Illustrated Glossary of Plant Pathology

A  |   B  |   C  |   D

A

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

abiotic   
pertaining to the absence of life; abiotic diseases are not caused by living organisms (pathogens), but by chemical and physical factors. (see also noninfectious) (contrasts with biotic, infectious)

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 (acronym 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)   
an erumpent, cushionlike fruiting body bearing conidiophores, conidia, and sometimes setae

acid
having a pH less than 7 (contrasts with alkaline, base)

acid precipitation  
precipitation (fog, rain, snow) with a low pH, due to the presence of nitric and sulfuric acids 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 acids 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 (contrasts with basipetal)

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

acute
pertaining to symptoms that develop suddenly (contrasts with chronic)

acute toxicity
the ability of a single dose of a compound to poison (contrasts with chronic toxicity)

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

ADI (acronym for acceptable daily intake)
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

adpressed 
closely flattened down or pressed against a surface; appressed

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      
the 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 (contrasts with anaerobic)

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

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

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

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

alate  
winged (contrasts with apterous)

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

alkaline
having basic (nonacidic) properties; pH greater than 7 (see also base) (contrasts with acidic)

alkaloid
a nitrogen-containing ring compound produced by plants that causes physiological effects in animals

allele 
any of one or more alternative forms of a gene

allelopathy (adj. allelopathic)
the 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

AM (acronym for arbuscular mycorrhiza)
the 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 also endomycorrhiza) (contrasts with ectomycorrhiza)

amino acid
an organic nitrogenous acids from which protein molecules are constructed

ammonification
the conversion of organic matter during decay by bacteria, fungi, and some other organisms into ammonia and ultimately ammonium, NH₄, which can be absorbed by plants

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 (contrasts with paragynous)

amphimixis
the 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 (contrasts with aerobic)

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

anastomosis   (pl. anastomoses)  
the 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 (contrasts with biennial, perennial)

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

annulations  
the 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 (contrasts with synergism)

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

anterior
toward the front or head (contrasts with posterior)

anther  
the pollen-bearing portion of a flower

antheridium   (pl. antheridia)     
a male sexual organ (male gametangium) found in some fungi and funguslike organisms

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

anthracnose     
a disease caused by acervuli-forming fungi (archaic 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)  
the blood serum containing antibodies

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

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

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

appressed 
closely flattened down or pressed against a surface; adpressed

appressorium   (pl. appressoria)     
a 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 (contrasts with alate)

aquifer
an area in which groundwater accumulates

arable
able to be cultivated for agriculture

arbuscular mycorrhiza (acronym AM)  
a 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 also endomycorrhiza) (contracts with ectomycorrhiza)

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

Area Under Disease Progress Curve (acronym AUDPC)
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

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

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

ascogenous 
pertaining to ascus-producing hyphae

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

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

ascomycete  (adj. ascomycetous)    
an informal term for a member of Ascomycota

Ascomycota 
a group of fungi that produce sexual spores (ascospores) within a saclike structure called an ascus; ascomycetes; sac fungi

ascospore    
a haploid (1N) 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 also pseudothecium)

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

aseptate     
having no cross walls; lacking septa (see also nonseptate; coenocytic) (contrasts with septate)

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 (contrasts with sexual reproduction)

AUDPC (acronym 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 (contrasts with heteroecious)

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

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

avirulence (avr) gene
a gene in a pathogen that causes the pathogen to elicit an incompatible (defense) response in a resistant host plant, and may enhance pathogen virulence in a susceptible host plant.  The outcome of the interaction of an avirulence gene product with its corresponding plant resistance (R) gene product is usually a hypersensitive reaction

avirulent   
unable to cause disease (see also nonpathogenic) (contrasts with virulent)

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

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

axenic
a 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   
a bud that develops in the axil of a leaf (see also lateral bud)

B

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 reproduces 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   
a structure at the base of a nematode stylet (see also stylet knob)

base
having a pH of greater than 7 (see also alkaline) (contrasts with acid)

basidiocarp       
a sexual fruiting body of a basidiomycetous fungus (see also basidioma)

basidioma     (pl. basidiomata)    
a sexual fruiting body of a basidiomycetous fungus (see also basidiocarp)

basidiomycete  (adj. basidiomycetous)   
an informal term for a member of Basidiomycota

Basidiomycota (adj. basidiomycetous)
a group of fungi that produce sexual spores (basidiospores) externally on a structure called a basidium; basidiomycetes; mushroom fungi

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

basidium   (pl. basidia; adj. basidial)    
a 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 (contrasts with acropetal)

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

berry
a fleshy fruit containing seeds

bicellular
two-celled

biennial
a plant that produces seed and dies at the end of its second year of growth (contrasts with 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 word designating the genus and the second word designating 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
the exploitation by humans of the natural competition, parasitism and/or antagonism of organisms for management of pests and pathogens (see also biological control)

biodiversity
the existing genetic variability among living organisms (see also biological diversity)

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

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

biological diversity
the existing genetic variability among living organisms (see also biodiversity)

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 (see also infectious) (contrasts with abiotic, noninfectious)

biotroph     
an organism that can live and multiply only on another living organism (see also obligate parasite) contrasts with 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 (contrasts with unitunicate)

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

bleeding  
the sap flow from a wound

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

blotch    
a necrotic area of tissue irregular in form

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

breeding line
a 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  
the profuse branching of woody stems from single stem position

brown rot (of wood)  
a 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 (contrasts with white rot)

