove the pathogenicity of an organism, i.e. its role as the causal agent of a disease

knot  
gall; localized abnormal swelling

L

lamina  
expanded part of a leaf (see petiole)

land race
plant stocks selected by farmers on a local basis over many years

larva (pl. larvae)  
immature stage of certain animals (especially insects) that undergo complete metamorphosis; the wormlike or caterpillar stage of the life cycle of such insects

latent
present but not manifested or visible, as a symptomless infection by a pathogen

latent infection
infection unaccompanied by visible symptoms

latent period
the time between infection and the production of new inoculum; the time after a vector has acquired a pathogen and before it can be transmitted

lateral bud (syn. axillary bud)  
bud that develops in the axil of a leaf

LD50 (abbr. for lethal dose 50%)
a measure of relative acute toxicity; the lethal dose of a compound that causes death in 50% of the test animals treated

leaf dip
inspection of a sample of sap from a suspected virus-infected leaf using an electron microscope

leaf sheath
lower, tubular part of a grass leaf that clasps the culm

leaf spot  
a plant disease lesion typically restricted in development in the leaf after reaching a characteristic size

leafhopper  
mobile insects with sucking mouthparts (order Homoptera)

leaflet  
one of the separate blades or divisions of a compound leaf

leafroll  
disease symptom in which the edges of the leaf roll or turn up; often a symptom of virus infection

legume
simple, dry, dehiscent fruit developed from a simple pistil and splitting at maturity along two seams;
a plant of the family Fabaceae (formerly Leguminosae)

lenticel
a natural opening in the surface of a stem or tuber, fruit or root for gas exchange

lesion  
localized diseased area or wound

life cycle
the cyclical stages in the growth and development of an organism

lignification
hardening of tissue through the deposition of lignin in the cell wall

lignin
complex organic substance or group of substances that impregnate the cell walls of xylem vessels and certain other plant cells; constitutes wood

local lesion  
small, restricted lesion, often the characteristic reaction of differential cultivars to specific pathogens, especially in response to mechanical inoculation with a virus

lodge  
to fall over

lumen (pl. lumina) 
central cavity of a cell or other structure

M

macerate
to cause disintegration of tissues by separation of cells; to soften by soaking

macroconidium (pl. macroconidia)  
the larger of two kinds of conidia formed by certain fungi (see microconidium)

macrocyclic
a rust fungus that typically exhibits all five stages of the rust life cycle (see demicyclic, microcyclic)

macronutrient  
an element needed in relatively large quantities for plant growth, e.g. nitrogen (N), phosphorus (P), and potassium (K); (see micronutrient)

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

marker-assisted selection
a procedure used in plant breeding in which genetic markers that are (a) easy to identify and (b) linked to desirable genetic traits that are difficult to identify (such as disease resistance) are used to aid in selection from a population

maternal inheritance (also cytoplasmic or extrachromosomal inheritance)
inheritance of non-nuclear genes, i.e. those in mitochondria and chloroplasts

mating types  
compatible strains, usually designated + and - or A and B, necessary for sexual reproduction in heterothallic fungi

mechanical injury  
injury of a plant part by abrasion, mutilation, or wounding

mechanical transmission  
spread r introduction of inoculum to an infection court (wounding) by human manipulation, accompanied by physical disruption of host tissues

medium (pl. media)  
a mixture of organic and/or inorganic chemical compounds and water that provides the nutrients needed for the growth of a microorganism in vitro; for higher plants, a mixture of fertilizers and other components in which a plant is growing

meiosis
process of nuclear division in which the number of chromosomes per nucleus is halved, i.e. converting the diploid state to the haploid state (see mitosis)

melanin  
brown-black pigment; common in sclerotia and other survival structures

meristem (adj. meristematic)  
plant tissue characterized by frequent cell division, producing cells that become differentiated into specialized tissues

meristem culture  
aseptic in vitro culture of a plant or plant part from a portion of the meristem; a method used to produce pathogen-free plants

messenger RNA (abbr. mRNA)  
a form of RNA that carries information to direct the synthesis of protein

metabolite
any chemical participating in metabolism; a nutrient

microbial
pertaining to or relating to microbes, or microorganisms

microclimate
weather conditions on a small scale, e.g. at the surface of the plant or within a crop

microconidium (pl. microconidia)  
the smaller of two kinds of conidia formed by certain fungi (see macroconidium)

microcyclic
describing a rust fungus that produces only teliospores and basidiospores (see demicyclic, macrocyclic)

