Among the nearly 160 varieties of citrus available for tasting at an event in Exeter, none appeared ready to upstage the seedless Tango mandarin that made its commercial debut about five years ago.

“I don’t think anything this year will take off as much as Tango did,” said Mikeal Roose, professor of genetics at the University of California at Riverside.

But that hardly dampened interest as scores of growers and their advisers sampled thousands of little pieces of fruit at the UC Lindcove Research and Extension Center.

Mandarins and navels were the leading categories in number of varieties. Others included satsumas, blood oranges, sweet oranges, tangelos, lemons, tangors, limes, pummelos, grapefruit, citron and kumquats.

Researchers including Roose have focused much of their attention on the mandarin and notably on varieties that have fewer or no seeds, said Beth Grafton-Cardwell, director of the Lindcove center.

She and Roose explained that irradiation of bud wood has been a key to stepping up mutation processes that can lead to a new desirable variety. The bud wood is grafted and planted at the Lindcove center for further evaluation.

Roose said it can take 15 years to come up with a commercial variety from the time irradiation is first done. He said one challenge now is to find varieties of seedless mandarins that could fill a gap in that variety from late February to early March.

Among new varieties of mandarins gaining favor is the Daisy, which has been available commercially since 2009. “It’s a marvelous piece of fruit, good color, fabulous flavor and low or no seeds,” said Tim Williams, a staff research associate in the Citrus Breeding, Genetics and Germplasm program at UC Riverside.

Williams said another lower-seeded variety of the Encore will be released in June 2012. It’s unusual in its appearance, bearing spots.

While those spots might be off-putting to shoppers, it’s not likely to be an absolute stumbling point, Williams said. Such distinctions can be used as an identifier to draw shoppers who recognize the fruit as good tasting. Williams cited the example of the Gold Nugget, which has a rough rind that has actually contributed to marketing success in Europe.

“It identifies that particular fruit,” Williams said.

Other relatively new low-seeded, irradiated mandarin varieties drawing interest include the Fairchild LS, released in January 2010 and the Kinnow LS, released in April 2011.

Dale Mehrten of Visalia talked with Williams about choices he’s looking at as replacements for flame seedless and red globe grape vines that he removed on 25 acres. He remarked that a neighbor has pummelos, and Williams said that can result in seeded citrus, making selection still more critical.

“I want something that comes off early,” Mehrten said.