variability: property or ability of an organism to change its characteristics from one generation to the other

vascular: term applied to a plant tissue or region consisting of conductive tissue

vector: animal able to transmit a pathogen

vegetative: asexual

vertical resistance: complete resistance to some races of a pathogen but not to others

virion: a virus particle

viroids: small, low-molecular-weight ribonucleic acids (RNA) that can infect plant cells, replicate themselves, and cause disease

virulence: degree of pathogenicity of a given pathogen

virulent: capable of causing a severe disease; strongly pathogenic

viruliferous: said of a vector containing a virus and capable of transmitting it

virus: a submicroscopic obligate parasite consisting of nucleic acid and protein

wilt: loss of rigidity and drooping of plant parts generally caused by insufficient water in the plant

witches' broom: broomlike growth or massed proliferation caused by the dense clustering of branches of woody plants

xylem: plant tissue consisting of tracheids, vessels, parenchyma cells, and fibers; woods

yellows: plant disease characterized by yellowing and stunting of the host plant

zoosporangium: sporangium which contains or produces zoospores

zoospore: spore bearing flagella and capable of moving in water

zygote: diploid cell resulting from the union of two gametes