Illustrated Glossary of Plant Pathology A

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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abaxial
directed away from the stem of a plant; pertaining to the lower surface of a leaf (see adaxial)

abiotic
pertaining to the absence of life, as diseases not caused by living organisms

abscise
to separate from a plant, as leaves, flowers, and fruits do when they fall

abscission
the shedding of leaves or other plant parts as the result of physical weakness in a specialized layer of cells (the abscission layer) that develops at the base of the structure

acceptable daily intake (ADI)
a measure of the level of a chemical, i.e. a pesticide residue, that is believed to be able to be consumed on a daily basis over a lifetime without harm; used by the U.S. government to establish safe levels of potentially hazardous substances in food

acervulus (pl. acervuli)
erumpent, cushionlike fruiting body bearing conidiophores, conidia, and sometimes setae

acid
having a pH of less than 7

acid precipitation
precipitation (fog, rain, snow) with a low pH, due to the presence of nitric and sulfuric acid formed by the reaction of air pollutants (N0_x and S0₂) with water

acid rain
precipitation with a low pH, due to the presence of nitric and sulfuric acid formed by the combination of air pollutants (N0_x and S0₂) with water

acropetal
upward from the base to the apex of a shoot of a plant; in fungi, the production of spores in succession in the direction of the apex so that the apical spore is the youngest (see basipetal)

actinomycete
a group of microorganisms similar to bacteria that produce long filaments

acute
pertaining to symptoms that develop suddenly(see chronic)

acute toxicity
ability of a single dose of a compound to poison (see chronic toxicity)

adaxial
directed toward the stem of a plant; pertaining to the upper surface of a leaf (see abaxial)

adpressed (syn. appressed)
closely flattened down or pressed against a surface

adventitious
arising from other than the usual place, as roots from a stem rather than as branches of a root

aecial host
the host plant on which a heteroecious rust fungus produces aecia and pycnia (contrasts with telial host)

aeciospore
dikaryotic spore of a rust fungus produced in an aecium; in heteroecious rusts, a spore stage that infects the alternate host

aecium (pl. aecia; adj. aecial)
the fruiting body of a rust fungus in which the first dikaryotic spores (aeciospores) are produced

aerial
occurring in the air

aerification
the act of infusing or forcing air into, for example, soil

aerobic
living only in the presence of oxygen

aflatoxin
chemical byproduct from Aspergillus flavus and A. parasiticus harmful to humans and other animals

agar
jellylike material derived from algae and used to solidify liquid culture media; term also applied to the medium itself

aggressiveness
relative ability of a plant pathogen to colonize and cause damage to plants (see virulence)

air pollution
any air contaminant that causes undesirable effects on living organisms or materials

alate
winged (see apterous)

albino (n. albinism)
white or light-colored; having a marked deficiency in pigmentation

alkaline
having basic (nonacidic) properties; pH greater than 7

alkaloids
nitrogen-containing ring compounds produced by plants that cause physiological effects in animals

allele
any of one or more alternative forms of a gene

allelopathy (adj. allelopathic)
ability of one species to inhibit or prevent the growth of another species through the production of toxic substance(s)

alternate host
one of two kinds of plant required by a heteroecious rust fungus to complete its life cycle; sometimes used as a general term for the aecial host, or for the economically less important host (see also: aecial host and telial host)

alternative host
a plant other than the main host that a parasite can colonize; alternative hosts are not required for completion of the developmental cycle of the parasite

alternation of generations
a reproductive cycle in which a haploid phase alternates with a diploid phase

amino acid
organic nitrogenous acids from which protein molecules are constructed

amphid
a chemosensory organ located in the anterior region of a nematode

amphigynous
having an antheridium through which the oogonium grows, as in many Phytophthora species (see paragynous)

amphimixis
copulation of two unrelated cells and nuclei, e.g., egg and sperm; reproduction by a sexual process

amphimobile
able to move both upward and downward in phloem, as do some pesticides

anaerobic
living in the absence of oxygen

anamorph (adj. anamorphic; syn. imperfect state)
the asexual form in the life cycle of a fungus, when asexual spores (such as conidia) or no spores are produced (see holomorph and teleomorph)

