Sweet Potato Varieties: Something Old, Something New

New sweet potato varieties will vie in the marketplace with heirloom varieties as producers seek profits in an ever-evolving market.

Read about: Exploring the Versatility of Growing Tomatoes

A major emphasis in the Louisiana State University AgCenter’s breeding program is improving shape quality. “The produce industry is looking for that cookie cutter shape,” said Don LaBonte, director in the Louisiana State University School of Plant, Environmental and Soil Sciences.

Newly released varieties like Orleans give produce buyers the shape and appearance they want in a good sweet potato.

“It is important to plant good, strong, quality plants,” said Tara Smith, regional director at the LSU Sweet Potato Research Station. The facility is on 300 acres in the northeast part of the state near Chase. It is the only facility of its kind devoted solely to sweet potato research.

Two new varieties are gaining traction in Louisiana – Bellevue and Orleans. Both are orange flesh cultivars.

“Orleans is similar to Beauregard but with somewhat higher yield and a higher No. 1 count,” Smith said of the disease-tolerant variety.

“Orleans is beginning to replace Beauregard in commercial production,” Smith said.

This year and last, Bellevue has found its way into limited commercial plantings. Growers are impressed with the 7 percent to 10 percent increase in No. 1 quality.

“Growers are still in the trial stage with Orleans. “We still have a lot to learn about it,” Smith said. Orleans is a rather attractive orange-flesh cultivar with a copper skin color.

LaBonte noted Bellevue does not produce as many plants out of the bed. “It is more sensitive to March weather in bedding,” he said. That said, Bellevue has “amazing shape quality,” he said. It thrives in sandy soil and it stores well.

“Anything that holds well in storage will not be a good sprouter. You can’t have it all,” LaBonte said. His recommendation to commercial producers is to bed later to avoid weather hassles. Instead of planting Bellevue the first week of March in the Gulf region, put in other varieties and hold Bellevue until later in the month.

Sweet Potato Varieties

Other well-accepted varieties dominate in the heart of Alabama’s sweet potato production region: Baldwin and Cullman counties. There, cultivars like Beauregard, Hernandez, Jewel, O’Henry and White Delight are good choices.

Oklahoma experts added a white variety, Bonita 2011, to their recommended list that includes Beauregard and O’Henry, which offer fusarium resistance; Jewel; Centennial, with wireworm resistance; and Cordner, Covington and Evangeline, which offer root knot nematode resistance.

They say variety selection should be based on factors like market preference, pest resistance, yield, quality and potential for slip production. As with all vegetable crops, market demands are a large factor in variety selection, Oklahoma extension experts noted, so is the potential for a variety to be productive. It is also important to try new varieties in on-farm trials, and if they are successful, introduce them to customers so they can provide input for future variety decisions.

Market preference is big. “If I were a producer, I’d be looking at some of the older, heirloom varieties,” said Joe Kemble, Alabama Cooperative Extension System extension horticulturist and associate professor at Auburn University. He is referring to the creamy white, purplish and rose-colored varieties that were popular years ago.

“Whether they cook better than Beauregard is debatable,” he said, noting that they can be stringy out of the pot. “But there is a market and a cachet for some of the older varieties.”

With much of Alabama’s production going to retail sales, and much of that moving through farmers markets, there is a good opportunity for producers to cash in on nostalgia.

LaBonte noted that a sweet potato, in 95 percent of the world, is white, not orange. With the influx of people from Latin America, Asia and Africa changing demographics, white varieties are in the ascendency.

“The growth in heirlooms is one of those things that is a bit surprising but still pretty cool,” LaBonte said. He notes that there has been excellent growth in that market sector in California and in other areas, too.

White sweet potatoes have a lower glycemic index than Irish potatoes, so people who cannot eat traditional spuds can substitute them. In another market segment, he notes LSU’s efforts to breed a sweet potato variety that tastes like an Irish potato.

