Iris Yellow Spot Virus (IYSV)
IYSV was first reported in Utah in 2001. It is a tospovirus that, in Utah, is transmitted by onion thrips (Thrips tabaci). Thrips have to acquire the virus as larvae to be able to transmit it to healthy plants. Once thrips larvae have acquired the virus they will transmit it for the rest of their lives. The virus has several known hosts related to onion, including shallots and garlic. The virus has also been reported in other parts of the world in iris and Lisianthus cut flower production. More recently, several weeds such as prickly lettuce, sowthistle, green foxtail, and saltbush have been reported as hosts. Common mallow has been identified as a potential host for the virus but has not been confirmed. Not all susceptible weeds show virus symptoms.
Symptoms of IYSV consist of lens-shaped bleached spots on leaves (Fig. 7.13) that sometimes have a green center (Fig. 7.14). In severe cases, the entire onion foliage will die back (Fig. 7.15).
Figure 7.13 - Iris yellow spot virus lesion.
Plants become infected when virus-carrying thrips feed on healthy plants, depositing virus particles. Infected plants may not show symptoms for several weeks and in some cases, symptoms may never appear. It is currently unknown what triggers symptom expression. Once a plant is infected, there is no cure and an infected plant can serve as an inoculum source for neighboring plants. The effect of IYSV infections on yield depends on how early symptoms develop. If symptoms develop while bulbs are still growing, bulb size and quality will be reduced.
Since there is no cure for infected plants, they should be removed and destroyed. The best management strategy is prevention.
- Control thrips (see insect management section).
- Good weed control. Weeds can be a host for IYSV and thrips reproduction where they acquire the virus. Research in Utah has indicated that fields with good weed control along field borders had lower IYSV infections than fields with weedy borders.
Figure 7.14 - Iris yellow spot virus lesion with green island.
Figure 7.15 - Severe leaf dieback due to iris yellow spot virus.
Figure 7.14, Cynthia Ocamb, www.pnwhandbooks.org