Don LaBonte

The development of high-yielding and delicious sweet potato varieties began right here in Louisiana more than 70 years ago. The late Julian C. Miller and others at the Louisiana Agricultural Experiment Station discovered how to induce flowering and seed set by trellising sweet potato vines onto fences. They found that this straightforward technique stressed the plant and caused it to flower. This opened a world of opportunity to develop new varieties for the industry by genetic recombination.

A fast-paced effort ensued to collect sweet potato varieties from throughout the world and to make as many crosses as possible. Many of the varieties resisted flowering, and just a handful of seed were produced. But ever so slowly new varieties were developed to replace the original varieties grown in Louisiana, which were from the Caribbean and had little or no disease resistance, poor and erratic yield, and a muted orange flesh.

Seventy years later the sweet potato breeding program continues. We still use the nursery started by Miller in the 1930s on the LSU campus in Baton Rouge. Though the technology has changed, the breeding program builds on a legacy of methodical progress toward developing the perfect sweet potato. Diseases that once were the scourge of the industry such as fusarium wilt and soil rot are forgotten as new varieties incorporating resistance to these diseases have replaced susceptible ones. But new threats to the industry invariably arise, and we adapt our breeding program to address these new issues.

Breeding programs take years of determined work to improve a trait, and we try to anticipate industry and regulatory changes. For instance, the federal government has pressed for years to reduce the number of pesticides used by growers, thereby delivering more pesticide-free crops to consumers and reducing pesticides in our environment. Consumers clearly prefer to have pesticide-free crops, and growers are anxious to reduce pesticide usage to save on production costs.

In the mid-1990s we set out to develop resistance to Rhizopus soft rot. This is a common disease that causes roots to break down after they are boxed and sent to market. A simple puncture wound and a few Rhizopus spores is all that it takes to drastically compromise the marketability of fresh market potatoes. A couple of roots infected with this disease are enough to get a load rejected at a distant shipping terminal. An effective fungicide (dicloran) is available, but we also had the foresight to realize its days were numbered. As recently as 2008, the European Union put a zero tolerance on this fungicide, and thus all sweet potatoes exported to the burgeoning EU market must be free of this fungicide.

Our efforts to breed for Rhizopus soft rot resistance are typical of the approach we use to improve all traits. We began with a group of genetically diverse individuals (parents or genotypes, as we term them) with good flowering attributes. We allow them to cross-pollinate in an open pollinated nursery. Because of the sweet potato’s complicated genetic structure, an individual genotype cannot cross with itself but must cross with another genotype. Thus, we arrange a nursery with 15-30 unique parents, and let bees to do the pollination. We then harvest the seeds in the fall and plant upwards of 2,000-4,000 seeds at the Sweet Potato Research Station in January each year in a greenhouse. In May, we transfer the resulting seedlings (progeny) to the field and allow them to grow and produce storage roots. At harvest we visually select those that have an attractive shape and decent yield. Through the course of this process, we usually winnow these to just a few hundred. Next we screen all those selected for resistance to Rhizopus soft rot.

In the beginning of this project few selections had notable levels of resistance. But by using an approach called mass selection, we select the best 10 percent and use these as parents in the next cycle of breeding. The entire process takes two years to complete and return to the nursery. This process is repeated, each time selecting the best of the best to use as parents. As it stands now, most of the progeny are resistant, and we are focused on selecting for all important traits including yield, appearance, resistance to all the other diseases, and eating quality. Our efforts are paying off. We are now poised to incorporate this resistance into varieties that can be commercially grown by the industry.

The LSU AgCenter breeding program has a similar effort to enhance resistance to the sweetpotato weevil, the most damaging insect pest of sweet potatoes in the world. We are also initiating efforts to improve virus resistance and skinning resistance in sweet potato. We are particularly keen on developing a sweet potato variety with Rhizopus soft rot resistance and a more durable skin. This combination will push us toward a more mechanized harvest, requiring less labor and less cost in production.

Trait breeding represents just a fraction of our breeding efforts. The bulk of our crossing efforts are among elite genotypes to produce a new variety with ever higher yield, improved quality and ease of production. We may generate 10,000 to 14,000 seeds every year from these nurseries and have only 10-15 suitable progeny after three years of trials. It can take another three years to determine if any of these have value for the industry. In a given year we are looking at progeny at all stages of the six-year process.

The breeding program is both an art and a science. It involves intuition that a given progeny has the right look, all the while using scientific investigative techniques to ensure good yield, disease and insect resistance, and eating quality. Once we identify a superior variety, we also need to figure out how to grow it – all sweetpotato varieties seem to behave a little differently, and they often require tweaking in-field variables, such as fertility, to maximize production. The LSU AgCenter Breeding Program is multi-disciplinary and taps into the expertise of many in the sweet potato research group. The team approach ensures that the LSU AgCenter will meet the ever-changing needs of producers and the industry.

Don LaBonte, Professor, School of Plant, Environmental & Soil Sciences, LSU AgCenter, Baton Rouge, La.

(This article was published in the spring 2009 issue of Louisiana Agriculture.)