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On-Line Glossary: P

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PAGE. Polyacrylamide gel electrophoresis. (16)

papilla. A hump or swelling. (Pl. papillae.) (2)

papillate. Bearing a papilla. (2)

paraphysis. Sterile, elongated cell that may occur in the hymenium, intermixed with asci or basidia, elongating apically and having a free apex. (Pl. paraphyses.) (15)

parasexual cycle. A sequence involving heterokaryon formation, diploidization, and haploidization, often resulting in the formation of recombinant nuclei. Unlike the sexual cycle, the parasexual cycle can occur at any point or continuously throughout the life cycle. (15)

parasite. An organism living in or on another living organism (host) from which it extracts nutrients. (3)

paratype. A specimen other than the holotype and its isotypes that the author cited at the time of publication of the original description.

parasitic. Having the characteristics of a parasite. (14)

parenchyma. A tissue composed of living, thin-walled cells that can continue to divide even when mature; parenchyma cells usually leave intercellular spaces between them. (2)

parthenogenesis. Process of reproduction by the development of an unfertilized egg.

parthenogenic. Pertaining to parthenogenesis. (14)

pathogen. An agent (biotic or abiotic) that causes plant disease. (3)

pathogenesis. That portion of the life cycle of a pathogen during which it becomes, and continues to be, associated with its suscept. (20)

pathogenic. Having the characteristics of a pathogen. (8)

pathogenicity. The capability of a pathogen to cause disease. (2)

pathology. 1. The study of disease. 2. The abnormal condition that constitutes disease. (20)

pathotype. An infrasubspecific classification of a pathogen distinguished from others of the species by its pathogenicity on a specific host(s). (3)

pathovar. In bacteria: An infrasubspecific group that can infect only plants within a certain genus or species. See pathotype. (2)

PCR. See polymerase chain reaction.

pellet. The material concentrated at the bottom of a centrifuge tube after centrifugation. (10)

peloderan. In nematodes: Caudal alae that meet posterior to the tail tip. (14)

penetration peg. In some plant parasitic fungi: The peg-like hypha emerging from an appressorium that penetrates the epidermal cell wall. (3)

perfect state. The state of a fungus characterized by sexual spores. (7)

peridermium. A blister-like aecium as in the form genus Peridermium. (Pl. peridermia.) (7)

peridium . A wall or membrane of sterile cells around a fruiting body (e.g., around a sporangium or delimiting an aecium). (Pl. peridia.) (7)

perineal pattern. Fingerprint-like pattern formed by cuticular striae surrounding the vulva and anus of the mature Meloidogyne female. (14)

periphysis. Short, hair-like filaments that line the canal of the ostiole in some Pyrenomycetes. (Pl. periphyses.) (15)

perithecioid. Like a perithecium. (17)

perithecium. A closed ascocarp with a pore at the top, a true ostiole, and a wall of its own. (Pl. perithecia.) (23)

peritrichous. Of bacterial flagella: Distributed more or less uniformly over the cell surface. (16)

Peronosporales. Specialized forms of the Oomycetes, including aquatic and terrestrial species; many species in this order are plant pathogens (damping-off fungi, downy mildews, and white rusts); unlike the true fungi, they lack chitin in their cell walls. (22)

persistent transmission. See circulative transmission. (5)

phage. A general term used for viruses isolated from prokaryotes including bacteria, blue-green algae (cyanobacteria) and mollicutes (phytoplasma and spiroplasma). The viruses from these different host groups are termed bacteriophages, cyanophages and mycoplasmaphages, respectively. (10)

phasmid. In nematodes: A pore-like structure located in the lateral field of the posterior region of nematodes belonging to the class Secernentea. Function is believed to be sensory. Sometimes called precaudal glands. (14)

phenotype. The observable characteristics of an organism, either in total or with respect to one or more particular named characteristics. (16)

phloem. Food-conducting tissue, consisting of sieve tubes, companion cells, phloem parenchyma, and fibers. (2)

phragmobasidium. A basidium that is divided into more than one cell by transverse or longitudinal septa. (Pl. phragmobasidia.) (15)

phragmospore. A spore having two to many transverse septa. (22)

