Acervulus (pl. Acervuli)—A saucer-shaped, spore-producing body of a fungus embedded in host tissue.
Actinomycetes—Filamentous bacteria that produce several antibiotics and give soil its earthy smell.
Alternate Host—One of two kinds of plants on which a parasitic fungus (e.g., rust) must develop to complete its life cycle.
Anamorph—Asexual stage of a fungus.
Anthracnose—Disease caused by acervuli-forming fungi (order Melanconzales) and characterized by sunken lesions and necrosis.
Antibiotic—A complex chemical substance produced by one microorganism that inhibits or kills other microorganisms (e.g., streptomycin).
Antibody—A specific protein formed in the blood of warmblooded animals in response to the injection of an antigen.
Antigen—Any foreign chemical (normally a protein) that induces antibody formation in animals.
Antiseptic—A substance that prevents, retards, or destroys microorganisms.
Apothecium—An open, cuplike, or saucer-shaped sexual fungal fruiting body containing asci.
Ascomycetes—A group of fungi characterized by the production of sexual spores within an oval or tubular membranous sac called an ascus.
Acus (pl. Asci)—A saclike cell in which meiosis occurs and which generally contain eight spores each.
Asexual—Vegetative; without sex organs, sex cells, or sexual spores, as the anamorph of a fungus.
Atrophy—The reduction in size of an organ by distributed metabolism.
Autoecious—The need of only one host for completing the life cycle of a rust.
Bacterium (pl. Bacteria)—Microscopic one-celled organism. Cell type lacks a distinct nucleus, sexual recombination, and chlorophyll. It does have cell walls and DNA.
Bactericide—Any chemical or physical agent that kills or protects plants from bacteria.
Basidiomycetes—A group of fungi characterized by the production of sexual spores on a club-shaped filament called the basidium.
Basidiospore—A haploid spore formed externally on a basidium.
Basidium (pl. Basidia; adj. Basidial)—Short, club-shaped fungus cell on which basidiospores are produced.
Blight—Any sudden, severe, and extensive spotting, discoloration, or destruction of leaves, flowers, stems, or entire plants, usually attacking young, growing tissues. (In disease names, often coupled with the name of the affected part of the host; e.g., leaf blight, blossom blight, shoot blight).
Blotch—A blot or spot, usually superficial and irregular in shape and size, on leaves, shoots and fruit.
Burn—The condition in which the cells of the host become reddish or dark brown and collapse.
Callus—Parenchyma tissue that grows over a wound or graft and protects it against drying or other injury.
Calyx—Outermost whorl of organs of a flower.
Canker—A dead area on a stem surrounded by living cortical tissues.
Carrier—A plant or animal that carries a virus or other infective agent without showing symptoms.
Chasmothecium (pl. Chasmothecia)—Closed, usually spherical, ascus-containing structure of powdery mildew fungi, also known as a Cleistothecium. A sexual fruiting structure.
Chemotherapy—Treatment of disease by chemicals (chemotherapeutants) working internally. Chemical agent has toxic effect directly or indirectly on the pathogens without injury to the host plant.
Chimera—A plant with several tissue sectors or layers differing in genetic or chromosomal constitution from the original plant.
Chlamydospore—A thick-walled asexual resting spore formed by the modification of a fungus hypha.
Chlorosis—The abnormal plant color of yellowish-white or gray condition of plant parts resulting from the incomplete destruction of the chlorophyll.
Cirrus (pl. Cirri)—A curl-like tuft; a tendril-like mass or “spore horn” of forced-out spores.
Cleistothecium (pl. Cleistothecia)—Closed, usually spherical, ascus-containing structure of powdery mildew fungi, also known as a chasmothecium. A sexual fruiting structure.
Conidiophore—The specialized fungal hyphal branch that bears the conidium.
Conidium (pl. Conidia)—Asexual spore formed by abstriction and detachment of part of a hyphal cell at the end of a conidiophore and germinating by a germ tube.
Coremium—A cluster of erect fungus filaments (hyphae) that are joined together to form a column and that bear asexual spores (conidia).
