Breeding new plants from seeds produces healthy seedlings, but after five to six years in a field, they all become infected. To answer the problem, researchers go back and “clean up” the plants using a process called tissue culture.
Louisiana’s sweet potato farmers soon will be planting their crop. And for most, the transplants, or “slips,” they put in the ground will be from seed roots that had their start at the LSU AgCenter Sweet Potato Research Station at Chase, La.
Sweet potatoes are vegetatively propagated, said LSU AgCenter plant pathologist Chris Clark. “Plants such as corn that are grown from seed start out disease-free because pathogens are kept out of the seed, so the plants are initially healthy.”
Grown from cuttings or roots, a vegetatively propagated crop accumulates pathogens in the plant naturally. Over time from year to year, the phenomenon of decline or “runout” causes the succeeding crops to go downhill, Clark said.
“Plants don’t always show symptoms, but a disease problem can result in a 20 to 40 percent yield loss,” Clark said. “It sneaks up on a farmer.”
To get the slips, farmers plant whole sweet potatoes — or “seed roots.” These “bedded” potatoes sprout, and the sprouts are collected and planted as transplants.
Breeding new plants from seeds produces healthy seedlings, but after five to six years in a field, they all become infected, Clark said. To answer the problem, researchers go back and “clean up” the plants using a process called tissue culture.
The process starts with a tip of the growth point of a plant about ∏0.5 millimeter around — about the size of a flake of pepper.
“It’s a small-enough piece so it has no virus,” Clark said. “It’s regenerated in the laboratory, tested for absence of the disease and then grown into larger plants that eventually become virus-tested stock plants.”
LSU AgCenter scientists culture sweet potatoes each year to provide seed roots for Louisiana farmers. Harvesting the growth tip for culture brings the sweet potato “back to its natural beginning as best we can,” Clark said. His lab maintains cultures of about 140 different sweet potato types that are provided to a private company that regenerates 5,000 to 10,000 cultures each year.
Plants from the laboratory cultures go to the greenhouse at the Sweet Potato Research Station, then cuttings go to the field to produce sweet potato roots. Those roots are planted the following year to provide the seed potatoes for growers. Finally, farmers plant them in their fields to produce enough to have seed for their crop the following year.
The seed potatoes the station provides to farmers are called foundation seed.
Researchers are looking for ways to keep seed cleaner longer. The virus is moved by aphids, and if their movement can be timed, it may be possible to control them and reduce the severity of the disease in any one year, Clark said.
The sweet potato industry used to rely on only one variety at any one time, he said. The sweet potato industry now has three major varieties. Two, Beauregard and Evangeline, were developed in Louisiana. The third, Covington, comes from North Carolina.
Louisiana growers lost significant sweet potato acreage in 2008 and 2009 because of drought followed by flood. The ability of a sweet potato in the ground to withstand one or two days of flood can be important, Clark said.
“Essentially, wet soils become ‘sour’ as they stay moist and cause soft rotting of the roots,” Clark said. “Evangeline withstands flooding better.”
Another area where sweet potatoes face diseases is in storage, Clark said. The No. 1 storage disease is a fungus that causes rhizopus soft rot. It can be controlled with a fungicide, but researchers are looking for fungicide-free products.
“The rot is extremely complex,” Clark said. “The problem becomes worse after 100-120 days in storage.”
Field conditions when the sweet potatoes were growing apparently affect the disease when the roots come out of storage. The fungus gets into the root through wounds, so the growers try to minimize wounding the produce.
As a plant pathologist, Clark’s task is to help farmers grow disease-free crops. “Plant pathology is both a science and an art. You use science in the art of diagnosing and managing diseases.”
“We want to increase resistance in new varieties,” Clark said. Beauregard is more resistant but not immune.
So plant breeders and plant pathologists are continually looking for new sweet potato varieties that are resistant to all of the diseases that affect the crop.
“We’re always trying to react to changes,” Clark said.