Iris yellow spot virus (IYSV)
Onion thrips (Thrips tabaci)
Infections remain localized and occur where thrips feed, resulting in an uneven distribution of the disease within the plant. Iris yellow spot virus (IYSV ) can only be detected in or adjacent to lesions. Infected leaves are generally dull in appearance. Initially, lesions can be irregular to diamond-shaped and chlorotic to bleached white in color. Distinctive, defined borders may or may not develop as lesions elongate. Leaves dieback as lesions enlarge and coalesce. Lesions may completely girdle the scape and cause lodging before seeds mature. Infected onion plants usually produce undersized, asymptomatic bulbs. Infected leek plants are stunted.
Conditions for Disease Development
Onion thrips (Thrips tabaci) transmit IYSV in a persistent manner. Disease severity is positively correlated with thrips populations in the field. This virus is not seed transmitted. Over-wintering onions, volunteers from prior productions, infected transplants and alternate hosts can all serve as sources of both vector and virus. Bulb to plant transmission of IYSV has not been demonstrated in bulbs collected from infected plants.
All onion and leek varieties are susceptible to IYSV ; however some varieties appear less susceptible than others. Many pesticides are available to help manage and control weeds, alternate hosts and the thrips vector. In addition, culled onions from packing operations should immediately be removed from the vicinity of all onion productions to provide further control of the thrips vector.