Soybeans, like all living things, cannot survive without water and nutrients. Two potential roadblocks to accessing both are the southern root-knot nematode (Meloidogyne incognita) and the reniform nematode (Rotylenchulus reniformis). LSU AgCenter nematologist Tristan Watson is trying to get the word out about the pests while also developing integrated methods to control them.

Watson, who has been conducting the lion’s share of his research at the AgCenter’s Northeast Research Station in St. Joseph and Macon Ridge Research Station in Winnsboro, said the two feed on the roots of plants, reducing their ability to access the life-giving water and nutrients.

“The symptoms look a lot like nutrient deficiencies and can easily be mistaken for such if not properly diagnosed,” Watson said. “This can lead to yield loss, which can get up to 50% in some cases, thus costing producers a significant amount of money.”

According to an in-state survey of fields, the root-knot and reniform varieties are two of the most damaging nematodes in Louisiana agricultural fields, with the root-knot variety in 20% and the reniform type in more than 60% of the fields examined.

Watson said the loss numbers last year were not insignificant. In Louisiana soybean production, the root-knot nematode was responsible for 2.25% yield loss, resulting in 931,628 bushels lost and $12,390,650. The reniform nematode was responsible for 1.90% yield loss, resulting in 786,708 bushels lost and $10,463,215.

“The root-knot nematode causes galls on the root system,” Watson said. “I encourage farmers who think they might be experiencing an infestation to uproot a plant, preferably mid- to late-season, and examine it. But take care not to confuse the pests for nodules associated with beneficial nitrogen fixing bacteria.”

Watson said to diagnose the reniform nematode, which is harder to detect, producers should take soil samples and send them to the LSU AgCenter Nematode Advisory Service where experts can extract the pests, quantify the number the sample has and inform the farmer how likely they are to have yield loss at that population level.

So, that’s the bad news. The good news is that last year Watson evaluated the chemical nematicide Velum as an in-furrow spray for nematode management on soybeans. The product has been used successfully in cotton and sweet potato production to control the same nematodes and was recently labeled for use on soybeans.

The preliminary trial showed moderate reductions in root-knot nematode populations relative to soybeans that were not treated. For the 2024 crop year, Watson is following up on this work, but from a more integrated approach.

Doctoral student Lucy Kiarie plants a reniform nematode variety screening trial at the LSU AgCenter Macon Ridge Research Station. Photo by Tristan Watson

In addition to nematicide applications, soybean variety breeding for resistance to the pests is an important route Watson has been researching in trials. Varieties found in Missouri seemed resistant to nematode reproduction in the 2022 growing season, as well as outyielding some of the commercial lines that are currently grown in state.

“During the 2023 growing season, we observed the exact same trend,” he said. “The reniform nematode resistant soybean varieties resulted in marked reductions in nematode population densities in soil and often outyielded the currently grown commercial varieties in Louisiana.”

Based on Watson’s previous work on the utility of soybean host resistance and nematicides for nematode suppression, he says it is becoming clear that there is the need to use a fully integrated management approach on soybeans. The use of just one management tactic alone often does not provide adequate protection from nematodes.

Watson’s team also is evaluating the best integrated nematode management tactics for Louisiana soybean growers. He hopes to figure out which combination of management strategies can provide adequate suppression of nematodes, high yields and minimize input costs.

“Our current trial is looking at combining three different nematode management tactics: deployment of host resistance, in-furrow application of nematicides and winter cover crop mixes,” Watson said.