Nematodes are microscopic roundworms that feed on plant roots, which reduces yields and poses a financial burden on producers. Tristan Watson, a nematologist and associate professor in the LSU AgCenter Department of Plant Pathology and Crop Physiology, has been working tirelessly to combat the pest.

Watson has been evaluating soybean varieties for resistance to the reniform nematode, which he estimates is in more than 50% of Louisiana soybean fields. He says he hopes to make recommendations to growers soon.

“We’ve already demonstrated that deploying experimental soybean varieties with host resistance can reduce reniform nematode populations and improve yield in our prior studies,” he said. “Now that reniform nematode-resistant soybean varieties are beginning to become commercially available to growers, we are shifting our focus to evaluating these new varieties for suitability as a nematode management tool in Louisiana.”

According to Watson, in 2025, the reniform nematode was responsible for a 1.25% reduction in Louisiana soybean yields. This amounted to a loss of more than 500,000 bushels and $5.8 million.

The reniform nematode is known to reduce the size of soybean root systems but does not cause obvious root deformation like other nematode species. Thus, it can be challenging to diagnose infestations, Watson said.

One way to spot a reniform nematode infestation is high variability in plant height, which gives rows a wavelike appearance. While most other nematodes have a patchier distribution, the reniform variety tends to be more broadly dispersed in infested fields.

“Although the overall damage on an individual plant is only moderate under severe infestations and rarely causes plant death, the cumulative impact of reniform nematode feeding across an entire field can cause serious yield losses for Louisiana growers,” Watson said.

Watson said that, in a related project, doctoral student Lucy Kiarie has identified new genetic regions in the soybean genome associated with reniform nematode resistance which can be utilized by soybean breeding programs around the country to develop new resistant varieties.

Another major threat to Louisiana soybean production, according to Watson, is the southern root-knot nematode.

Watson believes a multifaceted approach is crucial to managing this highly damaging nematode. He’s studying new nematicide chemistries and application methods to help protect young soybean roots from nematode feeding. With help from AgCenter soybean specialist David Moseley, he has set up trials across Louisiana to evaluate new resistant varieties.

“Although numerous soybean varieties are advertised as root-knot nematode resistant, most of these varieties only help to protect yield in infested fields and do not actively reduce nematode numbers,” Watson said. “This is a problem because growers then have to plant into a field with a high potential for root-knot nematode problems the next growing season.”

The southern root-knot nematode is present in about 15% of soybean fields in the state, Watson said. It causes root galling, severely stunts plants and can eventually lead to plant death.

The southern root-knot nematode diminished Louisiana soybean yields by 2%, or approximately 875,000 bushels, in 2025. That translates to an economic loss of about $9.3 million.

Iris Aguilar, a doctoral student, is playing a key role in this project. She is evaluating which combination of resistant soybean varieties and nematicide chemistries provide the best protection from nematode damage.

An experimental plot at the Northeast Research Station in St. Joseph is infested with the reniform nematode showing stunted plant growth, giving rows a wavelike appearance. Photos provided by Tristan Watson

The severely stunted plants and death of young seedlings in a patchy distribution are signs that this experimental plot at the Northeast Research Station in St. Joseph is infested with the root-knot nematode.