Beet Curly Top Virus Is An Unpredictable Disease

Beet curly top. Last year, when you uttered those words to California tomato growers, they shuddered, and with good reason. The crop took a major hit in 2013 as a result of this disease. The loss was upward of $100 million, all thanks to the beet leafhopper, the insect pest that vectors the beet curly top virus (BCTV), the cause of curly top disease.

Prior to 2013 the most significant losses due to the disease occurred in 2003, when growers in Kern County lost $5 to $10 million to curly top, says Bob Gilbertson, professor of plant pathology at the University of California-Davis. “The crop loss last year is far beyond any losses that the disease has caused in recent memory,” he adds.

In 2013, tomato fields with 30% to 50% incidence of beet curly top virus were observed. Afflicted plants can be scattered throughout the field, as shown here. Photo courtesy of Bob Gilbertson, University of California-Davis

In 2013, tomato fields with 30% to 50% incidence of beet curly top virus were observed. Afflicted plants can be scattered throughout the field, as shown here. Photo courtesy of Bob Gilbertson, University of California-Davis

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This year, it is unlikely there will be any significant economic loss from the disease, he says. However, the new strains of beet curly top that appeared in 2013 seem to be the same strains that are showing up this year.

“These strains have emerged and established themselves,” Gilbertson explains. “They certainly have the potential to cause severe disease, but they need other factors to allow them to do that. You can find curly top in most fields this year but at low incidence, less than 1%. Last year, fields with 30% to 50% incidence were observed.”

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Creation Of A Perfect Storm
What caused the huge losses in 2013? According to Gilbertson environmental conditions favored development of high populations of leafhoppers, plus most of those leafhoppers were carrying high amounts of virus. He adds that researchers are still trying figure out all the conditions that created the “perfect storm” for curly top last year.

One major problem was the annual spray program used to control curly top couldn’t handle the high populations of leafhoppers before they moved into tomato fields in 2013. The program, called the Curly Top Virus Control Program (CTVCP), is administrated by the California Department of Food and Agriculture (CDFA) and is totally supported by grower contributions, with most of the support coming from tomato growers.

“[CDFA goes] out with sweep nets in late winter and early spring, they identify areas that have a lot of hoppers, and then they call in a combination of aerial and ground sprays to control them,” he explains. “However, they haven’t been taking into account pockets of hoppers that have a lot of virus versus ones that don’t. The other problem is there are vast areas that need to be sprayed, and some acres are off limits to spraying, as the growers want to maintain them organically or have livestock nearby and they want the livestock to be organic. Plus, there are other environmentally sensitive areas that the Bureau of Land Management doesn’t allow to be sprayed.”

A Test Provides Answers
According to Gilbertson, curly top is very unpredictable, as reflected by the occurrence of the disease in 2013 versus 2014. The use of a polymerase chain reaction test (PCR), however, is providing more answers for growers. Specifically, the test can tell growers if a sample of a diseased plant they submitted is curly top or another disease. If it is curly top, the test will indicate which strain.

“The test can be run in 24 hours so growers know quickly what is in their fields,” he adds. Now Gilbertson and his team are working with CDFA to run the PCR test with the leafhoppers collected by the CTVCP.

To help determine the potential for the disease from year to year, Gilbertson says two factors come into play: the population of leafhoppers and the amount of virus they are carrying. By monitoring these factors over the winter and early spring, it is possible to predict the potential for curly top to be a problem in a given year.

The Migration Process
The leafhoppers migrate from the western foothills surrounding the Central Valley. As the foothills dry up, the leafhoppers are looking for anything green, he explains. If tomato fields are in their flight path, they will land there and can transmit the virus to plants.

Gilbertson points out that tomatoes are not a host for the leafhopper, and the pest will not complete its lifecycle on tomatoes, he says.

“[Leafhoppers] won’t lay eggs on tomatoes, but in the course of tasting the tomato plant, they transmit the virus. That is what makes it difficult to kill the leafhopper,” he says. “It takes about 30 minutes for a systemic insecticide to work, and it takes much less time for the leafhoppers to transmit the virus. Plus, the leafhoppers are just passing through looking for a suitable host, so the grower may not even see them in their fields.”

What researchers are learning, however, is there is a direct correlation between the distance between the tomato field and the foothills. If growers plant their tomato fields further away from the foothills, by the time the leafhoppers have migrated down they have hopefully found suitable hosts. These hosts are typically weed species on the valley floor, such as Russian thistle or various mustards, which will allow them to complete their lifecycle.

The Overwintering Process
In essence, the leafhoppers need a good place to overwinter to replenish their populations. According to Gilbertson, in the fall when tomatoes are being harvested and the amount of green plant tissue on the valley floor starts to go down, leafhoppers begin their migration back to the foothills in search of a place to overwinter.

As the females are the ones that overwinter and lay eggs — the males die off — their choice of weeds includes mustards, various species in the brassica family, and a number of invasive plants including saltbush.

Many of the weeds in the foothills are perennials and are infected with curly top, he says. As a result, it is important to spray this area to knock down leafhopper populations.

“In theory it is a good idea to spray the weeds in the foothills, and I think it does help, but in the years when the population is high, it is still a problem,” Gilbertson explains. “On the valley floor, there are additional weeds that can be hosts for the leafhoppers and they hang out there, too. One of the control recommendations is weed management on the valley floor to lower the populations.”

For leafhoppers to have a favorable habitat, a combination of environmental factors in the fall must occur, including the presence of host plants in the foothills that are able to germinate and produce new growth. This requires some degree of rainfall to maintain the plants — not too much but not too little, he explains.

Resistant Varieties, Possibly
Although researchers can’t put their finger on the exact conditions that are conducive to the creation of explosive populations of the leafhopper carrying high levels of the virus, there may be some good news for you — but it will be several years down the road.

Curly top hasn’t been a high priority for tomato breeders of the major seed companies, but it may be moved up the list given the $100 million loss last year, says Gilbertson. As private companies handle most of the breeding now, perhaps they will get involved, given the magnitude of the problem in 2013, he says.

The drawback is that it takes close to 10 years to bring a variety to market. On the upside, Gilbertson says researchers have been screening a number of tomato breeding lines that have genes for resistance to a related virus, tomato yellow leaf curl virus (TYLCV), which is vectored by whiteflies. He says a series of genes, TY1 through TY6, may confer resistance to curly top as well as TYLCV.

“This is exciting because it suggests that these breeding lines, which are being used to breed for TYLCV, can also be incorporated into breeding programs for curly top.”
In the meantime, he says, researchers will continue to examine the factors in 2013 and the conditions that favored the explosive leafhopper populations.

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