A survey on the infection of Onion yellow dwarf virus and Iris yellow spot tospovirus in seed and bulb productions systems of onion in Calabria, Italy

A survey on the incidence of onion yellow dwarf virus (OYDV) and iris yellow spot tospovirus (IYSV) was carried out over three production cycles of onion ‘Rossa di Tropea’ in Calabria, Italy. OYDV was found to be the prevalent virus. ‘Rossa di Tropea’ seed adjacent to OYDV-infected green onion field had seedlings with 1.76% infection rate determining 36.2% and 98.67% infected plants in the bulbs and in the subsequent seed harvesting times, respectively. When seedbeds were at least one km away from other onion crops seedlings and bulb cultivation had the infection rate close to zero. OYDV was detected in whole plants except the roots and outer desiccated bulb skins. Seed transmission was not detected in ‘Rossa di Tropea’. Early OYDV infection significantly reduced the number and weight of seeds/inflorescence compared to late season infection, while the weight of 100 seeds was not different in the two early and late OYDV infected plants. IYSV was never found in seedbeds. It was always detected first in seed crops (April) than in bulb crops (June), and the final infection rate was higher in seed (2.67%-3.33%) than in bulb crops (0%-0.87%), suggesting there was an internal source of viral inoculum in the field. IYSV was detected in 3/123 apex bulbs randomly collected from stored bulbs and in 12/12 apex fresh bulbs collected at harvest time from infected plants, showing the role of bulbs as IYSV inoculum source. On the contrary, randomly collected bulbs (N=109) from warehouse and bulbs of infected plants (N=22), transplanted after storage, did not result in IYSV-infected plants.