Cultivar (abbr. cv.)—A cultivated plant variety or cultural selection. Used synonymously with variety.
Curl—The distortion, fluting, and puffing of a leaf resulting from the unequal development of its two sides.
Damping-off—Decay of seeds in the soil or young seedlings before or after emergence.
Diagnostic—Distinctive. A distinguishing characteristic serving to identify or determine the presence of a disease or other condition.
Dieback—Progressive death of shoots, branches, and roots generally starting at the tips.
Diploid—Having a double set of chromosomes (2n chromosomes) per cell.
Disease—The sum of the deviations of the vital functions beyond the latitude of health. (This is just one of many definitions of disease.)
Disinfectant—Any agent for destroying the causal agent of disease after infection.
Disinfestant—Any agent that removes, kills, or inactivates disease-causing organisms before they can cause infection.
Dissemination—The spread of infectious material (inoculum) from a diseased to a healthy plant by wind, water, humans, insects, animal, machinery, or other means.
Dormancy—Nongrowing (inactive, quiescent) state of a plant.
Dwarfing—The underdevelopment of any organ of a plant.
Endophytic—Living within another plant.
Enphytotic—Plant disease that causes about the same amount of injury each year.
Epidemiology (adj. Epidemiologic)—The study of factors influencing the initiation, development, and spread of infectious disease.
Epinasty—An abnormal downward-curving growth or movement of a leaf, leaf part, or stem.
Epiphytotic—The widespread and destructive development of a disease on many plants in a community or communities.
Eradicant—Chemical used to eliminate a pathogen from a host or an environment.
Eradication—Control of disease by eliminating the pathogen after it is already established.
Escape—Plants in a given population that remain free of disease where it is prevalent, although they possess no natural inherent resistance to the disease. (See Klendusity).
Etiolation—Yellowing and long, spindly growth as a result of insufficient light.
Etiology—The description of the cause of disease.
Exclusion—Control of disease by preventing its introduction (e.g., by quarantines) into disease-free areas.
Exudate—A substance (usually liquid) formed inside a plant and discharged from diseased or injured tissue. The presence of an exudate often aids in diagnosis (e.g., fire blight bacteria).
Facultative Parasite—An organism that is ordinarily saprophytic but under proper conditions may be parasitic.
Facultative Saprophyte—An organism that is ordinarily parasitic but under proper conditions may be saprophytic.
Fasciation—A distortion of a plant caused by an injury or infection that results in thin, flattened, and sometimes curved shoots.
Flagellum—A long hairlike or whiplike contractile filament protruding from certain bacterial cells and spores of fungi and that enable movement.
Flagging—The loss of turgor and the drooping of plant parts, usually following a water deficit.
Fleck—A small, white to translucent lesion (spot) visible through a leaf.
Frass—Excrement of an insect, usually mixed with plant debris.
Fruiting Body—Any of various complex, spore-bearing fungal structures.
Fumigant—Vapor-active chemical used in the gaseous phase to kill or inhibit the growth of microorganisms or other pests.
Fungi Imperfecti—A major group of fungi for which no sexual production of spores is known.
Fungicide—An agent that inhibits or kills fungi.
Fungistat—A chemical or physical agent that prevents fungi from developing but does not kill them.
Fungus—A single- or many-celled, naked or covered, irregular or filamentous organism, usually with a chitinous cell wall. Lacking chlorophyll and incapable of manufacturing its own food, it feeds on dead or living plant or animal matter.
Gall—Outgrowth or swelling of unorganized plant cells produced as a result of attack by bacteria, fungi, or other organisms.
Germinate—To begin growth of a seed or spore.
Girdle—To circle and cut through; to destroy vascular tissue, as in a canker or knife cut that encircles the stem.
Giant Cells—Large, usually multinucleate cells formed by abnormal cell fusions or failure of proper cell wall formation following growth and nuclear division. Associated with nematode feeding.
Gram-negative (alternatively, Gram-positive)—A negative (or positive) reaction to the standard Gram’s stain for bacteria.
Haploid—The chromosome number of the gametophytic generation or phase or having a single complete set of chromosomes.