phragmosporous. Having transversely multiseptate spores. (22)

phyllody. A change of floral petals (leaves) to foliage leaves. (20)

phylloplane. The surface(s) of a leaf. (16)

physiological race. A subdivision of a species of pathogen, particularly fungi, distinguished from other members of the species by specialization for pathogenicity to different cultivars of a host. (5)

physiologic specialization. The existence of a number of races or forms of one species of pathogen based on their pathogenicity to different cultivars of a host. (20)

phytoalexin. A low molecular weight, antimicrobial compound synthesized by and accumulating in higher plants exposed to certain microorganisms (pathogenic and nonpathogenic). (16)

phytopathogenic. Of microorganisms: Capable of initiating disease in plants. (2)

phytoplasma. A prokaryotic, plant parasitic microorganism resembling a mycoplasma but not yet isolable in pure culture or characterized taxonomically. (21)

phytotoxic. Toxic to plants. (2)

phytotoxin. 1. A toxin produced by a microorganism and active against a plant or against plant cells/tissues. 2. A toxin produced by a plant. (16)

pileate. Possessing a cap or pileus. (17)

pileus. The expanded caplike portion of some basidiocarps or ascocarps that supports the hymenium. (Pl. pilei.) (15)

plasmalemma. The cytoplasmic membrane found on the outside of the protoplast adjacent to the cell wall. (2)

plasmid. In many types of prokaryotic and eukaryotic cell: a linear or covalently closed circular molecule of DNA, (distinct from chromosomal DNA, mtDNA, ctDNA, or kDNA and commonly dispensable to the cell), that can replicate autonomously (i.e., independently of other replicons). (16)

plasmodesma. A fine protoplasmic thread connecting two protoplasts and passing through the wall separating the two protoplasts. (Pl. plasmodesmata.) (2)

plasmodium. A multinucleated, usually naked (i.e., bounded only by a plasma membrane) mass of protoplasm that is usually motile and variable in size and form. (Pl. plasmodia.) (16)

plasmogamy. The fusion of two protoplasts. (23)

plasmolysis. The shrinking and separation of the cytoplasm from the cell wall due to exosmosis of water from the protoplast. (2)

plectomycete. A member of the Plectomycetes.

Plectomycetes. In general, a group of primitive or reduced ascomycetous forms that have an angiocarpous fructification without an ostiole, the entire interior of which is irregularly penetrated by ascogenous hyphae, with the result that the generally spherical asci, without accompanying paraphyses or other threads, lie scattered irregularly in a pseudoparenchymatous tissue composed of the ascogenous hyphae. (17)

pleomorphic. Exhibiting pleomorphism. (16)

pleomorphism. 1. In fungi: Having more than one independent form or spore stage in the life cycle. (7) 2. In general: An inherent variability in size and shape (e.g., among the cells in a pure culture or clone of a given organism). (16)

plerome. The plant tissues inside the cortex. (2)

plesionecrosis A symptom exhibited by tissues not yet dead but in the process of dying (e.g., wilting). (Pl. plesionecroses.) (20)

ploidy. The number of (complete) sets of chromosomes in a cell. (16)

polar. At one end or pole of the cell (e.g., a flagellum, spore inclusion, germ tube, etc.). (8)

polycyclic. Of a disease or pathogen: Producing many generations of inoculum and many cycles of infection during a single growing season. (See monocyclic.) (3)

polyetic. Of plant disease epidemics: Continuing from one growing season to the next. (3)

polymerase chain reaction. The selective amplification of DNA by repeated cycles of (a) heat denaturation of the DNA, (b) annealing of two oligonucleotide primers that flank the DNA segment to be amplified and (c) the extension of the annealed primers with the heat insensitive Tag DNA polymerase. (10)

polymorphism. Pleomorphism.

polygenic. A character controlled by many genes. (20)

polynucleate. Having more than one nucleus per cell. (14)

polypore. A member of the Polyporaceae; the hymenium forms tubes in the basidiocarp. (23)

poroconidium. See porospore.

