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.