Glossary Plant Breeding

Viroids. Very small particles consisting of genetic material alone (RNA) and lacking
enclosing membrane (of protein). It causes certain diseases in plants, potato spindle
tuber (the simplest infectitious agent), for example. Viroids are simpler than viruses.
Virulence. Capacity of a pathogen to incite a disease. A measure of degree of
pathogenicity of a pathogen. It is used to denote qualitative rather than quantitative
differences in the pathogenicity of races of pathogens. According to Van der Plank
(1975), virulence and avirulence in the pathogen are the counterparts of vertical
susceptibility and vertical resistance in the host, respectively. Virulence involves gene
diversity, probably largely through mutation.
Virulent Phage. A phage that cannot become a prophage. Such a phage after infection
always leads to lysis of the host cell.
Virus. Any of the infectitious, subcellular, and ultramicroscopic particles representing
potentially pathogenic agents. A virus consists of a nucleic acid (DNA/RNA) plus
protein (capsid). These are independent genetic systems (endowed with genetic
continuity and mutability) which possess their own evolutionary history. Viruses have
only their own genetic material; they lack in a cell membrane, a metabolic machinery,
and a biosynthetic machinery for the synthesis of proteins. Therefore, they are not
considered to be living organisms. They use the synthetic machinery of host cells for
their multiplication. Major groups of viruses are bacteriophages (bacteria “eater”),
animal viruses (DNA + protein), and plant viruses (RNA + protein). Viruses may be
as small as X174 (having only 12 capsomeres), and as large as adeno virus (252
capsomeres).
Virus ( of computer). A software (computer programme) written to destroy or corrupt
data in a computer. It is transferred from one computer to another through floppy
disks and compact disks (CD’s). When an infected floppy disk/CD is used, the virus
enters the random access memory (RAM) directly or by first entering the hard disk. It
either deletes the data or disables the computer hardware and prevents it from
working. Some common viruses are Yankee Doodle, April the first, July the 4 th , etc.
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