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Iris Yellow Spot Virus - Should Ontario Onion Growers be Concerned?

What is Iris yellow spot virus?

Iris yellow spot virus (IYSV) is a member of the Tospovirus family. Other examples of tospoviruses include tomato spotted wilt virus and impatience necrotic spot virus.

IYSV has been detected in a number of countries around the world including: Netherlands, Slovenia, India, Iran, Japan, Israel, Brazil and Australia. It was first detected and reported in the United States in 1991 in seed onions grown in Idaho. Since that time, it has been positively identified in Oregon, Washington, Nevada, California, New Mexico, Colorado, Utah, Arizona and Georgia.

How is IYSV transmitted?

The tospovirus is picked up by onion thrips larvae (specifically 2^nd instar). Infected thrips are able to transmit the tospovirus for the remainder of their life. Research has shown that IYSV is not distributed uniformly throughout the onion plant. In fact, the highest levels of IYSV are typically detected in the inner leaves where onion thrips tend to congregate and feed.

What symptoms should you look for?

While IYSV is characterized by straw coloured, diamond- and spindle-shaped lesions, researchers in Georgia have detected the tospovirus in plants with tip dieback and chlorosis in the absence of lesions. IYSV has also been detected in other allium crops (i.e. leek, garlic), iris, and weeds (i.e. redroot pigweed, Jimsonweed). Infected plants can produce good quality bulbs; however, under stressful conditions, plants will lodge and bulb sizing ceases.

Should Ontario growers worry?

• Once a plant is infected with IYSV there are no curative measures.
• In some affected areas in the U.S., 100% of plants are infected.
• Onion thrips, the only known vector, are prevalent in all onion-growing regions.
• Research has shown that some host plants, including Allium species, can be asymptomatic.
• It is unclear as to whether the virus can be spread through infected onion bulbs
• Transplants shipped from affected areas may harbor infected onion thrips.
• And finally, researchers in the U.S. are still unclear on how IYSV has spread throughout the country.

What can Ontario growers do?

Be vigilant. Avoid importing transplants from affected states.

Be aware. Familiarize yourself with disease symptoms, manage onion thrips populations throughout the season, and, keep a lookout.