Stave Off Sweet Potato Disease

Bacterial soft rot has proven to be a challenge for southeastern growers. Photo credit: Jason Chandler
Bacterial soft rot has proven to be a challenge for southeastern growers. Photo credit: Jason Chandler

Several key diseases of sweet potatoes affect growers in the Southeast, and tactics such as variety selection, sanitation, and proper postharvest handling have proven to be effective control measures.

Chris Clark, Professor at Louisiana State University (LSU) Agricultural Center’s Department of Plant Pathology and Crop Physiology, shows how using these management tactics can help keep key diseases at bay.

Assess Your Conditions
The first thing to do before choosing your varieties is to determine what diseases may be potentially present in your fields, Clark says.

The selection of resistant varieties is most often used to control soilborne diseases, as opposed to diseases associated with propagation, which are typically controlled by using virus-tested seed or certified seed, he explains.

“In the case of soilborne diseases, a grower may conduct soil tests, particularly for root knot nematodes, a pathogen that is common in the sandier soils of the Southeast,” Clark says. “By soil sampling and getting test results in the fall, growers can know whether or not they’re going to have to deal with it. If so, there are some varieties that have pretty good resistance and there are some that are susceptible.”

Increased Varietal Diversity
While the sweet potato industry was dominated by a few standout varieties, Clark explains how things are changing.

“‘Covington,’ a variety developed by North Carolina State University, has been predominant for the last few years, but there are other varieties that are being grown more so than in the past. Now we have ‘Bayou Belle,’ ‘Orleans,’ ‘Evangeline,’ and the newest one, ‘Bellevue,’” he says. These foundation seed varieties were developed by LSU.

When evaluating resistance among varieties, Clark and his colleagues have a different rating system for each disease.

“We evaluate them in controlled tests, comparing them to something we know is susceptible and something we know is resistant,” he explains.

Because root knot nematode is more common in Georgia and the Carolinas, many varieties adopted in that region will have resistance, particularly varieties ‘Bellevue,’ ‘Evangeline,’ and ‘Bonita.’

While disease resistance is key, Clark explains that most growers will not select a variety unless it is high yielding and high quality.

“For many years, they grew things that were susceptible even though there were resistant varieties available. That’s one of the critical things for us — trying to get everything in the same package,” he says.

Key Diseases And Non Varietal Management Methods
Clark says the potyvirus complex, which is a group of viruses comprised of sweet potato feathery mottle virus, sweet potato virus C, sweet potato virus G, and sweet potato virus 2, is a major threat to growers in the Southeast.

The best and only way to manage this complex is to start out with clean seed, he says. To support the distribution of clean seed, a sweet potato network was added to the National Clean Plant Network in 2015.

“One of the activities of the National Clean Plant Network is to develop tissue culture techniques to produce plants free of viruses. We then increase the tissue culture in greenhouses and produce foundation seed so farmers can buy clean plants of each of these varieties,” he explains.

After buying clean foundation seed, most growers increase the seed on the farm for an average of one year so there is enough to plant a crop.

“During this time, we encourage them to isolate the seed from their old plantings as much as they possibly can to avoid infection,” he adds.

Another key disease that has made an appearance in recent years is black rot, which can be carried in on seed roots.

“If it’s brought in, it can then spread onto the sprouts that come up from those seed roots that are used to transplant the crop. Again, the advice is to start clean and stay clean,” Clark says.

Although there are few management methods after infection occurs, crop rotation for up to three years may help. Selected crops for rotation will vary by state, but as long as it is not sweet potatoes, and there aren’t too many morning glories (a close relative of sweet potatoes) in the field, it should help prevent infection, Clark says.

While not as consistent, bacterial soft rot also has proven to be a challenge for southeastern growers. Bacterial soft rot tends to occur whenever oxygen is limited in the environment.

“The bacteria grow equally well in the presence or absence of oxygen, but to defend itself, the plant needs to have oxygen,” Clark explains.

Symptoms can be seen if the grower seals the plant bed too tightly with plastic and there is no air exchange, if potatoes are in storage with high temperatures and limited air exchange, or if there is a layer of moisture left on the sweet potatoes for an extended period of time.

Postharvest Management
To help control postharvest diseases or diseases that occur in storage, Clark recommends curing sweet potatoes immediately after harvest.

“With the curing process, we generally hold the sweet potatoes at 85°F to 90°F at about 85% humidity for anywhere between four to seven days,” he explains. “That helps promote healing of the wounds that occur when the sweet potatoes are being harvested, and it prevents pathogens from getting in and causing disease. It also helps prevent them from losing moisture.”

Improve Your Sanitation Practices
Sanitation is key when managing disease. While growers may have different systems for cutting seed, if using knives, dipping the knives in sanitizing solution with 10% bleach is helpful, Clark says.

When it comes to machinery, if a disease such as black rot is present, it is critical to sanitize all surfaces, however difficult that may be.

“You have to think about sanitizing the digging equipment, the pallet boxes, the packingline, and so on, from start to finish. You have to clean every surface that comes in contact with the sweet potatoes,” Clark explains.

For more information on sanitation and other postharvest management practices, you can consult the Postharvest Management of Sweet Potatoes publication at https://is.gd/sweet_potatoes.

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