Timothy Miller and Tristan Watson

Sweet potatoes have been a proud part of Louisiana’s agricultural legacy for many years. Louisiana and the LSU AgCenter remain national leaders in sweet potato research and cultivar development despite declines in total acreage in recent decades. In 2024, the estimated farm-gate value of sweet potatoes produced in Louisiana was more than $50 million. Today, breeders continue to innovate by producing cultivars that are higher yielding, better tasting and more resilient to diseases and pests. Among those pests, few are as pervasive as nematodes.

Nematodes are microscopic roundworms that inhabit nearly all soils. Most are beneficial, feeding on bacteria or fungi and improving soil fertility by releasing immobilized nutrients. A separate group of nematodes, known as plant parasites, poses a major challenge to farmers worldwide and can cause billions of dollars of yield loss globally every year. These parasites use specialized feeding strategies to tap into plant roots and draw out nutrients. Parasitized plants rarely die; however, their growth slows, and yields can drop dramatically.

When it comes to sweet potatoes, two nematodes in particular are responsible for significant economic losses: the root-knot nematode and the reniform nematode.

The root-knot nematode (Meloidogyne spp.) is easily recognized by the distinctive galls — or knots — that form on infected roots. It is a widespread pest capable of infecting hundreds of different crops, and in sweet potatoes, it causes severe disfigurement of both the nutrient-absorbing roots and the sweet potato itself. Several species of root-knot nematodes occur across the southern U.S., each varying in aggressiveness and damage potential.

The reniform nematode (Rotylenchulus reniformis), by contrast, is far less conspicuous but highly damaging. A tropical species, it parasitizes the roots of numerous high-value crops such as cotton, tobacco and soybean, in addition to sweet potato. The first nationwide survey for the reniform nematode in 1990 revealed its presence in 11 southern states, including Louisiana. Around that time, Louisiana researchers began investigating its impact on sweet potatoes. Early studies suggested that the reniform nematode could significantly reduce yield, but as root-knot nematode management took center stage, research into the reniform nematode remained minimal.

In 2020, the LSU AgCenter Nematode Advisory Service began surveying sweet potato fields across the state. The findings were striking. The reniform nematode was detected in more than 97% of the fields, while the root-knot nematode appeared in only 17%. A broader survey in 2023 across the entire southern U.S. showed that the reniform nematode had expanded its range into four new states and 140 new counties and parishes. Today, more than half of all counties and parishes that produce sweet potatoes are infested.

The reniform nematode is a particularly concerning pest because of its subtlety. Infected sweet potatoes often have no obvious symptoms. Vines may appear slightly chlorotic, a symptom easily mistaken for nutrient deficiency. Roots may develop tiny lesions that are indistinguishable to the naked eye. Female nematodes secrete a gelatinous substance that protects their eggs and binds soil particles, which can give the roots a dirty appearance even after washing. Despite the lack of clear symptoms, the damage is real. Field trials performed in Louisiana have shown that infestations can result in a 20% to 30% yield reduction.

Work in Tristan Watson’s nematology lab is currently focusing on better understanding how to manage this pest using both chemical and cultural strategies. Some nematicides have been shown to reduce both the number of nematodes in the soil and the number of eggs produced by females. Multiple years of field data have shown that nematicides can boost yield in reniform nematode-infested fields. Overall, effective management of the reniform nematode in sweet potato will require an integrated approach that relies on the use of host resistance, nematicides and effective crop rotations.

Timothy Miller is a graduate student in the LSU AgCenter Plant Pathology and Crop Physiology Department, and Tristan Watson is an assistant professor in the department.

This article appears in the winter 2026 edition of Louisiana Agriculture.

A microscope image of a mature female reniform nematode actively parasitizing a sweet potato root and laying eggs within a gelatinous egg mass. Photo by Timothy Miller

Relative yield of a sweet potato plant grown in a reniform nematode-infested field that has been left untreated, at left, or treated with a nematicide, at right. Photo by Tristan Watson