Many viruses are endemic, causing moderate losses each year.
Others, such as those causing rice tungro, give periodic severe epidemics. There are no fully collated figures for world-wide losses due to viruses but some examples has been listed, i.e., rice tungro in SE ASIA and african cassava mosaic in Africa with 1,500 and 2,000 millions dollars per year in losses respectively. However, in recent years the understanding of the genome organisation of plant viruses has increased in parallel with development of molecular biological techniques. The ability to obtain nucleotide sequences of complete viral genomes has also permitted the elucidation and understanding of expression strategies used by many different plant viruses. This review is aimed to summarise some aspects of the main strategies used by plant viruses to express their genomes.