anastomosis (pl. anastomoses)
fusion between branches of the same or different structures (e.g. hyphae) to make a network

annual
a plant that completes its life cycle and dies within one year (see biennial, perennial)

annual ring
single-year growth of xylem in a woody stem

annulations
grooved bands in the cuticle of some nematodes

antagonism
a general term for interference between organisms that may include antibiosis or competition for nutrients or space; action of two or more pesticides that reduces the effectiveness of one or all (see synergism)

antagonist
an organism or substance that limits or counteracts the action of another

anterior
toward the front or head (see posterior)

anther
pollen-bearing portion of a flower

antheridium (pl. antheridia)
male sexual organ (male gametangium) found in some fungi

anthesis
the period of the opening of a flower during which pollination can occur

anthracnose
disease caused by acervuli-forming fungi (order Melanconiales) and characterized by sunken lesions and necrosis

antibiosis
an association between organisms, or between an organism and a metabolic product of another organism, that is harmful to one of them

antibiotic
a chemical compound produced by one microorganism that inhibits growth or kills other living organisms

antibody
a specific protein formed in the blood of warm-blooded animals in response to the presence of an antigen

antigen
any foreign chemical (normally a protein) that induces antibody formation in warm-blooded animals

antiserum (pl. antisera)
blood serum containing antibodies

apex (pl. apices; adj. apical)
tip of a root or shoot, containing the apical meristem

aphid
small, sucking insect of the family Aphididae (order Homoptera) that produces honeydew and injures plants when in large populations

apothecium (pl. apothecia)
open, cuplike or saucerlike, ascus-bearing fungal fruiting body (ascocarp), often supported on a stalk

appressed (syn. adpressed)
closely flattened down or pressed against a surface

appressorium (pl. appressoria)
swollen, flattened portion of a fungal filament that adheres to the surface of a higher plant, providing anchorage for invasion by a fungus

apterous
wingless (see alate)

arable
able to be cultivated for agriculture

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

arbuscule (adj. arbuscular)
branched haustorial structure of certain endomycorrhizal fungi that forms within living cells of the root

arthropod
member of the phylum Arthropoda, which consists of animals with articulated bodies and limbs and which includes insects, arachnids, and crustaceans

ascocarp (syn. ascoma)
sexual fruiting body of an ascomycetous fungus that produces asci and ascospores; e.g. apothecium, ascostroma, cleistothecium, perithecium, pseudothecium

ascogenous
pertaining to ascus-producing hyphae

ascogonium (pl. ascogonia)
a specialized cell that gives rise to the hyphae that produce asci

ascoma (pl. ascomata; syn. ascocarp)
sexual fruiting body of an ascomycetous fungus that produces asci and ascospores; e.g. apothecium, ascostroma, cleistothecium, perithecium, pseudothecium

ascomycetes (adj. ascomycetous)
a group of fungi, also called the Ascomycota, that produces sexual spores (ascospores) within a saclike structure called an ascus

ascospore
sexual spore borne in an ascus

ascostroma (pl. ascostromata)
a fruiting body containing bitunicate (double-walled) asci in locules (cavities); usually dark with multiple locules, but sometimes single (see pseudothecium)

ascus (pl. asci)
saclike structure containing ascospores (typically eight) and usually borne in a fungal fruiting body

aseptate
having no cross walls; lacking septa; nonseptate; coenocytic

asexual
vegetative; without sex organs, gametes, or sexual spores; the imperfect or anamorphic stage of a fungus

asexual reproduction
any type of reproduction not involving the union of gametes and meiosis

AUDPC (abbr. for Area Under Disease Progress Curve)
a measure of the total amount of disease over a period of time, determined from graphs of disease vs. time, which can be used to compare epidemics quantitatively

autoecious
in reference to rust fungi, producing all spore forms on one species of host plant (see heteroecious)

autotroph
an organism that synthesizes its nutritive substances from inorganic molecules; e.g. plants capable of photosynthesis (see heterotroph)

auxin
plant hormone (growth regulator) influencing growth through cell elongation

avirulence (avr) gene
gene in a pathogen that usually causes a hypersensitive reaction, is associated with active plant defense reactions in a resistant plant, and causes disease in a susceptible plant

