Utah Pests News Fall 2008

Iris Yellow Spot Virus Surveyed in Utah Onion Fields

  Fig. 1. Straw-colored lesions of iris yellow spot virus on an onion leaf.

Iris Yellow Spot Virus (IYSV) is a new disease affecting onion crops worldwide.  It was first observed in the western United States in the 1980s but was not described until 1998.  It was found in a commercial onion field in Utah in 2000.

This summer, Kent Evans’ lab and the UPPDL surveyed 15 fields throughout northern Utah for the presence of IYSV.  In each field we noted disease severity ratings in 30 random locations, and collected an additional 30 random samples to test for the presence of the virus.  The test (ELISA, or enzyme-linked immunosorbent assay) involves a color change; those samples that change color are positive for the virus.  Early in the season, there were plants that were positive for the virus, but showed no visible symptoms on the leaves.  Symptoms didn’t become apparent in the field until early August.  We will continue monitoring the fields until they are harvested.

  Fig. 2.  Iris yellow spot virus lesions sometimes have green centers.
  Fig. 3.  A leaf infected with iris yellow spot virus showing large areas of necrosis.
Infection by the virus causes dry, straw-colored lesions on both leaves and scapes (Fig. 1).  The lesions will usually appear on the margins of the youngest, fully expanded leaves or on swollen portions of the scape.  Sometimes the center of lesions will be green (Fig. 2), or will have concentric rings of alternating green and straw-colored tissue.  The number and size of lesions will increase and the lesions will become necrotic as the disease progresses.  Large areas of foliage can become necrotic (Fig. 3), slowing plant development. Infection in seedlings can kill plants or severely stunt growth so that the field must be replanted or abandoned (which has not occurred in Utah).  Large necrotic lesions that develop on scapes will cause the scapes to lodge (fall over).  The highest incidence of symptomatic plants is usually found around the field margins.

Little is known about the disease cycle of IYSV.  It persists in areas where onion bulb crops, seed crops, or both are produced year-round.  The virus can also persist in volunteer onions, infected onion transplants, ornamentals, and weed hosts without causing symptoms.  IYSV is thought to be spread exclusively by onion thrips (Thrips tabaci), and a high population of this insect increases the risk of successfully transmitting IYSV in onion fields.

IYSV is a difficult disease to manage.  Using clean transplants free of virus and thrips is important.  Eliminating weeds, volunteer onions, or other alternate hosts will reduce the reservoir of IYSV and thrips.  Uniform, dense plant populations can reduce the incidence of IYSV.  Overhead irrigation seems to suppress thrips populations and decrease IYSV incidence and severity.  Use management practices that reduce plant stress; stressed plants will have higher disease severity.  Straw mulch can reduce thrips populations and increase moisture retention.

-Erin Frank, Plant Disease Diagnostician (No Longer at USU)