Grafting Research Could Rescue Watermelon Crop

The watermelon crop has declined dramatically in Washington because of disease. But Washington State University researchers are developing a solution that involves grafting watermelon plants onto squash and other vine plant root stocks.

“We’ve lost about a third of our state’s watermelon production over the last 10 years because of Verticillium wilt,” said Carol Miles, a professor of vegetable horticulture at the WSU Northwestern Washington Research and Extension Center in Mount Vernon. “Growers have switched to other crops that are less susceptible.”

Today, there are about 550 acres of watermelon grown in Washington, with a value of approximately $5 million.

Miles said growers can lose 25-75 percent of their yield to the disease – but this loss does not occur until the very end of the growing season. That’s when the damage from Verticillium appears.

The fungus also affects tomatoes, potatoes, eggplant and many other crops and plants.

Watermelon grafting used worldwide

Last fall, Miles received a $138,000 grant from the state agriculture department to look into grafting, a solution that doesn’t require fumigants. She is also working with a national team of researchers on a $3 million U.S. Department of Agriculture grant. Her portion is $171,000 to look at grafting tomato and eggplant.

Grafting involves cutting a young seedling from its roots and attaching it to the roots of a related plant that is disease resistant. The grafted plant produces fruits that are equivalent or better in quality than those of non-grafted plants.

“Grafting is very old technology, going back over 1,500 years in China,” Miles said. “Farmers in Japan have used grafted watermelon since the 1920's. In the Mediterranean region, farmers have been using grafted watermelon, tomato and eggplant for almost 20 years.

“We just need to find out what works best for our region and we’ll solve the Verticillium wilt problem,” she said.

Testing root stocks in the field

Her research involves testing which plants work best together under Washington growing conditions and which root stocks are most resistant to Verticillium wilt.

The first goal is to increase the survival rate for newly grafted watermelon plants. If only 25 percent survive, the effort is not worth it, Miles said.

The second goal is to find successful plant combinations that are disease resistant and have equivalent fruit yield and quality, compared to non-grafted plants grown in healthy soil. Miles and her team are testing watermelon grafted to pumpkin, squash and bottle gourd because they are all resistant to Verticillium wilt.

This year will be the second of a two-year field study. While these studies actually started about five years ago under a previous grant, Miles and her team are applying new information that they have learned along the way. They will have two full years of testing in commercial fields by the end of the grants.