Tristan Watson, a nematologist with the LSU AgCenter, collects a soil sample from a soybean field at the AgCenter Northeast Research Station near St. Joseph. Watson uses soil samples to determine the type and number of nematodes present in a production field.
Watson, right, and graduate student Caleb Hamm are involved in research designed to combat nematodes, which feed on the roots of plants and can reduce yields significantly if their populations reach a high density. Photos by Craig Gautreaux
Many people may not know that nematodes are the most numerous multicellular animals on earth, but you can be sure farmers are well aware of the damage these animals can cause on their crops.
Tristan Watson is a nematologist for the LSU AgCenter who is charged with helping farmers manage their nematode problems. His tasks include surveying fields across the state, identifying the types of nematodes found and watching for any new nematode species that might be encountered.
In Louisiana, the two most common nematodes are the reniform and Southern root-knot. Also known as roundworms, these parasites are found in the soil and can make waste of a crop in short time if they exist in high concentrations.
“The vast majority of nematodes damage crops by feeding on the root system, and this prevents plants from accessing water and nutrients,” Watson said. “So a lot of times, the symptoms of nematode damage look identical to nutrient deficiencies and water stress.”
Watson is currently doing research at the Macon Ridge Research Station near Winnsboro and the Northeast Research Station near St. Joseph on different soybean varieties. He is looking at the 20 most widely planted varieties grown in Louisiana and seeing if any provide resistance to nematodes.
“With every variety, we saw reniform nematode reproduction on every single one,” Watson said. “But there were some differences in the extent of nematode reproduction.”
Watson said five varieties seem to be the least susceptible to damage, but two varieties seemed to show high susceptibility.
Watson is also examining five new varieties that have confirmed resistance in Missouri and is working under the premise that this resistance would extend to Louisiana populations of reniform nematode.
Some populations of root-knot nematodes were able to overcome resistance in soybean varieties and build up to significant population densities. This discovery was concerning to Watson because there are very few nematicides to combat nematodes in soybeans.
One defense for reniform nematodes is to use a corn rotation, a commonly grown nonhost crop. This strategy seemed to reduce the population density in the field, which would mean lower numbers for the subsequent crops.
“Each crop in Louisiana has its own management strategy that can be utilized,” Watson said. “For instance, soybean and corn, we don’t have a lot of nematicides that are registered for use. But on soybeans we have some resistant varieties to the Southern root-knot nematode that can be utilized.”
The simplest way to determine how many nematodes are present and what species they are is to collect a soil sample.
“You need to go out and sample soil in your field and send it in to a diagnostic lab,” Watson said. “What you are going to get back is a list of nematode species you have and at what density.”
One trend Watson has been seeing in his research is the displacement of root-knot nematodes by reniform nematodes across the state. He has not been able to explain the phenomenon, but the reniform nematode does not seem to be as damaging to some crops as the root-knot. This may be a form of good news for farmers.
A third species of nematode Watson is on the lookout for is the guava root-knot nematode. It has been found twice in Louisiana. If this nematode becomes established, it could cause serious issues.
“We don’t have a lot of management tactics like we have for some of these other nematodes,” he said. “We don’t have any resistant varieties yet. We don’t really know if any of the nematicides that we have available will work for this nematode.”
Because nematodes feed on the roots of plants, it can turn sweet potatoes into an unmarketable product.
“The root-knot nematode, it causes deformation of your roots, and that’s what you’re trying to sell as a sweet potato grower,” Watson said.
If his research is successful, growers should have a good selection of varieties that can stand up to the attacks from these parasites to choose from. Craig Gautreaux