porospore. A conidium produced by an extension of the inner wall of the conidiogenous cell and extrusion through a pore in the wall of the conidiophore. (16)

potexvirus. (Siglum of potato virus X.) Member of a group of plant viruses that infect a wide range of hosts, including monocots and dicots; individual members infect only a narrow host range, typically causing mosaic and ringspot symptoms. Type member: potato virus X (PVX). (16)

potyvirus. (Siglum of potato virus Y). Member of a large group of plant viruses with flexuous particles containing a single molecule of linear RNA, most of which are transmitted by aphids in a noncirculative manner. (5)

predisposition. An increase in susceptibility resulting from the influence of environment on the suscept. (20)

primary cycle. Of plant disease: the first infection cycle to begin in a given season; usually occurring only once per season. (21)

primary infection. The first infection of a plant by a pathogen emerging from a dormant stage in its life cycle (overwintering or oversummering). (3)

primary inoculum. The overwintering or oversummering pathogen or its propagules that cause primary infection. (2)

probasidium. The cell in which karyogamy occurs in the basidiomycetes. (Pl. probasidia.) (15)

probe. A specific sequence of DNA or RNA used to detect complementary sequences by hybridization. (10)

procorpus. In nematodes: Cylindrical portion of the corpus anterior to the metacorpus. (14)

prodelphic. In nematodes: Having uteri parallel and anteriorly directed at the origin. (14)

proliferation. A rapid and repeated production of new cells, tissues, or organs; specifically, a hyperplastic symptom of plant disease in which organs continue to develop after they have reached the point beyond which they normally do not grow. (20)

prolepsis. A hyperplastic symptom of disease in which organs appear before the natural time. (Pl. prolepses.) (20)

promoter. A region of DNA, usually upstream of a coding sequence, that binds RNA polymerase and directs the enzyme to the correct transcriptional start site. (10)

promycelium. The short hypha bearing sporidia produced by the teliospore; the basidium. (Pl. promycelia.) (2)

propagative virus. A circulative virus that replicates in its insect vector. Such a virus is said to be propagatively transmitted (e.g., potato yellow dwarf virus). (16)

propagule. Any disseminative unit of an organism (e.g., a spore, a mycelial fragment a sclerotium). (16)

protease. A generic term for an enzyme that cleaves a polypeptide chain. (10)

protectant. Any chemical agent that interacts with a pathogen on the plant surface to inhibit infection before it takes place. Non systemic. (16)

protection. A principle of plant disease control in which a barrier is placed between suscept and pathogen (e.g., the use of protective chemical dusts or sprays). (20)

prototroph. A strain of microorganism whose nutritional requirements do not exceed those of the corresponding wild-type strain. (2)

protoplast. A plant cell from which the cell wall has been removed. (16)

pseudocoel. In nematodes: Body cavity containing a fluid in which the various internal organs are suspended. (14)

pseudoparaphysis. A sterile thread that grows downward in the cavity of some ascocarps, usually becoming attached at the bottom. (Pl. pseudoparaphyses.) (15)

pseudothecium. An ascostroma resembling a flask-shaped perithecium. (Pl. pseudothecia.) (15)

punctate. Having surface dot(s), pore(s), etc. (16)

pustule. Small blister-like elevation of the leaf epidermis created as spores emerge from underneath and push outward. (2)

pv. Pathovar. (21)

pycnidiospore. A conidium formed in a pycnidium. (16)

pycnidium. A closed sporocarp, usually opening by a pore, that contains a cavity bearing conidia. (Pl. pycnidia.) (15)

pycniospore. A spore (spermatium) borne in a pycnium in the Uredinales. (15)

pycnium. In the Uredinales, stage 0, consisting of the male fertilizing elements (pycniospores) and female elements, the flexuous hyphae. (Pl. pycnia.) (15)

pycnospore. An obsolete term for a pycnidiospore or a pycniospore. (16)

pygmism. The state of being dwarfed or reduced in size. (20)

pyrenomycete. One of the Pyrenomycetes. (7)

Pyrenomycetes. Class of Ascomycotina, traditionally based on taxa with perithecioid ascomata that are ascohymenial in ontogeny and have unitunicate asci, often with an apical annulus. (7)

pyriform. Pear-shaped. (17)

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