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

bush-type
a plant with determinate growth

C

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

callus    
the 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
the 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; the term canker also may be used to refer to the lesion itself, particularly in woody plants

canopy
the expanded leafy top of a plant or plants

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

capsule 
the 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  
te ovule-bearing structure of a flower in angiospermshe ovule-bearing structure of a flower in angiosperms

Casparian strips
the material between the cells of the root endodermis that blocks the passive movement of solutes and water

casting  
the premature loss of abscised leaves or twigs

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

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

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

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

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

center of origin  
the geographical area where a plant or other organism originated

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

certification  
describes 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 (acronym 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   
the movement or growth of an organism in response to changing concentration of a chemical stimulus, often in relation to food or for mating; chemotropism

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

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

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

chlamydospore    
a 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
a disklike organelle containing chlorophyll in which photosynthesis occurs in the cells of green plants

chlorosis   (adj. chlorotic) 
the 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, composed of DNA, 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 (contrasts with acute)

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

chytridiomycete  
an informal term for a member of Chytridiomycota

Chytridiomycota
a group of fungi that produce motile zoospores and resting sporangia; the plant-pathogenic species are all biotrophs that are restricted to the cells of their host; chytridiomycetes

circulative transmission 
a type of 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 (see also persistant transmission, propagative transmission) (contrasts with nonpersistant transmision, stylet-borne transmission)

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

clamp connection    
a bridge or buckle protrusion found at the septa of hyphae in basidiomycetous fungi and associated with cell division

clavate   (or claviform)
club-shaped

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

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  
to grow together into one body or spot

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

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 also triplet codon)

coelomycetes
an archaic term for a group of fungi in the deuteromycetes (imperfect fungi; Fungi Imperfecti) that produce pycnidia or acervuli (contrasts with hyphomycetes)

coenocytic  
having multiple nuclei embedded in cytoplasm without cross walls (see also aseptate; nonseptate) (contrasts with septate) 

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

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

companion cell
a specialized cell adjacent to sieve tubes in phloem

compartmentalization  
the 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 (contrasts with partial resistance)

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

concentric  
having one circle within another with a common center

conidiogenesis 
the formation of asexual spores (conidia)

conidiogenous 
producing and bearing asexual spores (conidia)

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

conidiophore    
a 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 (contrasts with induced)

contact fungicide
a fungicide that remains on the surface where it is applied and prevents infection often through the inhibition of spore germination; no after-infection activity (see also protectant fungicide) (contrasts with systemic fungicide)

continuous cropping
growing the same crop in the same location repeatedly

coppice
the regrowth of woody plants by sprouts from stumps or roots

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

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

cork cambium 
a cylinder of meristematic cells (lateral meristem) in the stems of woody plants that produces cork (bark)

corolla
the petals of a flower, collectively

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

cotyledon  
a 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 (contrasts with self-fertilization)

cross-pollination
the transfer of pollen from the anthers of a flower on one plant to the stigma of a flower on another plant (contrasts with self-pollination)

cross-protection 
the process by which 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  
the 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
a stem of grasses, cereals and bamboos

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

cultivar (abbr. cv.)
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) (see also variety)

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

cuticle   (adj. cuticular) 
the 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)
a plant type within a species, resulting from deliberate manipulation, which has recognizable characteristics (color, shape of flowers, fruits, seeds and height or form) (see also variety)

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

cytokinin 
a plant hormones (growth regulator) that controls cell division and is important for shoot stimulation of callus in tissue culture

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

cytoplasm
the living protoplasm in a cell, except the nucleus

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

D

damping-off    
the 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
the 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
a 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  
the 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) (contrasts with macrocyclic, microcyclic)

denitrification
the conversion of nitrate (NO₃) under anaerobic conditions by bacteria through several steps and ultimately to gaseous N₂

dentate
toothed

deoxyribonucleic acid (acronym DNA)  
the double-stranded, helical molecule that contains the genetic code; 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

desertification
the formation of deserts in areas that were previously productive

desiccate
to dry out

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

deuteromycetes
an archaic term for a group of fungi without a sexual stage; the asexual stage of members of Ascomycota (the ascomycetes) and Basidiomycota (the basidiomycetes) (see also Fungi Imperfecti, imperfect fungi)

diagnostic (n. diagnosis)
pertaining to 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 or dicotyledon 
a plant with two cotyledons or seed leaves (contrasts with monocot)

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

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

differential medium
a 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

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

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  
the 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) (contrasts with monoecious)

diploid  
having two complete sets of chromosomes (2N chromosomes) (contrasts with haploid, dikaryotic, polyploid)

direct penetration  
the penetration of plant tissues by a pathogen through a barrier such as leaf cuticle by chemical and physical means (e.g., infection peg, penetration peg) (contrasts with indirect penetration)

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

disease 
the abnormal functioning of an organism

disease cycle 
the 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
the number of plants affected by a disease within a population

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

disease pyramid 
a memory aid similar to the disease triangle but also including 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: a susceptible plant, a virulent pathogen, and a 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 or dissemination 
the 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 (contrasts with proximal)

DNA (acronym for deoxyribonucleic acid)
the double-stranded, helical molecule that contains the genetic code; 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  
a parasitic seed plant (Cuscuta spp.) without leaves; a yellow filamentous vine

dolipore septum  
a cross wall found in members of the Basidiomycota (the 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 (contrasts with 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 pathogen appears as a downy growth on the host surface; caused by a member of the Oomycota (the oomycetes)

drift (of pesticides)  
the 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
a resistance that remains effective during prolonged and widespread use in an environment favorable to disease (see also horizontal resistance, race-nonspecific resistance) (contrasts with  specific resistance, vertical resistance)

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