LSU AgCenter nematologist Tristan Watson is working on projects aimed at helping soybean farmers manage various species of nematodes — tiny roundworms that feed on plant roots, reducing yields.

Reniform nematode

Watson is evaluating soybean varieties for resistance to the reniform nematode, which he estimates is in 60% of Louisiana soybean fields, and hopes to make recommendations to growers soon.

“We have demonstrated that deploying host resistance can be an effective tool to reduce reniform nematode populations and improve yield,” he said.

In 2024, the reniform nematode was responsible for a 1.95% reduction in Louisiana soybean yields. This amounted to a loss of more than 1.1 million bushels and $11.75 million.

Despite these significant impacts, it can be challenging to diagnose infestations of the reniform nematode, Watson said. It reduces the size of soybean root systems but does not cause obvious root deformation like other nematode species.

One calling card of the reniform nematode is high variability in plant height, giving rows a wave-like appearance. It tends to be broadly dispersed in infested fields — another distinction from other nematodes, which usually have a patchier distribution.

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

In a related project, doctoral student Lucy Kiarie is trying to identify genes associated with host resistance and characterizing their resistance mechanisms.

Southern root-knot nematode

The Southern root-knot nematode represents another major threat to Louisiana soybean production.

Watson believes a multipronged approach is necessary to manage this nematode. He’s studying whether winter cover crops can suppress its activity. He also is evaluating new, in-furrow nematicides. And, with help from AgCenter soybean specialist David Moseley, he has set up trials across Louisiana to evaluate resistant varieties.

The Southern root-knot nematode is present in about 20% of soybean fields in the state, Watson said. It causes root galling, stunts plants and can even lead to plant death.

The nematode diminished Louisiana soybean yields by 2%, or 1.2 million bushels, in 2024. That translates to an economic loss of about $12 million.

Iris Aguilar, a doctoral student, is playing a key role in this project.

Nematode yield losses, damage thresholds

Watson is working to put a dollar amount on yield losses that result from nematode damage. He has established plots in soybean and corn fields statewide where he’ll monitor yields and nematode damage.

“This is critical to direct research priorities and aid in leveraging federal and private investment in development of effective management tactics,” he said.

In addition, Watson also is recalibrating the damage thresholds used by the AgCenter Nematode Advisory Service to accurately diagnose nematode issues in Louisiana.