Satsuma dwarf virus (SDV), Citrus mosaic virus (CiMV), Natsudaidai dwarf virus (NDV), Navel orange infectious mottling virus (NIMV), and Hyuganatsu virus (HV) are widely spread and cause serious damage to citrus production in Japan. SDV also occurs in some parts of China, Korea and Turkey. Biological, serological and molecular characterization of these viruses revealed both inter-relationships among these viruses and relationships with other plant viruses. SDV is a definite virus species in the genus Sadwavirus, and CiMV and other related viruses are classified as strains of SDV. Based on the serological and molecular biological information, useful detection methods utilizing DAS-ELISA, immunochromatographic assay (ICA), and RT-PCR have been developed. These detection methods will promote distribution of SDV-free nursery plants, which is a key control measure. Transgenic rootstocks which harbor coat protein gene and are tolerant to CiMV have also been developed.