avirulent (syn. nonpathogenic)
unable to cause disease (see virulent)

avoidance
principle of plant disease control in which plants are grown at times or locations where the pathogen is inactive or not present

awn
bristlelike structure at the apex of the outer bract of some cereal and grass flowers

axenic
culture in the absence of living bacteria or other organisms; pure culture

axil
the angle formed by the leaf petiole and the stem

axillary
pertaining to or placed within an axil

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

bacilliform
shaped like short rods with rounded ends

backcross
to cross (mate) an offspring with one of its parents

bacterial streaming
large populations of bacteria that exude from the cut surface of infected plant tissue when observed with a microscope

bactericide
a chemical or physical agent that kills bacteria

bacteriocin
a protein antibiotic, one or more types of which can be produced and excreted by certain strains of bacteria

bacteriophage
a virus that infects a bacterium

bacterium (pl. bacteria)
a prokaryotic, microscopic, single-celled organism with a cell wall that increases by binary fission

bark
all tissues outside the vascular cambium of a woody plant, generally including the cork layers, cork cambium (phellogen), and phloem

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

base
having a pH of greater than 7

basidiocarp (syn. basidioma)
sexual fruiting body of a basidiomycetous fungus

basidioma (pl. basidiomata; syn. basidiocarp)
sexual fruiting body of a basidiomycetous fungus

basidiomycetes (adj. basidiomycetous)
a fungus group, also called the Basidiomycota, characterized by the formation of external basidiospores on basidia

basidiospore
haploid (1N) sexual spore produced on a basidium

basidium (pl. basidia; adj. basidial)
specialized cell or organ, often club-shaped, in which karyogamy and meiosis occur, followed by production of externally-borne basidiospores (generally four) that are haploid. There are several types of basidia.

basipetal
downward from the apex toward the base of a shoot; referring to development in the direction of the base so that the apical part is oldest (see acropetal)

beetle
any insect of the order Coleoptera characterized by elytra (thickened outer wings), chewing mouth parts, and complete metamorphosis

bicellular
two-celled

biennial
a plant that produces seed and dies at the end of its second year of growth (see annual, perennial)

biflagellate
having two flagella

binary fission
a type of asexual reproduction in which two cells, usually of similar size and shape, are formed by the growth and division of one cell

binomial, Latin
the scientific name of an organism, composed of two names, the first designating the genus and second the specific epithet, together making the species name

binucleate
having two nuclei

bioassay
any test (assay) using a living organism

biocide
a compound toxic to all forms of life

biocontrol (syn. biological control)
use by humans of one species of organism to eliminate or control another species of organism

biocontrol (syn. biological control)
exploitation by humans of the natural competition, parasitism and/or antagonism of organisms for management of pests and pathogens

biolistic transformation (see gene gun)
a method used for genetic engineering in which plant cells are bombarded with metal particles coated with foreign genes using a "gene gun"

biological control (syn. biocontrol)
exploitation by humans of the natural competition, parasitism and/or antagonism of organisms for management of pests and pathogens

biotechnology
the development of genetically modified organisms through the use of modern technology and processes, including genetic engineering

biotic
relating to life, as disease caused by living organisms

biotroph (syn. obligate parasite)
an organism that can live and multiply only on another living organism (see necrotroph)

biotype
a subdivision of a species, subspecies, or race based on some identifiable physiological trait such as a specific virulence pattern

bitunicate
having a double ascus wall (see unitunicate)

blasting
a symptom of plant disease characterized by shedding of unopened buds; classically, the failure to produce fruit or seed

bleeding
sap flow from a wound

blight
sudden, severe, and extensive spotting, discoloration, wilting, or destruction of leaves, flowers, stems, or entire plants

blotch
necrotic area of tissue irregular in form

borer
insect or insect larva that forms tunnels or cavities in the bark or within the wood of trees

boundary layer
the calm layer of air on surfaces below the layer of more turbulent air

bract
a reduced leaf associated with a flower or inflorescence; modified leaf from the axil of which a flower arises

breaking
disease symptom, usually caused by a virus, involving addition or loss of flower color to create a variegated pattern. (see flower break)

