A new line item in surveys of low desert pest control advisors looks to help researchers better understand and document a lettuce virus that arrived in the desert three years ago on transplants out of Salinas.

For the last two decades, John Palumbo, an entomologist with the University of Arizona, has asked PCAs about their insecticide use and what pests they’ve treated against. One of these pests blamed for the cosmetic scarring of lettuce is now believed to vector a plant virus that can destroy entire lettuce fields. This happened a couple seasons ago in the Salinas Valley when INSV, or Impatiens Necrotic Spot Virus, was blamed for destroying much of the late summer crop there.

Palumbo, who specializes in vegetable crop pests in the Yuma region, says the western flower thrip is one of the most economically important pests infesting desert lettuce now for two reasons: the cosmetic scarring of plants, and the INSV can bring with it.

The issue is so important to lettuce growers that Palumbo added a line item to his annual spring survey of PCAs, asking them specifically about their efforts to control thrips and INSV in lettuce.

Controlling the pest typically requires two to three insecticide treatments each season to prevent feeding damage in lettuce.

Researchers continue to monitor thrips activity in lettuce, alternate crops, and weeds in the desert. Because INSV can be harbored outside of lettuce, particularly in weeds, a better understanding of the disease in weeds is needed, Palumbo says.

Host plants for INSV include lettuce, celery, broccoli, cauliflower, alfalfa, cotton, melons, durum wheat, Sudan grass, Teff grass, and several weeds. Cheeseweed, purslane, nettleleaf goosefoot, and sowthistle collected from Tacna and Bard in southwest Arizona were found in surveys to be infected with INSV, according to Palumbo.

How INSV spreads

Researchers believe that only the adult western flower thrip can spread the disease by feeding on lettuce. However, it is the juvenile thrip that contracts the virus by feeding on infested plants. For this reason, it is important to control all life stages of the western flower thrip.

Primary INSV infection or spread occurs where infected adult thrips migrate from an outside source into lettuce and feed on the lettuce plant. Secondary infection happens as these adults reproduce in infected lettuce and the developing larvae feed on the diseased plant.

Prior to 2021 INSV had not been seen in the Yuma region. Palumbo says in the 30 years prior to that, he never saw INSV, or a related tospovirus known as Tomato Spotted Wilt Virus (TSWV) in desert lettuce.

Palumbo says researchers demonstrated that INSV arrived in the desert through infected lettuce transplants or infected adults on these transplants arriving from Salinas.

“Given that Salinas has battled INSV each of the last three summers and produce a fair amount of lettuce and brassica transplants grown in Yuma, we must assume that INSV will again move into the desert at some point,” Palumbo writes in a report published last March.

Palumbo recommends several practices to reduce the risk of INSV outbreaks in desert lettuce:

It is important to test symptomatic plants immediately. Palumbo says the earlier INSV can be discovered, the better the chances are to reduce secondary spread within that field, and to surrounding lettuce. The UA Diagnostics Plant Pathology Lab and Pathologist Bindu Poudel-Ward can confidently confirm the presence of INSV on symptomatic plants.