Haustorium (pl. Haustoria)—A modified mycelial branch that grows into a plant cell, makes intimate contact with the protoplast, and absorbs food.
Heteroecious—Requiring two or more unrelated hosts for completing the life cycle of a rust.
Heterothallic—Producing fusing gametes on separate and distinct mycelia.
Homothalic—Producing fusing gametes on the same mycelium.
Host—The plant on or in which a parasite lives and from which it obtains its food.
Hydathode—An epidermal leaf structure specialized for secretion or exudation of water; leaf opening at terminus of vein.
Hyperplasia—The abnormal increase in the number of cells without their enlargement.
Hypertrophy—The abnormal increase in the size of cells, causing abnormal development of an organ or tissue.
Hypha—A single filament of a fungus mycelium.
Hypoplasia—The underdevelopment of cells, tissues, or organs.
Immunity—A relationship between a plant and a causal agent in which the plant does not become diseased.
Incubation Period—Time between infection by a pathogen and appearance of symptoms.
Indexing—Determining presence of disease in a plant by removing buds or other parts for inoculation of a susceptible indicator plant that exhibits specific symptoms of a transmissible disease.
Infection—Process in which a pathogen enters, invades, or penetrates and establishes a parasitic relationship with a host plant.
Infestation—Presence in numbers (e.g., of insects, mites, or nematodes). Do not confuse with “infection,” a term that applies only to living, diseased plants or animals.
Inoculum—Pathogen or pathogen part (e.g., spores, mycelium) that infects plants.
Intercellular—Between the cells.
Intracellular—Within the cells.
Klendusity—Ability of an otherwise susceptible variety of plant to escape infection because of the way it grows (e.g., early-maturing plants escape late-season diseases).
Latent—Present but not manifest or visible, as a symptomless infection.
Lesion—A local injury or delimited diseased area.
Local Necrosis—The death or disintegration of cells and tissues in a localized area of an organ.
Macroscopic—Visible to the naked eye, without the aid of a microscope.
Micron—A millionth of a meter (or, a thousandth of a millimeter).
Microscopic—Visible only with the aid of magnification.
Migratory ectoparasites—nematodes that feed from outside roots, moving from cell to cell and piercing them to feed without entering root tissue. (Examples: dagger nematodes, Xiphinema spp.)
Migratory endoparasites—nematodes that feed inside roots, tunneling inside and moving back into soil and to new roots at will. (Examples: root-lesion nematodes, Pratylenchus spp.)
Monotrichous—Having only one flagellum.
Mosaic—Disease symptom characterized by nonuniform foliage coloration, with a more or less distinct intermingling of normal and light green or yellowish patches. Usually caused by a virus.
Mottle—An irregular pattern of light and dark areas.
Mummification—The drying up and shriveling of fruits and other plant parts.
Mummy—A dried and shriveled fruit.
Mushroom—A conspicuous fleshy fungus fruiting body.
Mycelium (pl. Mycelia)—The mass of interwoven threads (hyphae) making up the vegetative body of a fungus.
Mycoplasma—Degenerate bacteria that do not have cell walls. Mycoplasmas are smaller than bacteria but larger than viruses. They cause animal and human diseases.
Mycoplasmalike Organism (abbr. MLO)—See Phytoplasma.
Mycorrhiza (pl. Mycorrhizae)—A symbiotic association of a fungus with the roots of a plant.
Necrosis (adj. Necrotic)—The death or disintegration of cells and tissues.
Nematicide—A chemical or physical agent that kills, inhibits, or protects against nematodes.
Nematodes—Generally microscopic tubular worms, usually living free in moist soil, water, and decaying matter, or as parasites of plants and animals.
Obligate—Necessary; obliged. An obligate parasite is an organism that can live only on living tissue.
Oogonium (pl. Oogonia)—Female egg cell of oomycete fungi.
Oomycete—A group of fungi that produce oospores such as Pythium, Phytophthora, and Aphanomyces.
Oospore—Thick-walled, sexually-derived resting spore of oomycete fungi.
Overwinter—To survive over the winter period.
Parasite—An organism that lives within or upon another living organism from which it derives nourishment and in which it may cause various degrees of injury.
