Viruses cause several destructive plant diseases and are accountable for massive losses in crop production and quality in all parts of the world. Infected plants may show a range of symptoms depending on the disease but often there is severe leaf curling, stunting (abnormalities in the whole plant) and leaf yellowing (either of the whole leaf or in a pattern of stripes or blotches).[25] Most plant viruses are therefore transmitted by a vector organism (insects, nematodes, plasmodiophorids and mites) that feeds on the plant or (in some diseases) are introduced through wounds made, for example during agriculture practices (e.g. pruning).