breeding line
plant strain used in a plant breeding program and usually containing one or more desirable agronomic or breeding characteristics

broadcast application
application by spreading or scattering on the soil surface

brooming
profuse branching of woody stems from single stem position

brown rot (of wood)
wood decay resulting from selective removal of cellulose and hemicellulose, leaving a brown amorphous residue that usually cracks into cubical blocks and consists largely of slightly modified lignin

bud
a terminal or axillary structure on a stem consisting of a small mass of meristematic tissue, generally covered wholly or in part by modified scale leaves

budding
a special type of plant grafting using a single bud on the rootstock of another plant; method of asexual reproduction in fungi, such as yeasts

bud scale
a specialized protective leaf of a bud

bud wood
wood consisting of strong, young shoots bearing buds suitable for use in budding

bug
any insect of the order Hemiptera characterized in part by piercing-sucking mouth parts, a triangular scutellum, two pairs of wings, and gradual metamorphosis

bulb
a short, flattened, usually globose or disc-shaped, underground, perennial, storage organ composed of concentric layers of overlapping fleshy scale leaves attached to a stem plate at the base; essentially a subterranean bud

bunt ball
smut sorus filled, with teliospores, that replaces a cereal or grass kernel but is covered by plant tissue at maturity

burr knot
a rough outgrowth, often present on the trunks or roots of certain trees

bursa
extension or flap of cuticle at the side of the male nematode sex organ, used for orienting during mating

bush-type
plant with determinate growth

callose
amorphous, hardened carbohydrate constituent of plant cell walls, commonly developing upon injury

callus
specialized tissues that form over a wound or cut in a plant; cork cambium may form and the cells produced will gradually seal the wound

calyx
outermost flower whorl; sepals, collectively

canker
a plant disease characterized (in woody plants) by the death of cambium tissue and loss and/or malformation of bark, or (in non-woody plants) by the formation of sharply delineated, dry, necrotic, localized lesions on the stem; "canker" may also be used to refer to the lesion itself, particularly in woody plants

canopy
the expanded leafy top of a plant or plants

capsid (syn. coat protein)
the protective layer of protein surrounding the nucleic acid core of a virus; the protein molecules which make up this layer

capsule
gel-like material surrounding a bacterial cell

carbohydrate
any of various chemical compounds composed of carbon, hydrogen, and oxygen, such as sugars, starches, and cellulose

carbon cycle
the continuous circulation of carbon atoms from inorganic carbon dioxide (CO₂) to organic molecules and back to CO₂

carcinogen
a substance or agent that causes cancer

carpel
the ovule-bearing structure of a flower in angiosperms

casting
premature loss of abscised leaves or twigs

catkin
conelike cluster of male or female flowers; conelike fruit of angiosperms

causal agent
organism or agent that incites and governs disease or injury

cell membrane
structure that bounds a cell and helps control the movement of substances into and out of the cell

cell wall
protective, resistant, but permeable structure secreted externally to the cell membrane in plants, bacteria, fungi, and certain other organisms

cellulose
carbohydrate polysaccharide composed of glucose units; major component of plant primary cell wall

center of origin
geographical area where a plant originated

centromere
the constricted portion of a chromosome to which, in mitosis, the chromosomal fiber is attached

certification
seeds, propagative plant material, or nursery stock produced and sold under inspection to maintain genetic identity and purity, freedom from harmful pathogens, insect pests, and weed seeds. It is approved and certified by an official certifying agency.

cfu (abbr. for colony forming unit)
the number of colonies formed per unit of volume or weight of a cell or spore suspension

chasmothecium
the ascocarp of powdery mildew fungi; it has no natural opening at maturity, but opens by the rupturing of its wall. See also cleistothecium

chemotaxis (syn. chemotropism)
movement or growth of an organism in response to changing concentration of a chemical stimulus, often in relation to food or for mating

chemotherapy
treatment of plant disease with chemicals (e.g. antibiotics or fungicides) absorbed and translocated internally

chimera (or chimaera)
plant or organ consisting of two or more genetically different tissues

chitin
complex polysaccharide carbohydrate in fungal cell walls, animal exoskeletons, and nematode egg shells

