Tomato yellow leaf curl virus, one of the world’s most damaging diseases of tomatoes, is being eyed closely by plant pathologists and regulatory agencies, who want to prevent it from moving into the crop in California’s Central Valley.

Robert Gilbertson, plant pathology professor at the University of California, Davis, says, even though TYLC is established in Imperial and Riverside counties, it could be kept out of the San Joaquin and Sacramento valleys, if care is taken with movement of tomato transplants and management of weed hosts that could act as a reservoir for infections.

Gilbertson detailed the disease and its history during a gathering of growers and other interested parties at the UC West Side Research and Education Center near Five Points.

One of the family of Geminiviruses, TYLC was discovered in Israel in 1940. It was inadvertently introduced to the Dominican Republic on Israeli tomato transplants in the early 1990s and eventually spread to Florida, Texas, Arizona, and Mexico before being found in Southern California in 2007. It goes to all tomato crops, including processing, fresh, and specialty varieties.

Infected plants show up-curled leaves, with bright yellow discoloration on leaf edges and interveinal areas. Early infection causes leaves to drop, a “bonsai” or broccoli-like form of stunted growth, and death before any yield.

Major processing tomato varieties grown in California are highly susceptible, although resistance could be bred into them if needed.

“We want growers and PCAs to be on the lookout for these symptoms in the field and bring in infected plants to local farm advisors, who can get them to us or the California Department of Food and Agriculture for testing,” Gilbertson said.

Symptoms can resemble those of tomato mosaic virus, but he urged everyone to submit samples when there is any question.

TYLC prefers tomato, but will also go to tobacco species and other solanaceous crops, some of which, along with weeds such as nightshade and jimsonweed, show no symptoms.

It is vectored only by sweet potato whitefly and silverleaf whitefly, and Gilbertson said that is an advantage in that those species do not naturally occur farther north than Kern County. The lack of tomato crops in the Central Valley during the winter is also a plus. Nevertheless, it can overwinter in symptomless weed species in the absence of tomato hosts. The virus is not transmitted by seed or touch.

Surveys for TYLC during 2008 showed no infections in Fresno, Kern, Kings, Merced, and Yolo counties, with the exception of a single transplant in Merced County.

When the whiteflies encounter an infected transplant, they pick up the virus and continue to transmit it through their lifetime. They move only about five to seven miles, although winds can carry them farther.

“Symptoms appear in plants two to three weeks after they are infected,” Gilbertson said. “The problem is infected transplants could be moved before the symptoms show. Long distance movement of infected plants is the key way the virus moves.

“I can’t emphasize enough the importance of not moving transplants from areas where the virus is established northward into main production areas.”

Gilbertson added, however, that tomato transplants from the Yuma and Imperial valleys could be allowed in the Central Valley without endangering the tomato industry if strict precautions are met.

“The transplant production season in those desert areas is when temperatures are cool and whitefly populations are knocked down significantly.

“Transplant growers there can produce plants that are whitefly-free and virus-free with an integrated management strategy using neonicotinoid drenches, monitoring for whiteflies with sticky cards, inspections of plants before shipment, and perhaps a contact spray before shipping,” he said.

Nevertheless, he added, CDFA officials, mindful of the economic significance of the tomato industry and how the virus has spread widely elsewhere, have sought to create a regulated program and allow counties to make decisions on transplant movement.

Carol Hafner, Fresno County Agricultural Commissioner, said the matter of regulating movement of tomato transplants for the 2010 season is being discussed by state, county, and industry officials, and at this point “nothing is set in stone.”

One issue is the lack of a statewide survey to substantiate that tomato production areas are free of the virus, she said. Furthermore, there is the potential reservoir of the virus in weeds, which do not show symptoms of the virus.

A survey and a statewide quarantine have been set aside because of the state’s and counties’ dire financial difficulties. Among alternatives to monitor transplants is a protected area system with inspection measures paid for by growers and/or nurserymen.

Hafner said her office is asking for input from growers and nurseries in Fresno County by contacting Tye Hafner at thafner@co.fresno.ca.us or at 1730 S. Maple Ave. Fresno, Calif., 93702. Those in other counties can contact their respective agricultural commissioners to make comments and learn of latest developments.

Turning to another virus that definitely exists in the Central Valley, tomato spotted wilt, Gilbertson said early management of the vectors, western flower thrips and other thrips species, is important.

The object is to reduce nymphal populations before they can transmit the virus as adults. Part of what makes the virus so difficult to control is it also infects onions, as well as many other crops, ornamentals and weeds.

The California Tomato Research Institute, Inc. has been funding efforts to monitor thrips on transplants to learn how the virus is getting to processing tomatoes.

“The goal is to develop regional integrated pest management strategies that will minimize the threat of spotted wilt to tomato production in all of Central California,” Gilbertson said.

The survey this year of greenhouses has indicated that transplants are not a significant source of thrips or the virus, based on sticky trap counts and fava bean indicator plants. Earlier observations showed that transplanted fields have no more of the virus than direct-seeded fields.

Sticky trap results in fields of processing tomatoes and other crops from Yolo County to Fresno County over the past three years showed buildups of thrips in April and May. “That’s why we are recommending controlling the immatures at about that time,” Gilbertson said.

He said investigation into where the virus overwinters has revealed that almond blossoms support very few western flower thrips populations.

Weeds such as London rocket, malva, prickly lettuce and sowthistle can have relatively low amounts of the virus, but when thrips pick it up, they can magnify it.

Tom Turini, Fresno County farm advisor, in evaluating various insecticides for thrips and gauging virus resistance of several varieties, noted that with the more susceptible varieties a more aggressive control program is needed.