“Look at the nuances,” LaBonte advised growers. Noting people are developing a palate for different flavors and nuanced taste, he said little can beat the amazing flavor of a Burgundy or Evangeline. “There is tremendous growth opportunity with new and novel types,” LaBonte added.

“I’d try to tap into those heirloom markets,” Kemble said. “There can be a 200 percent markup on them. A producer can make good money on a small acreage.”

Planting review

Sweet potatoes are cold sensitive and should not be planted until after the frost-free date. According to experts at Auburn, the optimum temperature for sweet potatoes is between 70 and 85 degrees Fahrenheit. Pick a field with well-drained sandy or sandy loam soil to produce the best-shaped sweet potatoes. Avoid clayey or other heavy soils.

Extension experts recommend planting into a soil that is well drained but not prone to drought. Waterlogged, poorly drained soils prevent roots from obtaining sufficient oxygen, which can cause “souring” of roots.

Test both for pH (lime may be necessary) and fertility. Optimum pH is in the 5.8 to 6.2 range. Add material as needed. A typical crop of sweet potatoes may need up to 90 pounds of nitrogen, 60 to 80 pounds of potassium and 150 to 160 pounds of phosphorus per acre.

Broadcast or band half of the required N before planting and then sidedress the remainder at layby when the vines begin to run. Horticulturalists caution producers to follow the recommended rate of fertilizer because excessive fertilizer concentrations can result in salt burn and plant damage. Additionally, surplus fertilizer causes excessive vine growth and just adds cost.

Experts at Oklahoma State University-Stillwater, say a typical sweet potato crop there will use roughly 110 pounds of nitrogen, 15 pounds of phosphorus and 150 pounds of potassium per acre from the soil.

Most producers who want assured quality produce their own slips. Sweet potatoes are propagated from sprouts or vine cuttings called slips. One bushel of sweet potatoes will produce 2,000 to 2,500 slips in two or three slip harvests.

Those who buy from a supplier should purchase certified, disease-free slips. About 500 slips can be produced from a bushel of seedstock. Extension experts emphasize that using quality roots for seed is essential for producing quality sweet potatoes.

Slips are ready to harvest when they have six to 10 leaves and a strong root system. Pre-sprouting roots produces roots ready for planting a week to 10 days quicker than usual and will give double the number of slips of non-presprouted roots. Waking up the sweet potatoes after they have been in storage decreases the amount of seed stock required.

Choose well-shaped roots that are free from insects and diseases as well as true to variety. Start by checking flesh color by cutting off a 1/2 inch of root at the stem end. Discard “off” types.

Yams or sweet potatoes?

Do not make the mistake of calling a domestic sweet potato a yam. In the United States, a typical sweet potato will weigh between 1 and 2 pounds. The sweet potatoes we know in the states have a nice, sweet, moist and orange-colored flesh. Elsewhere, most sweet potatoes are white. Yams — typically associated with the Pacific islands and major production centers in tropical Africa, like Nigeria — are dry, starchy tubers.

Yams are a tropical plant unrelated to our sweet potatoes. Yams are monocot vines. Types like Dioscorea rotundata and D. cayenensis are related to the morning glory, while sweet potatoes are a different genus: Ipomea batatus.

Yams do win the size battle, however. A typical yam will weigh somewhere between 3 and 8 pounds when mature. That is roughly five times the weight of a recently harvested sweet potato. A super-sized yam can go over 100 pounds.

Here’s some irony: Back in 1954, the Louisiana auto license plate promoted “Louisiana-Yams.” Today those plates fetch upwards of $100 on eBay.

Handle seed stock potatoes carefully. Oklahoma State University experts advise using cotton gloves. Harvest seedstock before frost, and cure and store separately from other sweet potatoes. And never let seed stock remain in the field unprotected from the sun after digging.