chlamydospore
thick-walled or double-walled asexual resting spore formed from hyphal cells (terminal or intercalary) or by transformation of conidial cells that can function as an overwintering stage

chlorophyll (adj. chlorophyllous)
one of a group of green pigments found in chloroplasts and important in photosynthesis

chloroplast
disklike structure containing chlorophyll in which photosynthesis occurs in green plants

chlorosis (adj. chlorotic)
failure of chlorophyll development, caused by disease or a nutritional disturbance; fading of green plant color to light green, yellow, or white

chromosome
the structure that contains the genes of an organism; in eukaryotes, chromosomes are in the nucleus and can be visualized with an optical microscope as threads or rods during meiosis and mitosis; in bacteria, the chromosome is usually a single circle of DNA that cannot be visualized with an optical microscope

chronic
pertaining to slow-developing, persistent, or recurring symptoms (see acute)

chronic toxicity
poisoning due to low levels of exposure to a compound over a period of time (see acute toxicity)

chytridiomycetes
a fungus group, also called the Chytridiomycota, characterized by the production of motile zoospores and resting sporangia; the plant-pathogenic species are all biotrophs that are restricted to the cells of their host.

circulative-propagative transmission(syn. 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

circulative transmission (syn. persistent transmission)
virus transmission characterized by a long period of acquisition of the virus by a vector (typically an insect), a latent period of several hours before the vector is able to transmit the virus, and retention of the virus by the vector for a long period, usually several days; the virus circulates in the body of the vector

cirrhus (pl. cirrhi)
a curled, tendril-like mass of exuded spores, held together by a slimy matrix

clamp connection
a bridge- or buckle-hyphal protrusion in basidiomycetous fungi, formed at cell division and connecting the newly divided cells

clavate (or claviform)
club-shaped

cleistothecium (pl. cleistothecia)
a spherical ascocarp that is closed at maturity

clone
n. a population of cells or organisms of identical genotype; population of recombinant DNA molecules all carrying the same inserted sequence;

v. the vegetative propagation of an organism to produce a population of identical individuals; the use of in vitro recombination techniques to insert a particular DNA sequence into a vector

coalesce
grow together into one body or spot

coat protein (syn. capsid)
the protective layer of protein surrounding the nucleic acid core of a virus; the protein molecules which make up this layer

coccus (pl. cocci)
a spherical (or near-spherical) bacterial cell

codon
a particular sequence of three nucleotides in a nucleic acid that codes for a specific amino acid or termination of a polypeptide chain (see triplet codon)

coelomycetes
a group of fungi in the deuteromycetes (Imperfect Fungi) that produce pycnidia or acervuli (see hyphomycetes)

coenocytic
having multiple nuclei embedded in cytoplasm without cross walls; nonseptate

collar
the portion of the seedling or plant near the surface of the soil; in grafted woody plants, the scion portion of the plant near the soil surface

collenchyma
supporting tissue in soft stems and other plant parts, composed of elongated living parenchyma cells with unevenly thickened primary walls, often bordering veins in dicot leaves; the "strings" in celery

colonization
establishment and ramification of a pathogen within a host plant

colonize
to infect and ramify through plant tissue with the growth of a pathogen

colony
growth of a microorganism in mass, especially as a pure culture

colony forming unit (abbr. cfu)
the number of colonies formed per unit of volume or weight of a cell or spore suspension

companion cell
specialized cell adjacent to sieve tubes in phloem

compartmentalization
isolation of a specific tissue area by host barrier tissues

complete resistance
a resistant reaction in which some aspect of disease development, usually symptom expression or pathogen reproduction, is completely stopped (see partial resistance)

compost
a mixture of organic residues and soil that is allowed to decompose biologically

concentric
one circle within another with a common center

conidiogenesis
the formation of asexual spores (conidia)

conidiogenous
producing and bearing conidia

conidioma (pl. conidiomata)
specialized conidia-bearing structure, e.g. acervulus, pycnidium, sporodochium, synnema

conidiophore
simple or branched hypha on which conidia are produced

conidium (pl. conidia)
an asexual, nonmotile fungal spore that develops externally or is liberated from the cell that formed it

conjugation
the temporary contact of bacterial cells during which genetic material is transferred

conk
a shelf-like, typically hardened basidiocarp of a wood decaying fungus, usually a polypore

constitutive
present or produced under all conditions (see induced)

