In processing tomatoes, what's the difference in yield between direct-seeded and transplants? Not that much, according to recent trials by University of California researchers in the Sacramento Valley.

Gene Miyao, farm advisor for Yolo, Solano, and Sacramento counties, set up plots near Davis in 2002 to observe stand establishment, spacing, vigor, and other performance of the two methods.

Speaking at the recent Kern County Vegetable Crops Meeting at Bakersfield about the tests, Miyao said the movement toward transplants, which require fewer irrigations to become established, was encouraged during drought years of the late 1980s and early 1990s.

“Another thing was the transition of the industry to hybrid varieties, almost exclusively. When the price of seed went up, there was some interest in transplants, which use less seed, to hold down costs,” he said.

Other reasons were use in fields with difficulties in stand establishment and lower costs for weed control.

In the trials with the variety Halley 3155 near Davis, he said, the transplants appeared less vigorous than the direct-seeded, although yields were quite similar. Yields, he added, were not tied to plant populations, and stands tended to compensate for various populations. Fruit quality was also similar.

Double vs. Single

In another trial with the same variety in a commercial field north of Dixon, Miyao evaluated double vs. single rows. Growers have chosen double rows for some varieties and some soil types, although single rows have some appeal to avoid fruit rot problems in Sacramento Valley fields harvested after mid-September.

For the single rows the primary spacing was 16 inches apart and for the double rows the spacing between plants varied from 24 to 48 inches.

In this trial, double rows yielded higher but the difference had more to do with plant population than row configuration, he said. With a population of 4,000 to 5,000 plants per acre, yields for both direct-seeded and transplant were similar. However, he recorded a gain of two tons for each additional 1,000 plants.

Miyao said corky root became a factor in the trial. He noted that the disease did not affect stands as they became established but sapped vigor later when fruit was beginning to size. The lower the population, the more disease pressure each plant has to sustain, so higher populations do help.

“In transplants, you really have to look at your costs,” he said. “You have a wide range of choices in plant populations and row configurations to reach similar yields. It costs about $44 for each 1,000 transplants.”

Some yield gain

Miyao said recent research in Colusa and Fresno counties on transplants having multiple plants per plug have shown some yield increases over conventional, single-plant plugs. “The cost for additional seed is not that much more, and some of the yield increases have been clearly economical.”

In a related talk, Tim Hartz, vegetable crops specialist at UC Davis, reminded that achieving acceptable soluble solids in processing tomatoes rests almost entirely with water management late in the season.

“Well-managed drip irrigation can give a lot of advantages in yields and management of other issues,” he said.

He also said applied potassium likely has no influence on soluble solids, although it may improve yield or color.

While many theories about how to raise soluble solids are in circulation, Hartz said the trade-off between concentration of solids and yields remains. “The only real tool growers have to manage their concentration of solids is irrigation.”

It's a matter of inducing some water stress, but not too much. But conventional irrigation is a blunt instrument, he said, and it becomes a gamble whether the effect is too early or too late for the desired effect.

Drip trials

For the Southern Sacramento Valley and the Northern San Joaquin Valley, this balance period can be from 10 days preharvest to 35 days preharvest.

Hartz said drip trials at UC Davis during 2000-2002 showed benefits in a cutback to 25 percent of the current ET at eight weeks after planting or about the time the first fruit shows color.

This deficit treatment gave the combination of minimal yield losses and increased solids, while maintaining the same Brix level as the full irrigation control. The reason is the later turning fruit that ripen under stress give a higher concentration of solids.

Stories are also circulating that potassium can increase yield and solids. Hartz said the potassium content of the soil has little to do with soluble solids, although color defects can be related to soil potassium status.

Several trials in conventionally irrigated fields, he added, show that once the soil has more than 130 ppm extractable K, a yield response is unlikely from applied potassium. Nevertheless, potassium should be monitored and maintained at adequate levels.