Biological and genetic characterization of carrot red leaf virus and its associated virus/RNA isolated from carrots in Hokkaido, Japan

Carrot motley dwarf (CMD) is caused by mixed infection of carrot red leaf virus (CtRLV) with either carrot mottle virus (CMoV) or carrot mottle mimic virus, and additional infection with CtRLV-associated RNA (CtRLVaRNA). Here, the author investigated the viruses or virus-like RNA isolated from carrots with reddening symptoms in Hokkaido, the northern island of Japan. Three types of infections were mainly detected: single infection with CtRLV, which was most prevalent; double infection with CtRLV and CMoV; and triple infection with CtRLV, CMoV, and CtRLVaRNA. Fields with the three agents were severely affected, with diseased plants showing mottling, whereas in fields where disease incidence was low and sporadic, CtRLV was often found alone in plants with mild symptoms. Inoculation tests using carrot plants showed that CMoV enhanced disease severity, and the RNA accumulation of CtRLV. However, in the presence of CtRLVaRNA (+ CMoV), distinct symptoms such as systemic mottling and stunting developed, while the enhancement of CtRLV accumulation was abolished. These results imply that CtRLVaRNA (+ CMoV) antagonizes CtRLV despite its dependence on CtRLV for aphid transmission, and that mixed infection with CtRLVaRNA is involved in the development of the conspicuous mottling. All agents detected in Hokkaido were very similar to European and American isolates in terms of their genomic sequences and host range. This represents the first report of CMD in Japan, and provides further information on the genetic and biological properties of CMD-associated agents, as well as the aetiology of the disease.