Parasitism—The phenomenon of the growth of one organism, the parasite, at the expense of another, the host.
Pathogen—An entity capable of producing disease.
Pathogenicity—An entity’s capacity for producing a disease.
Pectinase—The enzyme that breaks down pectic substances to simple carbohydrates.
Perithecium—A round to flask-shaped, thick-walled spore-case with an ostiole (pore) and containing asci.
Peritrichate (alt. Peritrichous)— Having flagella all over the outside of the cell.
Pesticide—Any chemical or physical agent that destroys pests (e.g., fungicide, insecticide, miticide).
Phycomycetes—A group of fungi that may consist of one cell or have filaments (hyphae) with few or no cross walls and that reproduce sexually by union of two sex cells.
Phyllody—Change from a normal flower to leafy structures. Characteristic of certain phytoplasma infections.
Physiogenic Disease—A disease produced by some unfavorable physical or environmental factors (e.g., light, temperature, water, soil nutrients, chemical, physical or mechanical injury).
Phytoplasma—Microorganisms found in phloem tissue that resemble mycoplasmas in all respects except that they cannot be grown on artificial nutrient media (yet). Formerly known as mycoplasmalike organism (MLO).
Phytotoxic—Injurious to plants.
Plasmodium—A naked, multinucleate, vegetative (fungal ) body capable of amoeboid motion.
Polymorphism—The existence of several asexual spore stages in the life cycle of an organism.
Primary Infection— The first infection of a plant, usually in the spring by an overwintering sexual pathogen propagule.
Primary Inoculum—lnoculum, usually from an overwintering source, that initiates disease in the field, as opposed to inoculum that spreads disease during the season.
Propagule—The part of an organism that may be spread so as to reproduce the organism.
Protectant—A chemical applied to a plant surface in advance of the pathogen to prevent infection.
Pustule—A local elevation of the epidermis that may rupture to expose the causal agent (e.g., rust, smut, white rust, etc.).
Pycnidium (pl. Pycnidia)—The asexual, globose or flask-shaped fruiting body of fungi that produces conidia.
Quarantine—Regulation forbidding sale or shipment of plants or plant parts, usually to prevent disease, insect, nematode, or weed invasion of an area.
Race—A strain of a pathogen characterized by the limitation of its host range to certain species and varieties of plants.
Resistance—The sum of the qualities of the host and causal agent that retard the activities of the causal agent.
Rhizoid—Intercellular thallus branch that absorbs food and provides anchorage.
Rhizomorph—An aggregation of hyphae into a cordlike or rootlike strand.
Rickettsia—A single-celled animal and human disease-causing organism with a partial cell wall that has not been grown in culture.
Ringspot—Symptom of a disease characterized by yellowish or dead (necrotic) rings with green tissue inside them, as in certain virus diseases.
Rogue (alt. Roguing)—To remove and destroy undesired individual plants from a planting on the basis of disease infection, not being true-to-type, insect infestation, or other reason.
Rot—Softening, discoloration, and often disintegration of succulent plant tissue as a result of fungal or bacterial infection.
Russet—Yellowish-brown or reddish-brown scar tissue on the surface of fruit.
Sanitation—Destroying all infested and infected plant parts during the season.
Saprophyte—An organism that derives its nourishment from dead organic matter.
Scab—Crustlike disease lesion.
Sclerotium (pl. Sclerotia)—A small, compact, hardened mass of hyphae that may bear fruiting bodies. Can help fungus survive adverse environments.
Scorch—“Burning” of plant tissue from infection, lack or excess of some nutrient, or weather conditions.
Secondary Infection—Infection resulting from the spread of infectious material produced after a primary infection.
Sedentary ectoparasites—nematodes that tunnel partially into roots, their heads entering to establish permanent feeding sites while their bodies remain outside. The nematode does not move after this. (Examples: citrus nematode, Tylenchulus semipenetrans.)
Sedentary endoparasites—nematode that tunnel into the roots, establishing permanent feeding sites from which they do not move. They may protrude from roots as they grow. (Examples: root-knot, Meloidogyne spp.; and cyst nematodes, Heterodera spp.)
