Tristan Watson, Josielle Rezende and Christopher Clark

Sweet potatoes are prone to several soilborne diseases, including those caused by nematodes and fungi. While some of the large acreage agronomic crops have had yearly systematic surveys of the incidence of diseases, there has not been such a program for sweet potatoes in the U.S. and much of what we know is from anecdotal records.

Root-knot Nematodes

Root-knot nematodes are damaging pests on a wide range of crops, including sweet potatoes. Recently, a new species of root-knot nematode (Meloidogyne enterolobii) has emerged as an important pest in the United States. This new species was first described parasitizing the pacara earpod tree in China in 1983 and was later reported in the United States in 2004 on ornamentals in Florida. In 2011, M. enterolobii was detected on soybean and cotton in North Carolina as well as on sweet potatoes in South Carolina in 2019. Recent soil surveys conducted in North Carolina suggest this nematode is well-established in North Carolina sweet potato production.

This is a problem for Louisiana growers because a small portion of planting material has been sourced from North Carolina. In 2018, M. enterolobii was intercepted entering Morehouse Parish on a shipment of Covington sweet potato seed roots from North Carolina. In response, Louisiana Department of Agriculture and Forestry (LDAF) established a quarantine on the importation of planting material from North Carolina, South Carolina and Florida; however, certified pest-free seed sweet potatoes can still be imported with proper inspections under a special permit.

Since this first introduction, LSU AgCenter researchers, in collaboration with LDAF, have subsequently detected M. enterolobii on ginger imported from Florida in Plaquemines Parish (2019), certified pest-free sweet potato planting material from North Carolina in Franklin Parish (2020) and potting mix from a foxtail fern imported from Florida in Natchitoches Parish (2021). In Louisiana, M. enterolobii was not detected in a recent survey of over 225 production fields, suggesting that this new nematode pest has not established in our state. Crop susceptibility studies have identified nonhost rotation crops (corn and grain sorghum) and winter cover crops (winter wheat and winter rye), which could be used to manage this nematode should it successfully establish in Louisiana.

The LSU AgCenter sweet potato team has evaluated sweet potato breeding lines for resistance to M. enterolobii, and several lines look promising. Recently, the team received a U.S. Department of Agriculture grant for over $5 million, in collaboration with North Carolina State University and other participating institutions, to characterize M. enterolobii resistance in sweet potatoes and develop resistant varieties for commercial use.

Black Rot Disease

Black rot is a disease caused by the fungus Ceratocystis fimbriata. It is more common in tropical areas that grow sweet potatoes throughout the year, but was once also a problem in the U.S. By about 1970, an integrated management program had reduced black rot such that for about 50 years it was only observed on rare occasions in some states and was not found in Louisiana. The program consisted of rotating production fields and plant beds out of sweet potatoes, selecting seed roots that were free of disease, treating the seed roots with an effective fungicide such as thiabendazole and cutting the slips at least an inch above the soil. Crop rotation reduced survival of the pathogen in soil, fungicide treatment reduced infection of seed roots during bedding, and cutting plants reduced spread in slips infected from growth of the fungus into the plants from the seed roots.

In about 2013, reports came from North Carolina of increased incidence of black rot, and by 2015 reports also came from other states. In 2018, black rot was found in one location in Louisiana. Unfortunately, roots used for sweet potato seed have included both certified seed as well as lots that were never certified for seed use, and there are not adequate inspection records to determine to what extent black rot may have spread among states on interstate movement of seed roots.

It is not yet clear why this once well-managed disease re-emerged. It is possible that farmers growing certain varieties that are slow to produce plants in plant beds may have succumbed to the temptation to pull rather than cut slips. There have also been suggestions that C. fimbriata may be growing further up the stems of slips than previously experienced, thus colonizing even those plants cut as recommended. Screening for resistance to black rot was active in the 1950s but abandoned after thiabendazole became available for disease management.

The LSU AgCenter sweet potato team began screening sweet potatoes for black rot resistance in 2016. So far, all but two breeding lines have been susceptible. Field research is not conducted in Louisiana to avoid re-establishing this pathogen where it does not presently occur, so it may take time to find a breeding line that can be used for effective black rot management in the field.

Tristan Watson is an assistant professor, Josielle Rezende is a research associate, and Christopher Clark is a professor in the Department of Plant Pathology and Crop Physiology.

(This article appears in the winter 2022 issue of Louisiana Agriculture.)

Tristan Watson, left, director of the LSU AgCenter Nematode Advisory Service, and Josielle Rezende, AgCenter research associate, examine soybean plants in a greenhouse at the AgCenter Central Research Station in Baton Rouge. Photo by Olivia McClure

Black rot disease of O’Henry sweet potato caused by Ceratocystis fimbriata. Photo by Christopher Clark

Abnormal growths on Covington sweet potatoes associated with Meloidogyne enterolobii parasitism. Photo by Charlie Overstreet