Necrotic: dead or discolored

Necrotroph: a microorganism feeding only on dead organic tissues

Nematicide: chemical compound or physical agent that kills or inhibits nematodes

Nematode: microscopic, wormlike animals that live saprophytically in water or soil, or as parasites of plants and animals

Obligate parasite: a parasite that can grow only on living organsims

Oogonium: female gametangium of Oomycetes containing one or more gametes

Oomycete: a fungus like chromistan that produces oospores

Oospores: a sexual spore produced by the union of two morphologically different gametangia

Parasite: an organism living on or in another living organism

Pathogen: an entity that can cause disease

Pathovar: in bacteria, a subspecies or group of strains that can infect only plants within a certain genus or species

Penetration: initial invasion of a host by a pathogen

Perfect stage: the sexual stage (or teleomorph) in the life cycle of a fungus

Perithecium: globular or flask-shaped ascocarp of the Pyrenomycetes containing asci, ascospores, and paraphyses. Generally hava an opening or pore

Phytopathogenic: term applied to a microorganism that can cause disease in plants

Polycyclic: complete many life cycles in one year

Polyetic: requiring many years to complete a life cycle

Polymerase chain reaction: PCR, selective amplification of DNA

Primary infection: first infection of a plant by the overwintering or oversummering pathogen

Primary inoculum: overwintering or oversummering pathogen or its spores that cause the primary infection

Race: subgroup or biotype within a species or variety, distinguished from other races by virulence, symptom expression, or host range, but not by morphology

Resistant: possessing qualities that hinder the development of a particular pathogen or the effects of other damaging factors. A plant may be slightly, moderately, or highly resistant.

Resting spore: a sexual or other thick-walled spored of a fungus that is resistant to extremes in temperature and moisture and which often germinates only after a period of time from its formation.

Rhizoid: a short, thin hypha growing in a rootlike fashion toward the substrate.

Ringspot: a disease symptom characterized by a circular area of yellowish or necrotic rings with green tissue inside the ring. This symptom is associated with many virus diseases.

Rosette: disease symptom characterized by a short, bunch growth habit due to subnormal elongation of internodes.

Rot: the softening, discoloration, and often disintergration of plant tissue by enzymes produced by fungal or bacterial infection. Rots may be hard, soft, dry, wet, black, brown, white, etc.

Russet: brownish, roughened areas on the surface (epidermis) of leaves, fruit, stems, and tubers as a result of abnomral cork formation; may result from disease, insects or spray or other mechanical injury.

Rust: a disease caused by one of the rust fungi (Uredinales); a disease giving a "rusty" appearance to a plant.