LSU AgCenter Nematode Advisory Service researchers are working to learn more about a nematode that’s new to Louisiana while also taking a fresh look at control strategies for two nematodes that have been around for years.

Tristan Watson and Josielle Rezende have been keeping a close eye on the guava root-knot nematode, which was first detected on a Louisiana sweet potato farm in 2018.

“It’s considered the most damaging root-knot nematode species in the world due to its ability to overcome nematode resistance in many important crops,” Watson said.

That means that varieties of cotton, soybeans, sweet potatoes and many other crops that have been bred to resist infestations of the more-familiar Southern root-knot nematode may not stand up to the new guava species.

Nematodes are microscopic worms that feed on and damage plant roots. They often cause galls to form on the roots, causing yield loss.

While it doesn’t seem to be spreading here, the guava root-knot nematode has gained a foothold in other states, including North Carolina, where some Louisiana sweet potato growers get planting material. This is how the nematode initially made its way into Louisiana in 2018.

The pest also has been carried into Louisiana twice on ornamental plants from Florida.

“Rotation and resistance are very important to controlling this nematode,” Rezende said.

She has found that corn, sorghum and even some species of winter cover crops are not good hosts for the nematode — meaning they could be used as rotational crops to manage an infestation. She and Watson also have found that certain soybean varieties offer some degree of resistance. They’re also working to examine the host status of various common weed species, which may act as a “green bridge” for this pest between growing seasons.

The scientists regularly survey fields around the state and receive samples in the mail. So far, the guava root-knot nematode has not been detected in any Louisiana soybean fields.

“This survey has also shed light on the current distribution of other economically important nematode pests,” Watson said.

One of those pests is the reniform nematode, populations of which have been growing and reaching damaging levels in soybean fields in several Southern states. In 2020, this nematode was detected in 113 out of 130 samples that the AgCenter collected from Louisiana soybean fields.

“Many soybean growers aren’t aware of just how severe the reniform nematode situation has become,” Watson said. “It’s also difficult to know when the pest is present. It can be rather elusive in that you don’t always see above-ground symptoms.”

The only clue may be yield loss, which is common to many pest issues, making it critical that growers send soil samples to the Nematode Advisory Service if they suspect a problem.

Watson recently planted a test plot of locally used soybean varieties to find which ones offer the best resistance to the reniform nematode.

“We’re currently using outdated reniform nematode resistance data for management,” Watson said. “And the resistant varieties that were recommended based on that data years ago are often no longer available.”

Another project is focused on managing the Southern root-knot nematode in soybeans. The researchers have found that this nematode behaves differently based on geography. Populations in certain parts of Louisiana seem to be more aggressive than others.

“Can we find a variety that protects against the nematode regardless of geographic locations?” Watson said.

For this study, Watson and Rezende are evaluating how 12 commercially available resistant soybean varieties respond to root-knot nematode populations that have been isolated from soybean fields across Louisiana.Olivia McClure

Tristan Watson, left, assistant professor and director of the LSU AgCenter Nematode Advisory Service, and Josielle Rezende, AgCenter research associate, examine soybean plants in a greenhouse at the Plant Material Center at the AgCenter Central Research Station in Baton Rouge. These soybeans are part of a project to learn more about how the reniform nematode affects the crop. Photo by Olivia McClure