(Jan. 12, 12:10 p.m.) California’s Salinas Valley, a prime source of spring, summer and fall lettuce crops, is under attack by impatiens necrotic spot virus or INSV, a disease that could affect future supplies.

“As of today, I believe there are only two places in the world where it’s occurring in lettuce — the Salinas Valley and in Italy,” Steve Koike, Monterey County plant pathology farm advisor for the University of California Extension, said Jan. 7.

As the name implies, INSV is known for attacking ornamental crops and has for many years been a challenge for greenhouse flower growers in Monterey County, Koike said.

The disease was not present in the area’s lettuce crops until fall 2006, he said.

Why the virus began to attack the vegetable crops remains a mystery.

“We thought it may be a change in the virus, maybe a mutation or a novel strain,” Koike said.

Samples were shipped to researcher Robert Gilbertson at the University of California-Davis. He compared the molecular fingerprint of the INSV in the lettuce crops with the virus in more conventional hosts.

“He found that, surprisingly, the two were identical,” Koike said. “Based on that information, it does not appear to be a new strain, and we have even less idea of why this problem surfaced.”

Thrips spread the disease

The virus is spread by thrips, an insect that grows to a length of about 0.04 of an inch, nearly invisible to the human eye. The size of the pest is part of the problem, Koike said.

“They are so tiny, they can be carried long distances on the wind, and they reproduce rapidly,” he said.

Making matters worse, the virus is quite aggressive on lettuce, and that translates to crop loss, Koike said. When the virus strikes a field, the infection rate is typically 3% to 5%, he said. But there have been fields where the disease has marred more than 25% of the crop, what Koike calls a wipeout.

Not toxic to people

Consuming diseased lettuce is not a health hazard for humans.

“It’s (mainly just) quality issues, stunted growth and yellow and brown spots,” Koike said. “Because of the damage, grower-shippers won’t harvest the infected vegetables, because they won’t make grade.”

If there is a silver lining, it is the resulting collaborative effort to fight the pest. The lettuce industry is financially supporting his work in the fields, Koike said, and the efforts of on campus researchers in the laboratories at U.C. Davis.

Grower-shippers do have some effective insecticides, he said, but not effective enough.

“It’s not uncommon for a grower to spray a lettuce field only to have the thrips back again in less than a week,” Koike said. “The pests have been reintroduced to the field.”

Thrips do have quite a few natural predators, but not enough of them to stem the tide, Koike said. Compounding the growers’ dilemma is that insecticides that kill the thrips also kill the predators, he said.

Thrip population explosion

Populations of thrips worldwide have skyrocketed in the past five years, Koike said. It is another mystery researchers have yet to solve, but a problem that may cure itself.

“With the elevated number of thrips, we may see in the next few years elevated predator numbers,” Koike said. “That’s often how natural systems work.”

Another mystery to be solved — the source of the virus. Suspects include weeds or other alternate hosts that are infected but show no symptoms, Koike said. Surveys have uncovered a few weeds that are INSV reservoirs, but too few to have caused the outbreak, he said.

Controlling the pest

In the short term, grower-shippers can hope that entomologists will get a handle on thrips’ management, Koike said.

“It’s not a matter of eradicating the thrips, but finding the right tools and combinations to keep them at a satisfactory level,” he said.

In the long term, researchers are attempting to breed lettuce varieties that can tolerate the disease or are virus resistant.

“We understand what’s happening, but we don’t have all the answers,” Koike said. “We’re missing some of the pieces of the puzzle.”

California growers fight lettuce-damaging virus
Impatiens necrotic spot virus, or INSV, stunts growth and causes yellow and brown spots on lettuce leaves but is not toxic to people.