microflora
the combination of all microorganisms in a particular environment

micron (syn. micrometer) 
one millionth of a meter

micronutrient  
an essential element needed by plants for growth and required in very small amounts (see macronutrient)

microorganism (syn. microbe) 
an organism of such small size that it can only be seen as an individual organism with the aid of microscope

microsclerotium  
microscopic, dense aggregate of darkly pigmented, thick-walled hyphal cells

middle lamella
the layer, consisting largely of pectic substances, between the walls of adjacent plant cells

midrib
central, thickened vein of a leaf

migratory  
migrating from place to place on a plant or from plant to plant when feeding (see sedentary)

mildew  
thin coating of mycelial growth and spores on the surfaces of infected plant parts

mitochondrion (pl. mitochondria)
cellular organelle outside the nucleus that functions in respiration

mitosis
nuclear division in which the chromosome number remains the same (see meiosis)

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

mold  
any microfungus with conspicuous, profuse, or woolly superficial growth (mycelium and/or spore masses) on various substrates; especially an economically important saprobe; molds commonly grow on damp or decaying matter and on the surface of plant tissues

mollicute  
one of a group of prokaryotic organisms bounded by flexuous membranes and lacking cell walls (see phytoplasma and spiroplasma)

molt  
to shed a cuticle or body encasement during a phase of growth

monoclonal antibody  
antibody produced from clones of a single antibody-producing cell (see polyclonal antibody)

monocot (syn. monocotyledon)  
plant with one cotyledon, such as grasses, grain crops, corn (see dicot)

monoculture  
the growth of the same plant species in close proximity, with few or no other types of plant present

monocyclic
having one disease or life cycle per growing season (see polycyclic)

monoecious
having male and female organs on the same individual; in plants, often refers to separate male and female flowers on the same individual (see dioecious)

monogenic
determined by a single gene (see polygenic)

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

monoxenic culture
a culture containing one species of organism growing in the presence of one other species of organism

morphology (adj. morphologic)
the study of the form of organisms; form and structure of organisms

mosaic  
disease symptom characterized by non-uniform coloration, with intermingled normal, light green and yellowish patches, usually caused by a virus; often used interchangeably with mottle

motile  
capable of self-propulsion by means of flagella, cilia, or amoeboid movement

mottle  
disease symptom comprising light and dark areas in an irregular pattern, usually caused by a virus; often used interchangeably with mosaic

movement protein
for viruses, a protein encoded by the pathogen that is required for spread within an infected plant

mRNA (abbr. for messenger RNA)  
a form of RNA that carries information to direct the synthesis of protein

mucilaginous  
viscous, slimy

mulch  
layer of material, such as organic matter or plastic, applied to the surface of the soil for purposes such as retention of water and inhibition of weeds

multicellular
composed of many cells (see unicellular)

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

multiline
mixture of cultivars that differ only in single, major genes for resistance to a pathogen, giving the opportunity to modify the mixture from year to year as needed

multinucleate  
having more than one nucleus per cell (see uninucleate)

multiparticulate virus
plant virus in which the genome is divided into more than one piece of nucleic acid, which are encapsidated in more than one particle

multipartite virus
plant virus in which the genome is divided into more than one piece of nucleic acid

multiseptate 
having many septa or crosswalls

mummy  
a dried shriveled fruit; plant part or organ partially or completely replaced with fungal structures

mushroom  
a fleshy fruiting body of a fungus, especially of a Basidiomycete of the family Agaricaceae

mutagen
a substance that causes a mutation

mutation (n.mutant)
an abrupt heritable or genetic change in a gene or an individual as a result of an alternation in a gene or chromosome, or of an increase in chromosome number

Mycelia sterilia  
A group in the Fungi Imperfecti that does not produce spores, but may produce chlamydospores or sclerotia, e.g. Rhizoctonia and Sclerotium; subsequent research has defined sexual reproduction for some species

mycelium (pl. mycelia, adj. mycelial)  
mass of hyphae constituting the body (thallus) of a fungus

mycology
the study of fungi

mycoparasite
a fungus that attacks another fungus

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

mycorrhiza (pl. mycorrhizae; adj. mycorrhizal)  
symbiotic association between a nonpathogenic or weakly pathogenic fungus and the roots of plants (see ectomycorrhiza and endomycorrhiza)

mycotoxin
poisonous compound produced by a fungus

mycovirus
a virus that infects fungi

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

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