University of California researchers have come up with a three-step, IPM approach to managing tomato spotted wilt virus (TSWV), an increasing threat to processing tomato crops in the Central Valley.

ROBERT L. GILBERTSON, UC Davis plant pathologist, left, talked about his TSWV research with Rod Clark and Russ Maly, both with CDFA, Fresno, during the vegetable crops research gathering.

Robert L. Gilbertson, associate professor in the Department of Plant Pathology at UC Davis, detailed the plan during a vegetable crops meeting at the UC West Side Research and Education Center near Five Points.

“This is a manageable disease in California, and we think the strategy will help keep the incidence of it in control,” said Gilbertson, who led a team of farm advisors and UC Davis specialists in the three-year project.

The research was funded by the California Tomato Research Institute and the California League of Food Processors.

TSWV, while not a new disease, has been on the increase, particularly in Fresno, Kings, and Merced counties, in recent years. It also occurs in Colusa and Yolo counties.

Vectored plant-to-plant mainly by western flower thrips, the virus, one of the family of Tospoviruses, also goes to peppers, lettuce, radicchio, fava beans, ornamental plants, and weeds, including malva, sowthistle, and prickly lettuce. These may allow it a “bridge” between tomato crops.

Generally, infection in the weed hosts has been low, although high rates can occur in weedy borders and fallow fields. No particular species has been found to be extensively infected by the virus.

However, during the 2009 survey of weed hosts, field bindweed, a known host of both the virus and thrips, was found to be TSWV-infected. That weed species will be monitored in the continuing surveys of areas where the virus has occurred.

Before explaining the management strategy, Gilbertson gave a background on the disease. Key in the program is detection and identification of TSWV, which can have some of the same symptoms as more than a dozen other viruses, including curly top or alfalfa mosaic, which may simultaneously infect the same plant.

“Depending on the virus, you may see all kinds of symptoms from curling, wilting, stunted growth, and yellowing, and it is very difficult to identify which virus by eye,” he said. Necrotic spots or rings on leaves are those mostly identified with TSWV.

Farm advisors can confirm suspected infections by laboratory diagnosis of samples submitted to them; or diagnostic immunostrip test kits, such as AgDia or EnviroLogix, can be rapidly used in the field.

Gilbertson said the virus shows different symptoms according to the age of its host when infected. Tomato seedlings or transplants can become extremely stunted, along with the symptoms above. Infection at early vegetative growth often shows as bronzing of leaves. If an infection occurs at flowering through fruit set, one or two shoots may exhibit the symptoms, plus discolored and distorted fruit.

The first step in the three-part IPM scheme is planning and preparation before the growing season. That includes planting one of the TSWV-resistant varieties. With these, which carry the resistant gene SW-5, Gilbertson said, thrips control is not generally required. Varieties without the resistant gene vary in susceptibility.

Use virus- and thrips-free transplants from greenhouses that monitor thrips and inspect transplants. If thrips are present on transplants, populations should be managed before planting.

The second point, during the growing season, is avoiding planting near established fields of susceptible crops that have confirmed TSWV infection.

Also during the growing season, monitor fields for thrips by using yellow sticky traps and watch for symptoms of the virus. Manage thrips with early treatments if they and the virus are detected. Consider removing plants infected at the seedling stage to limit further spread, and control weeds in and around fields.

Some insecticides for thrips management include spinetoram and spinosad, dimethoate, methomyl, and flonicamid.

The third step, after the growing season, is prompt removal and destruction of old tomato plants and other host crops or volunteers after harvest, on a regional basis. Control weeds and volunteers in fallow fields, non-cropped, or idle land near the next year’s tomato fields.

Touching on other viruses of interest to the processing tomato industry, Gilbertson said tomato yellow leaf curl virus (TYLCV), a potentially devastating disease, is being monitored by the California Department of Food and Agriculture.

Concerns were aroused in 2007 when it was identified in greenhouse tomatoes in the Imperial Valley, and it has become established in weed hosts there. In 2009, it was again found in Imperial County and also in Riverside County.

Gilbertson noted that it’s unlikely the virus’ vector, sweet potato whitefly, could travel over the mountain range between Imperial and Riverside counties. Therefore, the virus probably moved on plant material, and that is what agencies are focusing on.

General symptoms are stunted, upright growth and a bushy appearance. It causes flowers to drop before fruit set, and a heavy infection can mean complete loss of a field. Resistant varieties and an IPM program for this virus have been developed.

Gilbertson added that colder temperatures in Central Valley counties prevent the white fly from having indigenous populations there.

Unlike other viruses, TYLCV prefers tomato, and the natural, tomato-free period between seasons helps exclude it from that part of the state. Nevertheless, he and his colleagues are watching it.

Gilbertson said he has been working with Roger Chetelat of the C.M. Rick Tomato Genetics Resource Center at UC Davis, who has identified a source of resistance to curly top virus.

Obtained from the wild tomato species Solanum lycopersicoides, the resistance may be used to create new varieties resistant to the long-time economic pest of tomatoes, sugar beets, and other crops in the San Joaquin Valley.

A new virus, tomato necrotic spot virus, detected for the first time in 2008 from Yolo County to the Imperial Valley, is another look-alike to TSWV. It is being monitored with laboratory tests.

According to Gilbertson, it is thought to be spread by thrips after pollen from a presently unknown host plant blows onto tomato plants.

It was found in 2009 on a sporadic basis, and although it is not considered economic, its symptoms can be confused with those of TSWV.