Alabama extension horticulturalists note that conditions required for presprouting are similar to those required for curing sweet potatoes. Material goes into controlled storage or a curing room where temperature and humidity can be controlled. The room must have good ventilation – enough to turn over the air a couple of times each day. Presprouting requires setting out seed stock for three to five weeks at 70 to 80 degrees Fahrenheit and 90 percent humidity. If a sprouting room lacks a humidifier, spraying the walls and floors with water twice a day helps keep humidity levels where needed. However, if humidity approaches 100 percent, that is bad. You will start to see water on walls or wetting on the surface of the roots. This leads to onset of rots.

After presprouting, place roots into beds, covering roots with 2 or 3 inches of soil and add 4 to 5 pounds 8-8-8 or 10-10-10 per 100 square feet of bed area. A bushel of slips will require about 20 to 30 square feet of bed. Treat seed with fungicide to prevent bedding root decay. Keep the beds moist but not wet.

As a good rule of thumb, four to six bushels of foundation stock will grow vine cuttings to plant an acre of sweet potatoes for seed stock production. Usually, a two-row transplanter will get the job done. However, on smaller operations, one can transplant by hand.

Plant sweet potatoes on 12-inch spacing in 36- to 42-inch rows. This requires 12,500 to 14,500 slips per acre. Planting too close will produce undersize roots.

After planting, cover beds immediately with black or clear plastic to warm the soil. Punch holes in the plastic for ventilation as needed.

Scurf and other pests

“Growers across the country are aware of the different insect, disease and weed challenges and the need to be diligent and manage them appropriately,” Smith said.

Scurf, or soil stain, is one of the disease problems specific to sweet potatoes. The disease is characterized by potatoes with roughened skin. Soils with high levels of organic matter can promote scurf.

Most scurf is caused by using infected propagating material. The fungus will survive in crop debris. Disease severity is greater and persistence of the pathogen longer in fine-textured soils that have a high level of organic matter. The disease has a narrow host range that consists only of species in the genus Ipomea.

Experts recommend producers use long rotations to decrease the incidence of scurf and infection from Fusarium wilt. Any field that has been in sweet potatoes in the past two years should not be considered for planting, no matter how good the soil profile is.

Run a soil analysis and check for insects and nematodes in fields that have historically high nematode populations.

Black rot is the bane of storage areas, the transplant bed and in the field. The pathogen not only reduces yield and quality but also gives the sweet potato a bitter taste. It is initially marked by small, circular, slightly sunken, dark brown spots that enlarge and appear greenish black when wet and grayish black when dry. Eventually, the entire root may rot. Even roots that appear healthy at harvest are susceptible to rotting in storage, during transit or at market.

Black rot fungus survives in the soil in crop debris. Infected storage roots can escape detection at harvest and then colonize young shoots or infect stems. Transplants and daughter roots are then infected. When slips are pulled for transplanting, the stem carries the pathogen along with the plant.

Fusarium surface rot is another common problem with roots stored for any length of time after harvest. Surface rot occasionally occurs before harvest on roots that have been mechanically injured, split by growth cracks, or damaged by nematodes, insects or other soil pests. Lesions on fleshy roots are circular, light to dark brown, firm and dry. They appear solid brown and are often centered on a broken rootlet.

Surface rot typically occurs on sweet potatoes that are mechanically harvested, when soil is wet and cold at harvest or excessively dry before harvest. This causes increased skinning of sweet potatoes. It also can set in when sweet potatoes are exposed to high or low temperatures for extended periods after digging and before curing, or when conditions are favorable for desiccation of wounded tissue.

Fusarium root rot is another serious disease of sweet potatoes in the Southeast, according to Auburn University research. As with fusarium surface rot, root rot can persist in soil for years. The fungus invade roots through wounds that occur during harvest.

While insects do bug sweet potatoes, they are rarely an economic threat since the vines can outgrow any damage inflicted by the insect. Scouting for economic thresholds, then, is typically more economical than instituting a general spray program.

When the season is over, store sweet potatoes between 55 and 60 degrees Fahrenheit. Do not allow temperatures to fall below 55 degrees or chilling injury will result. Relative humidity should be maintained in the 75 percent to 80 percent range to prevent excessive water loss from the roots. Provide decent ventilation to prevent carbon dioxide buildup.