Population dynamics of iris yellow spot virus in onion fields and lisianthus greenhouses in Japan

Iris yellow spot virus (IYSV, genus Tospovirus) mainly occurs on onion and lisianthus plants (Eustoma grandiflorum) in Japan. The virus is genetically diverse and is classified based on its nucleocapsid protein sequence into two genotypes, IYSVNL (NL) and IYSVBR (BR), first isolated in the Netherlands and Brazil, respectively. Both genotypes are present in Japan, and they are sometimes found in the same field. Furthermore, the two genotypes are occasionally detected in a single necrotic lesion of an infected plant (designated as a ‘mosaic’ genotype). In this study, the population structures of these two genotypes and their dynamics in onion fields and lisianthus greenhouses from 2004 to 2006 were analysed using reverse transcription-PCR-restriction length polymorphism analysis (RT-PCR-RFLP). In onion fields the two genotypes were detected in all three years, and the BR genotype predominated within a year. On the other hand, both genotypes were detected at the first outbreak each year in almost all lisianthus greenhouses; however, the BR genotype was more widespread than the NL genotype. These results suggest that the incidence of IYSV disease in lisianthus during each year was triggered by the invasion of viruliferous Thrips tabaci that have acquired one or both genotypes from nearby onion fields. Although the BR genotype predominated in the last stages of cultivation, IYSV disease might be expanded by monocyclic patterns but may occasionally spread secondarily in lisianthus greenhouses.