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Protecting a Summertime Treat

New Test for Watermelon Seeds May Lead to Safer Fruit and Vegetables

Media Contact:
Jennifer Martin, (202) 720-8188

By Stacy Kish
August 18, 2009

Nothing says summer like a ripe, juicy slice of watermelon on a hot day. However, this important summertime fare is under assault and may not even make it to the table; a tiny pathogen is ravaging crops, causing the rind to weaken and allowing the fruit to ooze out into the field.

The disease, called watermelon fruit blotch, accounts for losses of up to 90 percent of marketable yield in some watermelon fields. Fruit blotch was first detected in Florida in 1989. Since that time, it has migrated along the eastern seaboard and into Indiana, affecting 11 states. The rapid migration and expansion of this disease may lie in its seeds.

The U.S. watermelon crop is worth $434 million; for each 1 percent of the crop lost to the disease, it accounts for an annual loss of $4.34 million. In addition, the average value of watermelon per acre of land is $3,488; if a farmer lost 90 percent of a crop to watermelon fruit blotch, the loss would be $3,139 per acre.

With funding from USDA’s Cooperative State Research, Education and Extension Service (CSREES), scientists in California have developed a test to detect the presence of this important fruit pathogen on watermelon seeds.

“Seed health testing is important in order to identify infected seed lots,” said Parm Randhawa, at the California Seed and Plant Lab, Inc. “[It allows us to] prevent the introduction of diseases by eliminating infected seed lots from sales.”

Since only one or two seeds in a lot consisting of thousands of seeds might be infected, sensitive tests are essential. Randhawa and colleagues have developed a Polymerase Chain Reaction (PCR) test that is sensitive enough to detect watermelon fruit blotch in watermelon and melon seeds. The test has the added benefit of determining if the pathogen is alive or dead.

The PCR test’s power comes from its ability to amplify a few copies of available DNA and replicate it by several orders of magnitude. By generating millions of copies, it is possible to easily test for the presence of the pathogen.

“In a blind test, the new test successfully detected the pathogen-infected seeds in 30 samples,” said Ronald Walcott at the University of Georgia.
The research team has successfully tested watermelon and melon seeds over the past 5 years, testing 1,925 seed lots, each consisting of 10,000 seeds. Their clientele include Shamrock seeds, Tokita seeds, Known-You seeds, United Genetics, Syngenta Seeds, and South Western Seeds.

Fruit blotch, transferred into a field by infected seeds or transplants, is first expressed as the fruit begins to develop. The disease appears as a coffee-colored stain on the rind of the fruit. The stain marks a point of weakness where the rind splits open allowing microorganisms to invade and rot the fruit.

The team plans to develop for growers and industry a PCR test kit for 10 of the most important bacterial pathogens to vegetable crops.

“The commercial application of this test kit will aid agriculture and the seed industry to detect bacterial pathogens easier, faster, and cheaper,” Randhawa said.

CSREES funded this project with Phase I and Phase II funds through the SBIR program. Through federal funding and leadership for research, education and extension programs, CSREES focuses on investing in science and solving critical issues impacting people’s daily lives and the nation’s future. For more information, visit