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Carrot motley dwarf disease: a good example for synergistic relationships between viruses and between virus and vector

Naoto Yoshida: HOKUREN Agricultural Research Institute

<div>Carrot motley dwarf disease is caused by a complex of two viruses: carrot red leaf virus (CtRLV, <em>Polerovirus</em>) and carrot mottle virus (CMoV, <em>Umbravirus</em>). Additionally, the diseased plants contain frequently a virus-like RNA called CtRLV-associated RNA (CtRLVaRNA). Those viruses are transmitted by willow-carrot aphid (<em>Cavariella aegopodii)</em>. Umbravirus and CtRLVaRNA do not encode a coat protein, and so use the coat protein of a helper virus CtRLV for aphid transmission. In Japan, we found CtRLV, CMoV and CtRLVaRNA-mixed infection and single CtRLV infection in carrot fields from different districts in Hokkaido. In our aphid inoculation tests using CtRLV isolates containing CMoV or CMoV + CtRLVaRNA, CtRLV with CMoV induced much severer symptoms in carrot plants than CtRLV alone, in which CtRLV accumulation increased. When co-infected with CtRLV with CMoV + CtRLVaRNA, accumulation of CMoV as well as CtRLV enhanced. The aphids transmitted more efficiently CtRLV with CMoV + CtRLVaRNA than CtRLV alone. In our field and laboratory observations, the aphids appeared to propagate more vigorously on mix-infected carrot plants than on non-infected plants. Together, we found that co-infections of polerovirus and umbravirus (and polerovirus-associated RNA) enhance synergistically their virus accumulation, the transmission efficiency, and the aphid feeding preference.</div>