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

continuous cropping
growing the same crop in the same location repeatedly

coppice
regrowth of woody plants by sprouts from stumps or roots

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

cork
external protective tissue of a stem or root, impermeable to water and gasses; the primary component of bark

corolla
petals, collectively

cortex (adj. cortical)
region of parenchyma tissue between the epidermis and the phloem in stems and roots; region beneath the rind of a sclerotium

cotyledon
seed leaf, one in moncots and two in dicots; primary embryonic leaf within the seed in which nutrients for the new plant are stored

crop rotation
the successive planting of different crop species; often used to improve soil fertility or to reduce disease and pest problems

cross-fertilization
a process in which sexual reproduction occurs as a result of the fusion of sex cells from different individuals (see self-fertilization)

cross-pollination
transfer of pollen from the anthers to the stigma of another flower (see self-pollination)

cross-protection
the process whereby a normally susceptible host is infected with a less virulent pathogen (usually a virus) and thereby becomes resistant to infection by a second, usually related, more virulent pathogen

crown
upper dome of tree, bearing leaves, flowers, and fruits; junction of root and stem of a plant, usually at the soil line; in grafted woody plants, the rootstock portion of the plant near the soil surface

crozier
the hook of an ascogenous hypha before ascus development; the curved apical portion of a blighted stem

cryptobiosis (hidden life):
the ability to enter a state of suspended metabolic activity during unfavorable environmental conditions (drying, cold); survival strategy of some plant pathogenic nematodes

culm
stem of grasses, cereals and bamboos

curl
distoration, puffing, and crinkling of a leaf resulting from unequal growth

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

cultural practices
the manner in which plants are grown, such as: application of nutrients, irrigation practices, type of cultivation; may be used for disease management

culture
growth and propagation of microorganisms on nutrient media; growth and propagation of living plants

cuticle (adj. cuticular)
noncellular outer layer of an insect or a nematode; water-repellent, waxy layer of epidermal cells of plant parts, such as leaves, stems and fruit

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

cyst
in fungi, a resting structure in a protective membrane or shell-like enclosure;
in nematode females, the egg-laden carcass of a female nematode;
in bacteria, a specialized type of bacterial cell enclosed in a thick wall, often dormant and resistant to environmental conditions

cytokinins
plant hormones (growth regulators) that control cell division and are important for shoot stimulation of callus in tissue culture

cytopathology
the study of changes induced by disease at the cellular level

cytoplasm
living protoplasm in a cell, except the nucleus

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

damping-off
death of a seedling before or shortly after emergence due to decomposition of the root and/or lower stem; it is common to distinguish between preemergence damping-off and postemergence damping-off

days to harvest
term for the prescribed minimum number of days required by U.S. government regulations between a pesticide application and the harvest of the crop

decay
the gradual decomposition of organic matter

deciduous
describing a tree that sheds its leaves completely at the end of its annual growth period

decoy crop
crop that stimulates germination of seeds of a parasitic plant such as witchweed (Striga spp.), but is not susceptible to infection by the parasitic plant; helps reduce seed populations of the parasite in soil so a susceptible crop can be planted

defoliation
loss of leaves from a plant, whether normal or premature

degree-day
the departure of the average daily temperature from a defined base (e.g. the minimum recognized temperature for the growth of a plant species). The number of degree-days may be totaled to assess the accumulated warmth of a particular year's growing season.

dehiscent
opening by breaking into parts

demicyclic
a rust fungus that lacks the urediniospore (repeating) stage (e.g. many species of Gymnosporangium) (see macrocyclic, microcyclic)

dentate
toothed

deoxyribonucleicacid (abbr. DNA)
the double-stranded, helical molecule that contains genetic code information; each repeating unit, or nucleotide, is composed of deoxyribose (a sugar), a phosphate group, and a purine (adenine or guanine) or a pyrimidine (thymine or cytosine) base

desiccate
to dry out

determinate
ceasing vegetative growth when the first flower or reproductive structure forms

