SALINAS, Calif. — Researchers in Salinas have found a new pest in lettuce and spinach, and they are not sure how or why it is invading a vegetable not considered a target for the disease.

Researchers identify new virus in leafy greens
                                      Photos courtesy Steve Koike
A thrip, about the size of a flea, is the vector for spreading impatiens necrotic spot virus in Salinas Valley lettuce crops.

Impatiens necrotic spot virus, a disease commonly associated with flowers, is cropping up on Salinas Valley spinach and lettuce and is puzzling researchers on how and why the virus is spreading to leafy greens.

Steve Koike, a plant pathologist and farm adviser with the University of California Cooperative Extension, said that before 2006 the virus was extremely rare in the Salinas Valley but is now becoming more common, though it still affects less than 5% of the area’s total lettuce acreage.

“It’s getting to where it’s not hard to find,” Koike, who is studying the disease in lettuce, said.

Beiquan Mou, a research geneticist with the U.S. Department of Agriculture’s agricultural research service in Salinas, said he first detected the virus in his research spinach crop in October and isn’t sure why the disease is now attacking spinach. It’s the first time the disease was found on spinach in California, Mou said.

“We are trying to understand what is going on,” Mou said.

The virus is spread by thrips, Koike said, which introduce the virus into the plant tissue as they eat lettuce or spinach leaves.

Researchers identify new virus in leafy greens
Lettuce damaged by impatiens necrotic spot virus. The virus causes cell damage in the infected leaves, which looks like chemical burn to the naked eye. Some Salinas Valley growers have reported the virus damaging some of their fields, though the problem is not widespread.

The virus causes cell death, which looks like chemical burn on the plants’ leaves, and makes the plant wilt as the disease advances, Koike said.

Thrips are not a new pest in the Salinas Valley, Koike said, and they haven’t been very active so far this summer because of cool temperatures. It’s unclear why the virus is now spreading to more lettuce fields, Koike said, but it corresponds with an increase in the virus worldwide among vegetable crops, including peppers in Georgia and lettuce in Italy.

Mou said he hasn’t received any reports of the virus affecting commercial spinach acreage, but said it’s only a matter of time before the virus appears in commercial production.

One theory on why the virus is impacting more lettuce production is an increase in the thrip population, Koike said.

Because thrips are a pest commonly treated by growers through chemical pesticides, he said that might be a reason why the virus is spreading slowly, though there hasn’t been a noticeable difference between the number of conventional acres and organic acres hit with the virus.

Some fields have been ruined by the virus, Koike said, with one grower reporting that more than half a lot of lettuce was lost to the virus.

Josh Ruiz, Tanimura & Antle’s director of lettuce, said thrips are a pest for lettuce but haven’t caused any problems or introduced the virus into any fields this year.

No growers contacted for this story reported the presence of the virus in their fields so far this summer.

The virus found last year on spinach was the same virus that’s been infecting lettuce, Mou said, though it’s unclear if the disease can spread from plant to plant.

Mou said he is working on an experiment with other researchers to see if a lettuce plant can be inoculated with the virus from a spinach plant.

“We don’t know if this will be a problem or not,” Mou said.