Senesce (n. Senescence; adj. Senescent)—To decline with maturity or age, often hastened by stress from environment or disease.
Shothole—Disease symptom characterized by the dropping out of small, round fragments of leaves, making them look as if riddled by shot.
Sign—The manifestation of disease by the presence of structures of the causal agent.
Soilborne—Refers to many fungi able to survive in the soil as saprophytes. Also called “soil inhabitant.”
Sorus (pl. Sori)—A compact aggregation of spores and/or sporophores growing out to the surface of the host.
Spiroplasma—A single-celled, wall-less, spiral, filamentous organism associated with corn stunt and citrus stubborn disease.
Sporangiophore—A sporangium-bearing hypha.
Sporangium (pl. Sporangia)—A fruiting body that produces asexual spores within a more or less spherical wall.
Spore—Reproductive body of fungi and other lower plants, containing one or more cells; a bacterial cell modified to survive an adverse environment.
Sporodochium (pl. Sporodochia)—A cushion-shaped spore-producing body of a fungus.
Sporogenous—Capable of forming spores.
Sporulation—The process of producing spores.
Sterilant—Any agent or chemical that destroys all living organisms in a substance such as soil.
Streak—An elongated lesion with irregular sides.
Stroma (pl. Stromata)—A compacted mass of hyphae that supports sexual fruiting bodies.
Stunted—An unthrifty plant reduced in size and vigor due to unfavorable environmental conditions.
Stylet—Slender, tubular mouthparts in plant-parasitic nematodes or aphids.
Substrate—The substance or object on which an organism lives and from which it gets nourishment.
Sun Scald—Plant tissues burned or scorched by too much sun exposure and other unfavorable conditions.
Susceptibility—The sum of the qualities of a plant and causal agent that allows the development of the causal agent.
Symptoms—External or internal physical characteristics of disease expressed by the host plant.
Systemic—Pertaining to a disease in which an infection leads to general spread throughout the plant body. Also, a chemical that spreads internally through a plant.
Teliomorph—Sexual stage of a fungus.
Teliospore—Thick-walled resting spore produced by some fungi, notably rusts and smuts, that germinates to form a basidium.
Thallus—The vegetative body of the lower plant that has not differentiated into stems and leaves.
Tolerance—Ability of the plant to endure the development of the parasite without showing marked symptoms of disease.
Tylosis (pl. Tyloses)—A bladderlike intrusion of the protoplasm from a parenchymatous cell through a pit into the lumen of a xylem cell.
Variety—One or more races of a pathogen that are characterized by the limitation of their host range to a certain genus or genera. Also, a group of closely related plants of common origin and similar characteristics within a species (see also Cultivar).
Vector—An agent, such as an insect, nematode, or fungus, that may transmit a pathogen.
Vein-banding—Symptom of a virus disease in which regions along the veins are darker green than the tissue between the veins.
Virescent—A state or condition in which normally white or colored tissues, such as flower petals, become green.
Viroid—An infectious nucleic acid without a protein coat that causes hop stunt, potato spindle tuber, or chrysanthemum stunt.
Virulent—Strong ability to produce disease.
Viruliferous—Capable of transmitting a virus.
Virus—Submicroscopic, infectious agent, too small to be seen with a compound microscope, that multiplies only in living cells. A virus consists of nucleic acid (DNA or RNA) surrounded by a protein coat.
Water-soaked—Describing plants or lesions that appear wet and dark and are usually sunken and translucent.
Wilt—Lack of freshness and turgor and drooping of leaves from lack of water; a vascular disease that interrupts the plant’s normal uptake and distribution of water.
Witches’ Broom—Abnormal, brush-like development of many weak shoots.
Yellowing—The yellow color of plant parts resulting from the excessive proportion of yellow pigments, in turn produced by the underdevelopment or partial destruction of the green pigments.
Yellows—A disease characterized by yellowing and stunting of affected parts (caused by fungi, virus, bacteria, or deficiency of essential elements).
Zoospore—Fungus spore with flagella, able to move in water.
Zygospore—A fungal resting spore produced by the fusion of equal gametes.