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Can watermelons be grafted?

For years South Carolina peach growers have planted grafted trees, the union of fruit wood to rootstock which is resistant to soilborne diseases and nematodes. Watermelon growers may one day follow suit.

Richard Hassell, a Clemson University horticulturist at Coastal Research and Education Center, is at the center of a four-state project which is adapting a watermelon production system widely used in Asia for use in this country.

“The Chinese grow more watermelons than anybody else. They don’t have a lot of land to rotate melons with other crops, so soilborne diseases have built up into a big problem,” said Hassell. “To get around that they graft watermelons onto rootstock of squash and gourd varieties that are resistant to fusarium and other soilborne diseases that would devastate watermelons.”

South Carolina producers and growers in other Southeastern states have a similar problem with diseases, particularly a fusarium organism which causes vascular wilt. To combat the problem growers use fumigants under plastic mulch prior to planting their melons.

Grafting may provide an alternative, according to Hassell.

This spring, with the aid of staff and students at Coastal REC, he prepared 7,000 grafted transplants for five test sites — 1,400 for each site. The first planting was at Quincy, Fla. The next was in Tifton, Ga., then Coastal REC, followed by Edisto REC and Kinston, N.C.

Each batch of grafts was given a week to heal and a week to grow in the greenhouse before being shipped for planting.

“We have two projects going,” said Hassell. One tests the compatibility of seven different squash and gourd rootstocks with two different watermelons — how easily they graft and how disease resistant they are. The other looks at in-row spacing, using two rootstocks (a gourd and a squash) and a seedless watermelon variety.

The National Watermelon Association and grower associations in each state support the grafting work, looking to answer several questions.

Can the cost of grafted transplants be reduced or, more importantly, justified? Each one costs about $1, compared to 25 cents for regular transplants, according to Hassell.

“Preliminary research last year showed an increase of one fruit per plant. With mini-watermelons we got at least three more fruits per plant,” he said. “That begins to increase the justification for grafting.” More fruit set may mean growers will need fewer plants, which helps control cost.

“We are also looking at whether the fruit flesh is firmer. The fresh cut industry is interested from that angle,” said Hassell. “If there isn’t as much leakage, the cut fruit will last longer on salad bars and in fresh-cut packs at the supermarket.”

Researchers at the five test sites will also be looking for compatibility problems between cultivars and rootstock.

“We have had reports of rootstock changing shapes and flavors in the rootstock grafted onto them,” said Hassell. Most of the rootstock being tested this year comes from Japan, Korea and China.

“They do well there, but we’re looking at how well they do here in the different growing areas,” he said.

Watermelons were harvested from about 7,500 acres in South Carolina in 2006, according to the National Agricultural Statistics Service, worth approximately $14 million to the state’s economy.

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