A thrips-transmitted disease first identified in onions 16 years ago by University of Idaho plant pathologist Krishna Mohan is exploding this year in Treasure Valley onion fields.

"Any plant stress aggravates the problem," says Mohan of iris yellow spot virus, which has infected onion seed crops in the Treasure Valley to varying degrees since 1989 and commercial onion bulb crops since 2001. "Any situation that favors the buildup of onion thrips also increases the symptoms."

Lynn Jensen, Oregon State University's Extension educator in Malheur County, says the disease has been "showing up fairly extensively throughout the whole valley" since late July. He believes the crop's health was compromised by early-season rains, which left fields so wet that necessary springtime farming activities compacted the soil. That compaction is now interfering with the onion roots' ability to take up critical moisture, increasing the crop's vulnerability to plant diseases. Populations of the culprit insect are also unusually high, which Jensen suspects is weather-related as well. "We've had more thrips than we've seen in many, many years and they've been almost impossible to control," he says.

Iris yellow spot virus-also found in irises in other countries but not yet in the U.S.-can render onion seed fields unharvestable. Infected flower stalks die and fall over, leaving "a whole tangled mass," says Mohan. In commercial onions, the disease decreases bulb size and slashes the percentage of the crop that qualifies for the "colossal" grade. "Our onion trade is mostly based on the size of the bulb," says Mohan. "This disease affects the size, and that in turn affects the marketable yield."

Jerry Neufeld, University of Idaho Extension educator in Canyon County, says iris yellow spot can be "pretty devastating." Neufeld warns area growers of its presence through the Web-based service www.tvpestalert.net or www.pnwpestalert.net. Indeed, iris yellow spot virus and onion thrips were recently ranked the No. 1 and No. 3 research priorities, respectively, for dry bulb storage onions in the West by developers of a pest-management strategic plan for that crop.

Since Mohan first reported the disease in the valley's onions in 1989, it has been identified in onion-growing areas throughout the West and, last year, even in Georgia. "Every time I present this at a national onion conference, people say, "Oh, I have seen this,'" says Mohan. "I think it has been here for quite some time."

According to Mohan, iris yellow spot virus is characterized by distinctive-"almost artistic"-tan or straw-colored diamond-shaped lesions on the flower stalks of biennial onion seed crops and by elongated dry lesions or flecks on the leaves of annual bulb crops.

Currently, no chemical treatments are available to curtail the disease, although a number of insecticides are labeled for onion thrips control. Jensen says the insects appear to be developing resistance to the commonly used insecticides and "have been getting harder and harder to control every year."

In addition to keeping a lid on thrips populations, the valley's onion growers have been advised to promote crop vigor through good fertilizer and water management, to control volunteer and wild members of the onion family, to destroy cull onions and to plant varieties less susceptible to thrips.

At OSU's Malheur Experiment Station each growing season, Mohan and Jensen make systematic observations of onion variety trials, rating varieties for the severity of iris yellow spot virus symptoms. No varieties are currently resistant, but Mohan says these annual disease evaluations "may help us gain some insight into the relative susceptibility of varieties and give us some initial advantage in recommending less susceptible varieties for future planting.

Soon, Mohan hopes to conduct far more extensive research into the disease and its transmission. "We have a lot of questions about where the virus is and what is transmitting it," he says. Is it overwintering in the Treasure Valley in wild onions, irises or weeds, for example? Is any other thrips species, besides onion thrips, involved in spreading it?

Mohan and Jensen are among a group of Western scientists who are seeking $2.37 million in federal funding for a multi-year, multi-state project led by Colorado State University. Should funding be appropriated, the study will focus on identifying onions with tolerance to thrips or resistance to iris yellow spot virus, quantifying the impacts of straw mulching and novel chemical treatments, and determining the disease's relationship to soil properties, plant stress and management practices.