Leek (Allium porrum)-Iris Yellow Spot

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Cause Iris yellow spot virus (IYSV) is new to leek production in the Pacific Northwest, first confirmed on leek seed crops during 2006 in the Willamette Valley. This virus is known to infect other members of the Allium group, including onion and garlic. A number of weeds and ornamentals may also be infected by IYSV.

This virus is transmitted by onion thrips (Thrips tabaci) but has not been shown to be transmitted by other thrips species. Recently it has been found in Colorado that onion transplants can be infected with IYSV as well as carry high populations of onion thrips. Studies have not shown this virus to be transmitted by seed.

Symptoms Affected leek seed crop plants have yellowish or tan diamond-shaped or irregularly-shaped lesions on flower stalks. Necrotic areas develop on leaves, and can be elongated brown lesions or brown flecks that resemble thrips injury.

Cultural control

  • Promote a healthy vigorous crop and avoid stress. Practice good fertility and water management.
  • Control volunteer and wild alliums.
  • Plant as densely and uniformly as possible.
  • Overhead irrigation can provide some suppression of thrips and iris yellow spot.
  • Eliminate weeds in and around onion fields.

Chemical control Monitor the population of onion thrips and control thrips with labeled insecticides. Consult the PNW Insect Management Handbook for currently effective materials.

References Kritzman, A. et al. 2001 Distribution and transmission of Iris yellow spot virus. Plant Disease 85:838-842.

Schwartz, H.F., Otto, K,. and Pappu, H.R. 2007. First Report of Iris yellow spot virus in commercial leek in the United States. Plant Disease 91:113.