California's tomato industry is battling one of its usual culprits — the Tomato Spotted Wilt Virus (TSWV), and is threatened with a relatively new one — Tomato Yellow Leaf Curl Virus (TYLCV).
“TSWV which causes tomato spot wilted disease has been here in California for many years,” Bob Gilbertson, University of California, Davis plant pathologist reminded growers and others at the research section of the California Tomato Growers Association's (CTGA) annual meeting. “It comes and goes in terms of its severity on tomatoes. TYLCV, on the other hand, is a newly introduced virus. It was first found in the Imperial Valley in March 2007, so it is a new potential problem that our industry faces.”
These viruses are most commonly spread from one plant to another via insects, according to Gilbertson. They are very difficult to diagnose and control. TSWV and TYLCV are particularly concerning due to their wide host range, potential to spread quickly and their potential to wreak havoc on yield and quality.
“TSWV is spread exclusively by thrips, especially the Western flower thrips in California,” Gilbertson says. “For the adult to spread the virus, it must acquire it in the immature form.”
That's important from a management standpoint. “The larvae must feed on an infected plant to contract the virus before they can transmit it as adults,” he says. “That means you want to control early, immature populations so that you don't get large populations of adults that can vector the virus.”
Tomato spot wilt disease, which is caused by TSWV, has increased in incidence in California in recent years and was particularly worrisome in 2005. That has prompted concern the disease might become a major threat to the tomato industry. To address that concern, the California Tomato Research Institute (CTRI) funded a study in 2007 to better understand where the virus originates, how it spreads and how management strategies could be developed to address it.
Researchers monitored thrips populations and virus incidence in greenhouses and commercial fields in 2007. They found relatively low levels of thrips in greenhouses, and no evidence of disease, which led them to believe the virus is not being introduced into the field by transplants.
Once tomatoes are in the field, however, the story changed. Researchers monitored fields (mostly in the Fresno area) that were direct seeded as well as transplanted.
“Thrips pressure was low to moderate last year,” Gilbertson says. “The disease was first detected on April 20 in a direct seeded field. Eventually, spotted wilt appeared in all the monitored fields, but it stayed at relatively low levels. The incidence was slightly higher in direct seeded fields than transplant fields. That's consistent with the idea that transplants are not bringing the virus into the fields, but there are sources of inoculum that are bringing the virus into the fields at a later date.”
Thrips larvae were also found in monitored tomato fields indicating that reproduction was occurring in tomatoes — something that has been disputed in the past, but appears to be happening based on researchers' observations.
Tomato spotted wilt disease didn't blow up into a major problem last season. Part of the reason could have been that CTRI blasted e-mail alerts to tomato growers as soon as thrips populations were detected.
“This gave growers the opportunity to get out and start implementing thrips management strategies immediately,” Gilbertson says. “Many of the growers in the Fresno area did this. We believe that by getting out early and managing the larval populations, they were reducing the opportunity for those populations to grow into adults that were carrying the virus. Early intervention and knowing when the thrips are beginning to build up in the field seems to be key in slowing down the spread of this virus.”
Another area of last year's research concentrated on potential sources of inoculum. Nearby radicchio fields were observed with similar symptoms and then confirmed as positive for TSWV. Researchers also looked at weeds, but didn't find the virus present.
“We think it is bridge hosts such as radicchio, more than weeds, that are harboring thrips and loaded with the virus that are a more serious threat to nearby tomato fields,” he says.
An integrated management program is the best approach to keep the problem from getting out of control, according to researchers. Work by Tom Turini, Fresno County UC farm advisor, has shown that an assortment of chemical control options are effective at knocking down thrips populations temporarily, but the effect is relatively short-lived — about 7-10 days. However, it can help reduce the number of virus carrying adults.
An integrated approach includes measures that can be taken before planting, during the season and after harvest, according to Gilbertson. “It's worth noting that some tomato varieties do have resistance,” he says. “However, it doesn't appear that it is necessary that growers plant these resistant varieties if they use some of these other strategies.”
Clean transplants are important. Avoiding “hotspots” or areas that have bridge hosts such as radicchio can help decrease pressure.
“During the season, it's important to monitor for the thrips and the virus,” he says. “Know where the thrips are and where the virus is increasing and implement thrips management early to minimize adults carrying the virus. After harvest, prompt sanitation is important to keep thrips from moving into younger fields.”
The newest threat to the Valley's tomato production is Tomato Yellow Leaf Curl Virus (TYLCV). It is widespread in northern Mexico and was found in March 2007 in the Imperial Valley for the first time. Researchers are very concerned it has the potential to continue to move northward into California's prime processing tomato production areas.
“Whitefly is spreading this virus very effectively,” Gilbertson says. “It's a devastating virus, one of the worst tomato viruses in the world. With early infections you can get yields losses of 100 percent.”
TYLCV primarily infects tomatoes. The good news for the northern area is that it is only transmitted by whiteflies and only transmitted by biotypes of the sweet potato whitefly — Bemisia tabaci, which is not currently found in Northern California. It is not transmitted by seed or mechanical means.
TYLCV was detected in late December in a semi-commercial planting of tomatoes in Niland and in transplants in Thermal. Currently, the California Department of Food and Agriculture (CDFA) is trying to contain the infestation through various quarantine efforts. There is hope that it will not become a major problem in most of California's processing tomato growing areas due to cold winter temperatures and the three-month tomato-free period that exists in these areas.
However, growers are strongly encouraged to monitor tomatoes, not only in the southernmost California counties, but also in Kern and Fresno counties. “Avoid bringing in transplants from areas known to have established TYLCV,” Gilbertson says. “And take proactive measures if you're using transplants from Southern California and Yuma.”
Those proactive measures include treating transplants with systemic neonicotinoids, monitoring for whiteflies and virus-like symptoms as well as testing for TYLCV and treating with a contact insecticide prior to transport.