Ag Alert June 19, 2024

Until this year, processing tomato grow- ers could use neonicotinoid insecticides in-season to manage beet leafhoppers as long as they followed the product label. On Jan. 1, the California Department of Pesticide Regulation significantly restrict- ed use of four neonicotinoids on a wide variety of crops as part of pollinator pro- tection measures. For processing tomatoes, growers could make applications only from preplant un- til bloom. But the new restrictions allow neonicotinoid applications past that time- frame for a local emergency or to control a quarantine pest. The cause for concern is the 0.13-inch- long beet leafhopper can transmit beet curly top virus, which may kill young toma- to plants. In older plants, it may distort and discolor leaves, stunt growth and reduce fruit yield and quality. Depending on the weather and leafhop- per pressure, the effectiveness of an early season neonicotinoid application may wane two to three weeks after transplant- ing tomatoes into a field, said University of California Cooperative Extension vege- table crop advisor Tom Turini, who serves Fresno County. This may leave the plants at risk later in the season should infected beet leafhoppers migrate into the area. Based on results of ongoing field trials of alternative insecticides at the UC West Side Research and Extension Center, he said neonicotinoids are the most effective products at preventing leafhopper feeding and thus virus infection. Scott Stoddard, UCCE vegetable farm advisor for Merced and Madera counties, is conducting simi- lar trials this season. Turini works closely with area pest con- trol advisors to collect and submit leafhop- per samples to UC Davis plant pathology professor Bob Gilbertson for virus testing. He also submits weed samples from the valley floor for testing, since numerous asymptomatic weeds may still be infected. Leafhopper Continued from Page 6

Beet curly top virus, spread by the the sugar beet leafhopper, inset, may stunt processing tomato plant growth and reduce fruit yield and quality. Agricultural commissioners in Kern, Fresno, Kings, Madera, Merced and Stanislaus counties declared emergencies this spring to control the pest.

As of late May, the percentage of infect- ed leafhoppers has remained relatively low, while a larger percentage of weeds such as goosefoot and lambsquarter has come back positive for curly top virus, Gilbertson said. Based on this year’s results, Turini de- scribed the tomato curly top risk as mild to moderate. “Usually, the hills will dry much ear- lier than they did this year, so we did see the increase in beet leafhopper later than we usually do,” he said. “We have generations of leafhoppers on the valley floor, and we know (the virus) is in the weeds, and we can expect those popu- lations to move.” Turini said last month, “I think there’s a risk as we get further into the season

because there are still tomatoes going in at the end of May right now.” Beet leafhoppers overwinter in the coastal foothills, where they likely pick up beet curly top virus by feeding on infected weeds. As the foothills dry in the spring, the insects migrate into the San Joaquin Valley looking for green plants on which to feed. The insect pest has a wide host range, in- cluding numerous weeds. In fact, tomatoes aren’t even a preferred food. As they probe to taste tomato plants, the insects may in- ject virus and infect them before flying off to other plants or crops. Historically, severe curly top virus outbreaks have occurred in processing tomatoes when both leafhopper populations and the percentage of infected leafhoppers are high, Gilbertson said. This

occurred in 2006 and 2013. Although beet curly top disease out- breaks are unpredictable, he said he ex- pects 2024 to be a relatively low disease year based on high winter leafhopper mortality from wet weather, foothills leafhopper sampling data and virus test- ing results to date. Nevertheless, some fields, particularly along the west side of the San Joaquin Valley, may become infected, he added. “From western Fresno and Kings down into Kern and maybe into southern Merced County historically have been our greatest challenges,” Turini said. “Even in a mild to moderate year, you can still have fields that have damage.” (Vicky Boyd is a reporter in Modesto. She may be contacted at vlboyd@att.net.)

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June 19, 2024 Ag Alert 7