A potential solution to one of the most damaging diseases in California processing tomatoes and other Western crops, a vaccine to protect plants against curly top, has been successful in advanced laboratory evaluations.
William Wintermantel, USDA-ARS research plant pathologist based at Salinas, Calif., says the concept behind the vaccine is “gene silencing,” by which treated plants can repel the virus before it can become established.
Wintermantel described his research progress at the University of California West Side Research and Education Center near Five Points to a group of tomato growers, PCAs, and others.
Basically, the laboratory process amounts to “blasting” tomato plants with bacterial DNA particles carrying material to silence the curly top gene. The plants are then exposed to the curly top virus for monitoring of results by molecular tests.
In the field, however, it might be accomplished by treating tomato transplants in the nursery with a spray containing the gene-silencing material before planting and potential exposure to curly top.
Wintermantel said he has begun work on possible methods of delivery and noted that some sort of vaccination method to available varieties would be more likely to be approved by regulatory authorities than development of genetically-engineered varieties.
“If we can treat the plants early enough, we are seeing very promising results in control,” he said. Tomato plants treated 18 days before exposure to the virus in the lab have been found to be healthy.
In a discussion of beet curly top virus, Wintermantel noted the disease has been a problem for crops on the West Side of the Central Valley for generations. Vectored by the beet leafhopper, it overwinters in the western foothills of the Valley and then is spread during the spring as the insects move into cropland. The disease can be taken up by tomatoes, sugar beets, wild mustard, plantago and some 300 other susceptible plant species in as little as one hour after contact from the vectors. Amount of infection can vary from year to year according to weather and vector populations.
It is actually composed of four species of viruses, known as the curtoviruses and including beet curly top, beet severe curly top, beet mild curly top, and pepper yellow spot. Beet severe and beet mild, so named for their effect on sugar beets but not tomatoes, are the most common in California.
The Beet Curly Top Virus Control Program has been the primary means of defense. Resumed this year after applications ceased during 2008, the program treated nearly 35,000 acres of rangeland in western Fresno, Kings, and Tulare counties for concentrations of leafhoppers during March and April. Spot treatments are also being made by the program, and growers are encouraged to clean up weeds and volunteer crop plants as a further effort.
Wintermantel’s multi-year research is being funded by the California Tomato Research Institute, California Beet Growers, and the Western Sugar Cooperative.
Cooperating on the project are plant pathologists S.P. Dinesh-Kumar of Yale University and Robert Gilbertson, University of California, Davis.
Gilbertson, a professor in plant pathology specializing in virology, was also on hand for the gathering. He praised the effort in trying to find a vaccine for curly top. For the time being, however, the long-term outlook for disease is it is likely to “always be here on the West Side,” he said, “as long as susceptible tomato varieties are grown.”
The sporadic intensity of the disease and attention to other diseases may be reasons, he pointed out, that seed companies have not devoted greater priority to curly top resistant tomato varieties.
Although the reduction in sugar beet acreage in the San Joaquin Valley has eased disease pressure somewhat, the Five Points area continues to be a target for control measures. The disease is difficult to predict, but assessments of beet leafhopper populations in January can help some clues for what to expect the following spring, he said.
A key in managing curly top, Gilbertson noted, is detection and identification since its early symptoms before fruit develops can be confused with tomato spotted wilt virus, even to a trained professional. Molecular tests are needed for confirmation.
Although the two share similar early symptoms, later on, tomato spotted wilt develops necrosis of leaves and ring-spots on fruit, while curly top shows stunted growth, dying plants, and small, ripe fruit. One early symptom to watch for is the upward curling of curly top-infected plants.
Resistant tomato varieties, he said, fend off both beet mild and beet severe, the two species of concern in California.
Since the curly top program endeavors to spray for beet leafhopper before they can migrate into crop land, Gilbertson said a major interest is to know where virus carrying hoppers are coming from.
Gilbertson’s laboratory has developed sophisticated PCR testing methods to determine whether sample leafhoppers from the field are infected. It is working with control program personnel to chart the hoppers’ movements. Year-to-year, the susceptible crops in the Five Points area are a common destination, or “hot spot.”
Surveys of other counties have shown nowhere near the intensity of infection. Infections have been found as far north as Yolo and Colusa counties, but they are comparatively few.
“Those flying down from the foothills into the San Joaquin Valley in March and April are loaded with the virus. We’ve detected it in various counties this season, but fortunately the incidence, from what we’ve seen this year, is no more than 1 percent. There may be some fields with higher incidence.”
Gilbertson said questions have arisen about mixed infections of curly top and tomato spotted wilt. “We thought we might see more of that, but we’ve rarely found the two together in the same plant. Interestingly, it may be that once one virus gets into the plant, the other can’t. Somewhere in the future that might have some use in developing resistant plants.”
For the moment, he said, one management step might be avoiding planting more susceptible varieties closer to the foothills. Another might be modifying plant densities, so losses could be better tolerated.