Had California’s 2013 tomato crop not been hit so hard by the Beet curly top virus (BCTV) it could have been a banner year for growers. The drought and water challenges for 2014 could prove interesting as tomato processors say they will contract for a record 13.5 million tons in the coming season.
Bob Gilbertson, plant pathologist with the University of California, Davis, estimates last year’s tomato crop was reduced by about a million tons because of BCTV, which is vectored by the Beet leafhopper.
The disease “is perplexing to work on,” Gilbertson told growers and industry officials at the California Tomato Growers Association (CTGA) annual meeting in Modesto in late January.
BCTV is one of several diseases affecting processing tomatoes in California, but is the only one Gilbertson says is transmitted by the leafhopper. Aphids, thrips and whiteflies still transmit more diseases in tomatoes.
Combating the Beet leafhopper in California is not easy, Gilbertson said.
Leafhoppers tend to migrate quickly. Even though tomatoes are not a favorable host for the pest, they can stop just long enough to quickly feed and transmit the disease with great efficiency through phloem feeding.
“This is one of the challenges, because growers will come into their field and not see any leafhoppers in their fields,” Gilbertson said. “It’s because the leafhopper has come down, landed in the field, fed briefly and moved on,” he said.
Chemical controls fail
Further frustrating growers is that systemic insecticides like neonicotinoids, which are very good at controlling aphids and white flies, do not provide high levels of leafhopper control. “Leafhoppers can transmit the disease in five- to 15 minutes, which is too little time for the systemic insecticides to work against the leafhopper,” he said.
Symptoms can be difficult to read early in the disease phase, he said. Early-season symptoms between spotted wilt and curly top can be similar, according to Gilbertson. This is why diagnosis becomes very important for growers to determine what they’re up against and how to combat it.
“Diagnosis becomes important because the management strategies for dealing with them are different,” he said.
Tomato plants with BCTV become stunted and develop curled leaves. Upon closer inspection of the undersides of tomato leaves the veins appear swollen and they turn purple, Gilbertson said. Plants also turn a dull green-to-yellow and the fruit is small and tends to ripen prematurely.
If plants are infected at a young stage of growth they will turn yellow and die in the field,” he said. “Older plants will not die; they will get this upcurling of the leaves and the purple veins, the stunting and the yellowing. So the time of the infection is very important, particularly in terms of the yield loss you’ll experience.”
Disease transmission begins early in the season as leafhoppers migrate from the foothills to the agricultural valleys, but can also happen during the growing season. It is transmitted solely by the leafhopper and not mechanically or by seed.
Adult leafhoppers tend to overwinter in the foothills. In the spring the females lay eggs on the green plants in the foothills and acquire the virus during feeding. As new leafhoppers become adults they then migrate to the valley.
There are generally 3-5 generations of leafhoppers on the valley floor, Gilbertson said.
The California Department of Food and Agriculture has a curly top spray program where the state sprays the foothills for the Beet leafhopper.
“That has had some impact on reducing the curly top, though obviously in 2013 when the spray program was going on it wasn’t sufficient to stop the outbreak we saw last year,” Gilbertson said.
Cultural practices to prevent spread of curly top can help, such as not planting near the foothills or with heavy plant populations, but even these were not entirely effective in 2013, according to Gilbertson. There are no commercially available curly top resistant tomato varieties.
New detection tools
New detection tools for curly top virus offer opportunity for improved understanding of the disease and its management strategies. PCR tests can quickly detect the virus in plants. The CDFA can also sample leafhoppers for the virus.
Gilbertson said researchers have determined that leafhopper population numbers and the presence of the virus can be determining predictors in whether BCTV will be a significant issue in the following growing season.
For instance, in March and April of 2013 the CDFA was collecting five times more leafhoppers than they normally collect in the foothills. That was the first bad sign, Gilbertson said.
Tomatoes with curly top symptoms began showing up in late March of last year. Losses were the highest in Fresno, Kern and Kings Counties. Yields losses appeared far beyond the western foothills, and even into San Joaquin County.
“This was very unusual in that regard because we were seeing curly top in areas where we typically don’t see it,” Gilbertson said.
Gilbertson noted that the disease was not widespread, but affected different areas to varying degrees. While particularly bad in counties such as Kern, Kings and Fresno, it was seen to a lesser degree in the rest of the San Joaquin Valley. The virus was not seen in the Sacramento Valley.
“To illustrate just how bad it was for curly top last year, we saw it in cucurbits, which we never see,” Gilbertson continued. “I’ve been working here over 20 years and this was the first year I’ve seen this develop in crops like cantaloupe and honeydew.”
Why was it so bad?
Gilbertson suspects a combination of favorable conditions for the leafhopper and hosts for the virus in the foothills. There are also hypotheses of new, more virulent strains of BCTV that have a wider host range or are able to be transmitted more effectively.
“The state’s spray program was certainly not enough to manage the disease,” he said.
There are limitations on the spray program, which could have helped leafhopper populations thrive.
“The spray program is being constrained now by certain farmers who want to do organic production in the foothills,” he said.
A comprehensive research program to address these issues has been initiated with the goal of applying new approaches and technologies to develop an effective integrated pest management program for curly top.
One such thought is the possible development of a spray that will act as a deterrent to keep leafhoppers from landing on tomatoes and feeding upon them.
Gilbertson is cautiously optimistic that the coming growing season won’t be as bad as last year’s.
“Clearly it’s been very dry,” he said. “There are not a lot of green plants out there for these leafhoppers to feed on to survive the winter and lay the brood for the coming season.”
State tests for the curly top virus in leafhoppers and host plants are coming back negative so far.
“I would say that it is looking promising that the pressure of curly top this year does not look like it’s going to be anything like it was in 2013,” he said. “Things can change in terms of the weather.”
Historically the BCTV was a severe problem in sugar beets until the sugar beet industry developed resistant beets, he said.
“When we had a major sugar beet industry here in the early 1900s through the mid-1900s curly top was one of the most devastating diseases of sugar beets,” he said. “One of the reasons for that is Beet leafhoppers love sugar beets. They really don’t like tomatoes.”
Most thought that the incidents of curly top diminish as sugar beet production declined, but curly top did not disappear.
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