Christopher Clark, Tara Smith, Arthur Villordon and Don La Bonte

The orange, moist-fleshed sweet potato varieties developed at the LSU AgCenter have become the standard American perception of a sweet potato. Early on it was realized that mutations could occur in orange-fleshed sweet potatoes that resulted in roots that were off-type and light yellow in color. In the late 1940s, LSU pioneered the production of foundation sweet potato seed (storage roots) by developing a method to minimize such mutations.

The land for the Sweet Potato Research Station, near Chase, Louisiana, was purchased in 1948, and operations in research and foundation seed production began in 1949. Two important practices were followed to eliminate mutations from sweet potato seed: single hill (plant) selections and cutting and culling off-type seed roots. At harvest, individual hills were selected that represented the best traits of the variety being harvested. Traits evaluated included uniform yield, consistent shape and overall appearance of the storage roots. During the winter storage period, seed roots were cut in cross section, and any that showed flesh color mutations were discarded. The remainder of the seed roots that were free of mutations were bedded in the greenhouse to produce plants that would be the source of the next foundation seed crop. Throughout this time, the research station served multiple roles, also serving as a center for the sweet potato breeding and cultural practice research programs.

Virus-like symptoms have been observed on sweet potatoes for as long as they have been grown as a crop. In the U.S., symptoms on the leaves are mild, and viruses were not historically accorded much importance. However, in the 1970s, California grew a sweet potato variety that was susceptible to russet crack, a virus disease that disfigures the storage roots and renders them unmarketable. California developed a seed program to produce virus-tested sweet potatoes to minimize the russet crack problem, which at that time was not as serious in other sweet potato-producing states. During the 1990s, research conducted at the LSU AgCenter demonstrated that while the symptoms from viruses that commonly infect sweet potatoes in Louisiana are mild, they can reduce yields by 25% to 40%. In 1999, the LSU AgCenter integrated virus-tested tissue culture technology into the foundation seed program.

Because viruses that infect sweet potatoes spread rapidly in the field, breeding lines were almost always infected following two to three years of field evaluation. This necessitated therapy to generate plants free of viruses, which was accomplished using meristem-tip culture in the sweet potato pathology laboratory in the Department of Plant Pathology and Crop Physiology. By cutting a tiny piece about half of a millimeter in length from the growing tip or axillary bud of a vine, it is possible to regenerate complete plants in tissue culture, most of which are free of viruses. However, because not all such plants are free of viruses, it is necessary to do virus testing to be certain that only those that are not infected are selected for seed purposes. The mericlones (all plants derived from a single meristem) are maintained permanently in tissue culture, where they are not exposed to re-infection.

In the early days of virus-tested seed programs, the primary means of virus testing was to graft the sweet potato that was to be tested to a seedling of the Brazilian morning glory, Ipomoea setosa, and observe for virus-like symptoms on new growth. Over the past 25 years, research around the world has led to very sensitive molecular tests for viruses’ nucleic acids, and these have been incorporated into the LSU AgCenter virus testing program. In 2015, sweet potatoes were added to the USDA-sponsored National Clean Plant Network (NCPN-sweet potato), with centers at the AgCenter and in Arkansas, California, Hawaii, Mississippi and North Carolina.

This has proved invaluable because it has provided each center with access to the technologies and practices of all the centers and has provided funding critical for improving the infrastructure of these programs. As one result, each of the centers follows the same minimum set of practices and standards that includes tests that allow us to produce plants free of the potyviruses that we know can reduce yields of sweet potatoes in Louisiana as well as to prevent two other viruses, Sweet potato leaf curl virus and Sweet potato chlorotic stunt virus, from getting into our seed system and causing even greater losses.

The National Clean Plant Network has nearly doubled production of greenhouse foundation plants to 636,000 plants since its inception. The industry, however, plants about 2 billion plants per year. This requires growers to increase clean stock in year one and then use this build of storage roots for year two. Viruses and disease can reinfect during field increases, so a major goal of the National Clean Plant Network is to increase the amount of clean seed produced while simultaneously striving to improve the quality and assurances of that quality. The LSU AgCenter foundation seed program is actively examining innovative alternatives to using tissue culture to propagate the initial starter stock of plants for greenhouses at the Sweet Potato Research Station and for providing clean vine cuttings to growers. The overarching goal is flexibility in providing virus-tested plants and seed stock to meet the changing needs of the Louisiana sweet potato industry.

Christopher Clark is a professor in the Department of Plant Pathology and Crop Physiology; Tara Smith is the director of the Central Region and an extension sweet potato specialist; Arthur Villordon is a professor at the Sweet Potato Research Station, Chase, Louisiana; and Don La Bonte is a professor in the School of Plant, Environmental and Soil Sciences.

(This article appears in the spring 2021 issue of Louisiana Agriculture.)

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Read the story of sweet potato breeding.

Cathy DeRobertis, research associate in the Department of Plant Pathology and Crop Physiology, works in the tissue culture lab, which is part of the foundation seed program to provide clean, virus-free plants for the sweet potato industry. Photo by Randy LaBauve

Tissue culture technology is used to ensure sweet potato plants are virus-free for the foundation seed program. Photo by Olivia McClure