A bothersome plant virus first discovered in 2006 in the Salinas Valley of California was discovered last season near Tacna, Ariz., a farming community east of Yuma, and in five different fields in California's Imperial Valley.

The disease is like others in lettuce that cause decay and render the crop unmarketable, except that it is not caused by a soil-borne pathogen but vectored by a common insect.

Impatiens necrotic spot virus, or INSV for short, can resemble burn damage caused by chemical applications, according to Steven Koike, a plant pathologist and director of TriCal Diagnostics in Hollister, Calif. Koike was a farm advisor with the University of California when the disease was first discovered in California.

By itself, INSV in lettuce does not cause wilting, root rot or discoloration, crown rot or vascular discoloration, according to Koike. In combination with other pathogens, a confusing combination of symptoms can arise, necessitating lab tests to determine the cause.

At the end of the day INSV does the same thing to lettuce that other plant viruses do – it renders the crop unmarketable. Symptoms can vary depending on when the plant was infected, the type of lettuce involved, and the environmental conditions, he said.

Transmission

Unlike the soil borne pathogens that lead to Fusarium and other plant diseases in produce crops, INSV is vectored by the Western flower thrips.

The Western flower thrips has long been a pest of concern for lettuce, spinach, and cabbage growers because of the feeding damage it left behind, according to John Palumbo, Extension entomologist with the University of Arizona. That feeding damage also leaves crops unmarketable, particularly for fresh market uses.

According to Palumbo, the feeding damage may be more problematic than the INSV it vectors because of the biology of how that disease is vectored.

Daniel Hasegawa, a research entomologist with USDA-ARS in Salinas, says the Western flower thrips and its ability to vector tospoviruses – INSV and Tomato spotted wilt virus (TSWV) are examples of this – is limited. The virus must be acquired as larvae for the insect to vector the disease. Larvae alone cannot transmit the virus; only adults can transmit the virus. Additionally, the virus is not passed from adult to offspring.

This suggests that good thrips control can break this cycle because only the larvae, feeding on infected plant material, can acquire the pathogen and vector it once the insect reaches adult stage.

Palumbo says thrips control can be achieved in part through good weed management. Keeping fields, the edges, and nearby areas free of locations for the pest to migrate is helpful. There are effective insecticides that are also helpful, he said.

Ongoing research

Scientists still do not know how INSV found its way to Arizona. Researchers with the University of Arizona, including Stephanie Slinski, a plant pathologist, and the associate director of the Yuma Center of Excellence for Desert Agriculture, continue to seek answers.

Slinski saw thrips in alfalfa last year and wondered if INSV could exist there and act as a "green bridge" of sorts between the growing seasons. She had similar questions about weeds that serve as a good thrips host. Her surveys turned up no positive INSV cases in the nearby alfalfa fields, but they did turn up common infections in nearby weed banks. Not all weeds showed positive INSV infections, leading to more questions.

Researchers with the University of California are likewise looking at the disease as it was found late in the desert growing season last year.

According to Apurba Barman, Extension integrated pest management advisor in Imperial County, Calif., the detection came at the end of the desert growing season, and "did not seem to be widespread and of significant concern this year."

Managing thrips populations in lettuce will be a priority this season, as will addressing weeds that can host thrips and/or the disease.