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

diagnostic (n. diagnosis)
a distinguishing characteristic important for the identification of a disease or other condition

diapause
a period of spontaneous dormancy, independent of environmental conditions, interrupting developmental activity in an embryo, larva or pupa

dichotomous
branching, often successively, into two more or less equal arms

dicot (syn. dicotyledon)
plant with two cotyledons or seed leaves (see monocot)

dieback (v. die back)
progressive death of shoots, leaves, or roots, beginning at the tips

differential host (syn. differential cultivar)
a plant host that on the basis of disease symptoms serves to distinguish between various strains or races of a given plant pathogen

differential medium
culture medium that is used to distinguish between organisms that can grow on it

differentiation
the physiological and morphological changes that occur in a cell, tissue, or organ during development from a juvenile state to a mature state

digitate
having lobes radiating from a common center

dikaryon (adj. dikaryotic)
having two sexually compatible haploid nuclei per cell, that divide simultaneously; this phase is called the dikaryophase

dilution plating
a method to obtain pure colonies of bacteria and fungi in which infected plant material or infested soil is diluted in sterilized water. Small samples of the water are spread on the medium surface of several petri plates to find which dilution will produce pure colonies.

dilution streaking
repeated streaking of bacteria on the surface of a nutrient medium with a sterile metal loop to allow pure colonies to grow

dimorphic
having two distinct shapes or forms

dioecious
having male and female organs on separate and distinct individuals (used primarily for plants) (see monoecious)

diploid
having two complete sets of chromosomes (2N chromosomes) (see haploid, polyploid)

direct penetration
penetration of plant tissues by a pathogen through barriers such as leaf cuticle by chemical and physical means (e.g. penetration peg)

Discomycetes
a group of the ascomycetes in which the hymenium is exposed at maturity; one in which the fruiting body is an apothecium or discocarp

disease
abnormal functioning of an organism

disease cycle
succession of all of events and interactions among the host, parasite and environment that occur in a disease, from initial infection of the plant by a causal agent, through pathogenesis, to over-seasoning, until another infection occurs

disease incidence
number of plants affected by a disease within a population

disease progress curve
graph of some measure of disease (i.e. severity) over time

disease pyramid
a memory aid similar to the disease triangle but including, in addition, the factor of time in the development of a disease

disease severity
the measure of damage done by a disease

disease triangle
a memory aid that diagrams the three important components necessary for disease: susceptible plant, virulent pathogen and favorable environment

disinfect
to eliminate a pathogen from infected plant tissues

disinfest
to kill pathogens that have not yet initiated disease, or other contaminating microoganisms, that occur in or on inanimate objects as such soil or tools, or that occur on the surface of plant parts such as seed

dispersal (syn. dissemination)
spread of infectious material (inoculum) from diseased to healthy plants

dissemination (syn. dispersal)
spread of infectious material (inoculum) from diseased plants to healthy plants

distal
far from the point of attachment or origin; in a direction away from main body (see proximal)

DNA (abbr. for deoxyribonucleic acid)
the double-stranded, helical molecule that contains genetic code information; each repeating unit, or nucleotide, is composed of deoxyribose (a sugar), a phosphate group, and a purine (adenine or guanine) or a pyrimidine (thymine or cytosine) base

dodder
parasitic seed plant (Cuscuta spp.) without leaves; a yellow filamentous vine

dolipore septum
cross wall found in basidiomycetes and characterized by special swellings and membranes in association with the septal pore

dominant
describes a phenotypic trait that is expressed in hybrid progeny of diploid organisms even when contributed by only one of the parents (see recessive)

dormancy (adj. dormant)
a condition of suspended growth and reduced metabolism of an organism, generally induced by internal factors or environmental conditions as a mechanism of survival

downy mildew
a plant disease in which the fungus appears as a downy growth on the host surface; caused by a member of the oomycetes

drift (of pesticides)
movement of airborne particles of a spray, dust, or vapor away from the target area during or shortly after an application

drought
insufficient soil water for normal plant growth

durable resistance
resistance that remains effective during prolonged and widespread use in an environment favorable to disease

dwarfing
underdevelopment of a plant or plant organs, which may be caused by disease, inadequate nutrition, or unfavorable environmental conditions

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