Curly top virus pressure seen in Fresno County tomatoes

Curly top virus (CTV) in tomatoes and other susceptible crops in Fresno County is significant this year, says a University of California, Davis plant pathologist.

Robert Gilbertson detailed his surveys recently at a tomato production discussion for growers and PCAs at Five Points. He said the disease, vectored by sugarbeet leafhopper (BLH), “is not epidemic this year, but it is more common than during the years 2004 through 2007.” Infections are particularly “hot” in western Fresno County along Interstate 5.

CTV, actually composed of at least three viruses of varying severity, strikes sugarbeets, tomatoes, melons, and other crops. Symptoms include stunted growth, upcurled leaves, purple veining, and death of infected plants. Fruit is small and ripens prematurely.

Control has traditionally been by the California Department of Food and Agriculture’s Curly Top Virus Control Program. The program is based on sprays of selected concentrations of BLH on western foothill areas of weed hosts to reduce populations before they move into crops and spread the infections through their successive generations during the growing season. However, the spray program was halted in 2007 because of issues with permitting.

Gilbertson’s surveys earlier this year of the severity of curly top carried by samples of BLH pointed to medium to high pressure in Fresno County, medium to low in Kern County, and virtually none in the Imperial Valley. These levels were consistent with those of previous years.

Infection was found widely in Fresno County tomato fields, prompting concerns the processing tomato industry would be in peril. As of mid-July, Gilbertson said it remains to be seen how great the economic loss will be. Indications are less, however, than the severe season of 2002, he added.

After sampling infected plants to confirm they had curly top and not tomato spotted wilt virus, which has similar symptoms, Gilbertson said most of the plants he tested had “the same curly top we’ve known for years.”

Tests to learn if both viruses were present on the same plants were negative, he added.

Infections were also found in late-planted, fresh-market tomatoes in Merced County, and Gilbertson attributed those to CT transmitted by generations of BLH that developed on the valley floor. “So the infection concern is not just the leafhoppers that come down from the foothills.”

Turning to management of CT, Gilbertson said there are no resistant tomato or pepper varieties, but in the absence of the CDFA control program this year, in-field management of the BLH has to be done with various insecticides. Cultural practices such as field location and plant spacings can also help. Resistance exists in sugarbeet and common bean varieties.

The eventuality of resistant tomato material could be hastened, he said, with the use of new DNA tests that can be used for rapid screening in greenhouses or growth chambers of plant material for resistance to the virus.

A new test has also been developed for rapid identification of CT in BLH, as well as tomato spotted wilt virus in its vector, thrips.

In a presentation on deficit irrigation in processing tomatoes, Tim Hartz, Cooperative Extension vegetable crops specialist at UC, Davis, gave some cautionary tips for this water-scarce season.

In short, he said, “it’s not a good idea to cut back on water during fruit set,” since even moderate levels of soil moisture deficit during fruit set can substantially reduce set and cause blossom-end rot.

A processing tomato crop typically has a consumptive use of 24 to 28 inches of water. Daily water use is governed by reference evapotranspiration, or ETo, values, specific to several sites in the SJV, and the amount of crop foliage covering the ground.

Crop ETo for a drip-irrigated field with no stress rises to about 1 percent about 80 days after transplanting and then declines.

Sprinkler or furrow-irrigated fields, however, take more water early before the crop canopy develops. By midseason crop water use can be slightly higher than ETo, but as the crop matures water use tends to decline.

“A tomato crop is most sensitive to water stress during fruit set,” Hartz said, “and attempting to save water by reducing irrigation during fruit set is strongly discouraged.”

But once fruit set is complete, at about five to six weeks before harvest or when the earliest fruit are at the mature green stage, “a substantial level of moisture stress can be imposed with minimal loss of productivity.” Although fresh fruit yield may decline by a few tons per acre, an increase in soluble solids concentration usually results in little or no decline in Brix yield.

The amount of deficit irrigation possible without loss of Brix yield depends mainly on the soil water-holding capacity and the presence or absence of a shallow water table.

During that final 40 or so days during fruit ripening, Hartz said, average crop water use in a fully watered field is about 80 percent to 90 percent of ETo. “Most fields can tolerate irrigation of only 40 percent to 60 percent of ETo during this period with minimal problem. Fields with high water-holding capacity and good rooting depth may be able to deal with as little as 25 percent of ETo over the final six weeks.”

Deficit irrigation with drip can be controlled easily by reducing the hours of operation to deliver the desired percentage of ETo. Furrow irrigation, however, is more difficult and requires risks in manipulating the irrigation cutoff date.

Hartz said where the water table is within 2 to 3 feet of the surface, deficit irrigation can cause the crop to draw several inches from the water table, allowing for a more severe cutback than would otherwise be appropriate.

“If the water table is non-saline, late season deficit irrigation poses little risk of serious yield decline,” he said. “However, if the water table is saline, a much larger yield loss is possible with an aggressive irrigation cutback. Also, deficit irrigation at the end of the 2008 season will leave the root zone with a high EC, thereby increasing next year’s water requirement.”

Noting that San Joaquin Valley groundwater can be of poor quality, making for problems in some crops, Hartz said tomatoes, once established, are quite tolerant of high salinity irrigation water.

Trials at the Westside Research and Education Center at Five Points showed that irrigating with drainage water of 8.0 EC after flowering had begun did not affect fruit yield.

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