Role of onion bulbs in Iris yellow spot virus spread: a contribution to the debate

Iris yellow spot virus (IYSV), genus Tospovirus, is an important pathogen causing severe disease in onion crops worldwide. In Italy, it is an emerging virus in several regions but it is supposed to be present all over the country. The virus is transmitted by onion thrips (Thrips tabaci Lindeman), not seed-transmitted while bulb to plant transmission has not yet been conclusively demonstrated even if IYSV location in bulb was reported by some authors. During a three years of epidemiological study on viral diseases affecting a local cultivar in southern Italy, IYSV life and overwintering cycle was investigated over the complete growing biennial cycle of onion, examining the possibility that bulbs may act as infection pathway to seed crops. For two years, dormant bulbs during storage in warehouse were randomly collected before transplanting for seed crops. The stocks were split into two groups, one was used to assess the virus presence in apex inside the bulb while the remaining bulbs were potted for growing in greenhouse to analyze the presence of the virus in the resulting plants. Virus detection was performed using both real time RT-PCR and nested PCR. The first method showed amplification reactions at late cycles (> 38 Ct) in 18 out of 232 tested samples whereas only 3 out of the 18 samples were amplified in nested PCR. The sequencing of the obtained amplicons confirmed the virus identity. These results suggest that IYSV migrates in bulb and remains quiescent in the apex. Confirmation of IYSV migration in bulb was achieved by detecting the virus in all tested apexes of fresh bulbs collected in the field from green infected plants. When results of IYSV detection in dormant bulbs were related to incidence and appearance of disease in the cultivations for seed production, this analysis led to assume again a role of bulbs as IYSV inoculum source in the field. Unfortunately, incidence of the disease during this study, in particular in the first-year bulb crops, was very low, and therefore, further investigation is necessary to increase the number of infected material tested.