MISSISSIPPI STATE – In the ongoing attempt to put the best seed possible in the ground every time they plant, sweet potato growers often turn to virus-tested foundation seed for their next crop.

Many crops today are grown from genetically modified seed engineered to resist certain pests, diseases or weed-control chemicals. For most crops, growers must buy seed every year, not holding seed back from the previous year’s harvest to plant the coming year.

Sweet potatoes are an exception to this rule, at least some of the time. Benny Graves, executive director of the Mississippi Sweet Potato Council in Vardaman, said growers do hold back seed to plant the next year’s crop.

“At harvest, they hand-select the best potatoes in some of their best fields to save for seed,” Graves said. “They have to save 5 percent to 10 percent of the crop for seed for the coming year.”

Most growers save their own roots, but sometimes they purchase fresh roots from a certified seed potato grower. They also purchase a small percentage of foundation seed potatoes on a regular basis to preserve the quality of their potatoes.

Mark Shankle, a Mississippi Agricultural and Forestry Experiment Station researcher at the Pontotoc Ridge-Flatwoods Branch Experiment Station, said Mississippi State University is a vital part of the sweet potato foundation seed program in the state.

“The foundation seed program keeps the sweet potato cultivar true to character so you have a good-quality seed source that is available to producers at all times,” Shankle said.

Growers commonly buy and plant foundation seed one year and save the best of their own potatoes for seed the next two years. Then, the grower buys more foundation seed and starts the process again.

“Growers can save seed back for two years, but they keep early-generation seed from the virus-tested foundation program coming through the pipeline,” Shankle said.

Ramon Arancibia, a MAFES sweet potato researcher at the Pontotoc station, said the foundation seed going to producers is true to type and tested against the most common viruses that infect sweet potatoes.

“All plants have diseases from different types of fungi, bacteria and viruses,” Arancibia said. “There are pesticides that control fungi, bacteria and insect pests, but viruses can’t be controlled with these products.

“In addition, plants don’t have immune systems, so the best way to avoid the infection of the virus or the loss of productivity is to obtain and propagate virus-tested material,” he said.

In the laboratory, plants are subjected to high temperatures that kill viruses but allow the plant to survive. Germplasm, or plant material, is taken from the tip of the plant, which is typically disease-free. This material is grown in the sterilized lab conditions of a tissue culture and tested against known viruses. The next stage is propagated in greenhouses to supply sweet potato growers.

“Many of the viruses in plants are transferred by insects. Aphids and whiteflies spread a lot of the viruses that infect sweet potatoes,” Arancibia said. “We grow plants from tissue culture in enclosed greenhouses that are screened to keep insects out.”

In addition, sweet potatoes are prone to mutations, so the seed foundation program also focuses on maintaining the germplasm true to type by discarding material with obvious alterations.

Arancibia said researchers refer to the plants that result by their generation number.

“Generation 0 is the tissue culture and the first stages that go to the greenhouse,” Arancibia said. “Once they come out of the greenhouse and into the field, that is Generation 1.”

Seed roots from Generation 1 are saved and sold to growers. Growers plant Generation 2 and 3, then have to go back to the foundation seed and start again.

“The plants become reinfected in the field, and the load of viruses they carry is so high that it lowers productivity after just a few generations,” Arancibia said.

In addition to producing foundation seed that has been tested and found to not carry viruses, MSU researchers also test between five and seven experimental varieties of sweet potatoes each year in on-farm research plots. Results are made available to growers and breeders, helping them determine what varieties perform best under Mississippi growing conditions.

EDITOR’S NOTE – A related story on the unique process of sweet potato production can be found at http://extension.msstate.edu/news/feature-story/2011/two-crops-needed-harvest-sweet-potatoes. Photos are also available.