As drip-irrigated acreage for processing tomatoes in the San Joaquin Valley increases, growers and researchers are flattening the once-steep learning curve, and results of 2004 trials show that deficit irrigation on green fruit is a management tool for improved soluble solids concentration (SSC).

According to Tim Hartz, vegetable crops specialist, University of California, Davis, results of trials with cooperating growers on seven drip fields in Fresno and Tulare counties showed that once ripening occurs, SSC cannot be increased by water stress.

“This suggests that the only way for a grower to substantially increase SSC is to begin to dry down the field while most fruit are still green,” he reported.

Collaborating with Hartz were Paul Johnstone, post graduate researcher at UC, Davis, and Michelle LeStrange, farm advisor for Tulare and Kings counties.

In the 2004 research, the latest in a series of trials investigating the response of tomatoes to water stress, irrigation flow was reduced by 25 percent to 50 percent and fruit quality measurements were recorded during the four to seven weeks prior to harvest. Six cultivars were observed on sandy-clay loam and clay soil textures.

None of the cooperating growers used any prolonged irrigation cutoff, and they continued to make light irrigations to within 14 days of harvest.

Earlier trials indicated the crop water requirement for full irrigation over the last four to six weeks of the season would require about 80-90 percent of CIMIS ETo.

“SSC of early-maturing fruit varied from 4.2 to 5.9 degrees Brix among the fields,” Hartz said. “This range was partly attributable to cultivar differences and partly to field-specific factors.”

However, he added, “The pattern of subsequent SSC development was quite consistent across fields. Later-maturing fruit, or those exposed to deficit irrigation while still green, showed substantially increased SSC.”

SSC measurement

SSC was measured in the various plots first by collecting 20 pink fruit from each plot the day the reduced irrigation treatments began. These were blended and analyzed by refractometer for SSC. Another set of fruit was collected and analyzed two to four weeks later, and the process was repeated at harvest.

Although high yields were recorded at all sites, the shorter water regimes did cause slight reductions in marketable fruit, but the yield loss was offset by a corresponding increase in SSC. No significant differences in cull rates or blended fruit color were noted.

“These results,” Hartz said, “indicated that processing tomatoes can tolerate a significant degree of water stress before Brix yield is compromised. Irrigation regimes as low as 20 percent to 40 percent of ETo over the fruit-ripening period did not significantly reduce Brix yield when compared to wetter treatments.”

He went on to say the findings show that “growers have substantial flexibility in late-season drip irrigation scheduling, enabling them to tailor irrigation based on field-specific factors such as the SSC of early-ripening fruit.”

Field variations

The consistent pattern of increasing fruit SSC in response to deficit irrigation points to SSC monitoring of ripening fruit as a useful tool for irrigation scheduling.

Hartz also noted the large variation among fields in the SSC of early-ripening fruit underscores that some fields may require more severe deficits than others to reach acceptable SSC.

The researchers pointed out their trials have not been in fields where severe vine decline occurred. They warned that the deficit irrigation method could be compromised where foliar or root pathogens, nutrient shortages, or other problems are present.

Although it was not an a concern in these trials, the team advised that starting deficit irrigation during the six weeks before harvest may invite root intrusion into the drip tape.

Separate UC research on the West Side of the SJV in recent years has shown the benefits of drip on certain soils over sprinkler or furrow irrigation, mainly in lower labor and land preparation costs.

Studies estimated that buried drip systems for processing tomatoes cost about $850 per acre, based on a five-year service life for the tape and 15 to 20 years for other equipment. The overall cost came to $155 per year, versus $300 to $500 per acre in increased returns.

Dodder management

Tom Lanini, Extension weed ecologist at UC, Davis, recently reviewed steps to manage the age-old threat of dodder, the stringy, yellow, parasitic plant that infests about 30,000 acres of California tomato acreage, as well as other row crops and even roadside weeds.

Dodder strands sap host plants of water and nutrients and can reduce tomato yields by 75 percent. Field dodder is the most common species, although several others exist.

“Dodder control can be achieved by avoidance, hand removal, resistant tomato varieties, and herbicides. None of these methods work 100 percent of the time, but all offer some hope for managing this native, parasitic weed,” he said.

One way to avoid the pest is to plant after mid-May, although, he adds, this may not always be possible to meet cannery delivery dates. The larger the plant, the better it can tolerate emerging dodder.

Shallow cultivation can destroy dodder seedlings, which have no roots, before they can attach to the crop. It can also promote drying of the soil surface and prevent further emergence.

Hand roguing of tomato plants with dodder attached is “a viable but expensive option, when infestations are small,” says Lanini.

Removal from field

In a trial, he observed that a hand crew removed about 90 percent of the dodder, the remainder left because it was too small to detect. He suggests a second roguing about 15 to 21 days after the first to remove the remaining dodder before it sets seed.

He added that infested plants do not need to be removed from the field unless they carry viable dodder seeds. If kept at least six inches from a host, dodder will be unable to attach to it.

Tomato varieties known to resist dodder in field trials include the Heinz quartette: H9492, H9553, H9992, and H9888. Among varieties showing resistance in the laboratory are CDX 233, H1100, H9997, and SVR 924 0 0665. Varieties to avoid in the presence of the pest are AB2, ENP 113, APT 410, CDX 222, H2501, H2601, H8892, and H9665.

Herbicides give marginal performance against the parasite. Pre-emergence products used in tomatoes are not effective. Preplant metam-sodium controls many weed seeds but not the hard-shelled dodder. Split applications of Matrix have been shown to give no more than 50 percent control. Treflan and Tillam at layby are too late for control of dodder.

Lanini reminded that dodder is spread by contaminated equipment, and as more custom operations are performed, the chances increase. Equipment leaving an infested tomato field should be washed down before entering another.

The best course is to destroy dodder before it can produce seed. “Patches can be removed by hand, spraying both host and parasite with a contact herbicide, or by searing with a flame-throwing torch or hand burner,” Lanini said.