Learning from the tree-fruit industry

03/01/2009 02:00:00 AM

By Vicky Boyd

Seed companies and university researchers have borrowed the theory behind grafting scion varieties onto rootstocks from the tree-fruit industry and applied it to vegetables.

The result, they say, is a vegetable plant that resists many soil-borne pests and yet produces high-quality fruit.

One of their first candidates is melons, particularly triploid watermelons that have poor germination in the field.

But Bob Dyer, president of Mission, Texas-based Grower Select Product Inc., who’s experimented with grafting in Mexico since 1997, says he’s given up because of the cost and the lack of consistent fruit quality.

“[Grafting] will work where you have diseases in your soil—Fusarium problems and stuff like that,” Dyer says. “But you’ll never, in my experience, get the quality of fruit that I’m looking for. You can cut pieces of fruit off the same plant and have night and day differences.”

Instead, he says he will focus on improving soil health and trying to eliminate or reduce Fusarium-related issues in the soil.

Looking to the East
Although the techniques for grafting melons onto squash and gourd rootstocks have been used for decades in Asian, they are still in their infancy in the United States.

“In Korea, 95 to 100 percent of the watermelon are grafted,” says David Baquerizo, eastern production specialist for Syngenta’s Full Count Plant Program. “They’ve been using this technique since the early 1900s, and the reason why is they’ve been farming the same land for so long that they were battling soil-borne diseases and soil exhaustion.”

Richard Hassell, a Clemson University horticulture professor in Charleston, S.C., has studied cucurbit grafting for several years as part of a four-state research project.
The main benefit, he says, is the squash and gourd rootstocks resist soil-borne Fusarium.

In addition, some of the rootstocks are resistant to Southern rootknot nematode and Phytophthora.

Grafted plants also could offer growers an option to soil fumigation to address soil-borne pests, Hassell says.

But the effects of the rootstock go beyond the roots and impart beneficial qualities to the fruit, he says.

Hassell’s research has shown that watermelon from grafted plants contain significantly more lycopene—a phytochemical that may have health benefits—than those from own-rooted plants.
“Grafting also increases the firmness of the flesh, so the fresh-cut people are very interested in this,” Hassell says. “It firms up the flesh so you don’t get as much leakage and it’s crunchier to the taste.”

In addition, fruit from grafted plant have increased shelf life compared with fruit from own-rooted plants, he says.

Squash-like qualities in melons
Dyer says he didn’t have the same outcome, citing his own work. He says the fruit from the grafted plants typically had undesirable squash-like characteristics that were presumably imparted by the rootstock.
“You could have stuff that was almost perfect, but even the best of the stuff will have an off taste to it,” he says. “At first you won’t notice it, but as you eat on it, you’ll start to taste it.

“The melon will be hard and have a rind on it like a squash. And more times than not, instead of a red color, you’ll end up with an orangey red color.”

Research shows benefits
Based on his research, Hassell says he believes that growers can get by with fewer grafted plants per acre because they yield more fruit. In fact, he has found each grafted plant produces an average of one more marketable melon than non-grafted plants.

But the benefits don’t come without a cost. Because grafting is a time-consuming, intricate operation, grafted plants cost more than three times as much as own-rooted plants, Hassell and Baquerizo say.

“You need skilled people,” Baquerizo says. “Not everybody can graft. It’s almost like an art.”

Hassell agrees: “It’s not something somebody who has a small operation can do because you have to have healing chambers to put the plants into after they’re grafted for over a week. They have to be close to 100 percent humidity. Then you bring them out of it gradually. It’s quite a process.”

Dyer agrees and says the cost was one reason he decided to drop his work on perfecting grafted plants.

“All of [the grafting] is done by hand, so it’s very labor intense and a very costly process to do,” he says.

Because of the delicate squash stems, Syngenta uses a silicone-like clip—rather than the plant tape used on trees and vines—to hold the graft union together as it heals.

The grafted plants also are slightly taller than own-rooted plants because the graft union needs to be kept away from the soil, Baquerizo says. Otherwise, adventitious roots from the scion could move into the soil, becoming infected with soil-borne diseases and negating the benefits of grafting.

A marriage of seed and transplants

Through its Rogers Brand Vegetable Seeds, Syngenta has offered the Full Count Plant Program since 2002. So adding options for grafted watermelons seemed like a natural extension, says Craig Mathis, Full Count Eastern business manager based in Paducah, Ky.

The Full Count program allows producers to order greenhouse-grown transplants produced from Rogers varieties. The program is especially attractive for hybrids, such as triploid watermelon, which have poor germination in the field, Mathis says.

By using transplants, he says growers can plant to stand and not have large gaps in their rows left by weak seedlings.

Syngenta contracts with several greenhouses throughout the nation, so growers don’t have to. Instead, growers place their orders through their seed salesperson as they’ve always done.

Mathis says Rogers also guarantees plant quality and on-time delivery and can provide back-up transplants should weather take out part of a field.

This spring marks the first season that the Full Count Program will offer grafted watermelons on a limited commercial basis in the Southeast, says David Baquerizo, Full Count Plant Program Eastern production specialist. Eventually, the program will expand nationwide.

Customers can choose a watermelon scion variety, and Syngenta will recommend the appropriate rootstock, based on soil type, pest spectrum, growing climate and scion-rootstock compatibility.

The rootstocks were developed by Syngenta-Korea and have been used successfully there for years. Before Rogers introduced them to the United States, Baquerizo says they were tested under U.S. field conditions to ensure they’d provide similar benefits.

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