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Know your nematode numbers

TAGS: Soybeans
Ginger Rowsey woman in laboratory
Heather Kelly grinds a soil sample at the West Tennessee AgResearch and Education Center. It’s part of the process of screening for nematode eggs.
Experts say there’s still time to submit samples for nematode screening. It’s the first step for managing this silent yield robber.

Soybean cyst nematode is the number one yield-robbing pathogen in soybean nationwide. Depending on the severity of the SCN infestation, experts say soybean yield loss may be up to 50 percent when population levels are very high. This fall pathologists are again urging producers to have soil samples screened for SCN. 

“If you’re taking soil samples for nutrition or soil fertility anyway, why not send them for nematode screening,” posed Heather Kelly, associate professor in the Department of Entomology and Plant Pathology with the University of Tennessee. “A small fee for a test could end up saving you lots of money in the long run.” 

In Tennessee, growers can have soil tested for 14 pathogenic nematodes at a cost of $15 per sample. A simple SCN soil screening is free. 

“For the free screening, we’re just catching the SCN cyst, which is the egg-filled body of a dead female,” said Kelly. At her lab at the West Tennessee AgResearch and Education Center, she and technician Susan Thomas-Laird show how they crush the cysts open and then count eggs under a microscope to get a field-level threshold.  

“The $15 test looks at adult or juvenile nematodes. The only way you can tell a difference between these microscopic worms is by looking at their mouthparts,” Kelly said. “That takes a lot of time and expertise, which is why it costs more money if you want the full pathogenic screening.” 

Which screening do I need? 

“SCN is just affecting soybean, so if the soybean crop is your main concern, the free screening may be fine for you,” Kelly said. “But if you are looking at corn or cotton or other specialty crops in the rotation, then you’ll probably want to do the full pathogenic nematode screening.” 

While Kelly says SCN is the most common type of nematode in Tennessee, in areas of high cotton production, the reniform nematode has developed into a major pest causing damage in cotton fields and even some yield loss in soybeans. 

“Another nematode that can cause a lot of damage across a lot of crops is root knot nematode. It really prefers sandy soil, so if you do have a field that’s sandy, that’s one of the most concerning nematodes. It’s harder to manage because it has such a wide host range. Just rotating between soybean and corn will not control root knot nematode.” 

Ginger RowseySampling for nematodes in laboratory.

Susan Laird prepares a soil sample for SCN screening at the West Tennessee AgResearch and Education Center.

When should I sample? 

Kelly advises producers to act quickly to get accurate nematode readings and recommendations for next year’s crop. 

“We don’t put down a deadline, but really after the ground freezes the samples are not going to be as representative or as trustworthy for your population. You’ll have to go deeper into the soil to get to those nematodes. It’s really best to try to sample in the fall before the ground freezes to look at that population at its peak to be able to make recommendations going into next year.” 

Kelly says collect 10-20 soil cores in a zigzag pattern across the field. Avoid sampling in between the rows of the recently harvested crop since the nematodes will be under the roots. Insert the soil probe at a 45-degree angle about 4-6 inches off the row. 

On average it’s recommended to screen for nematodes once every 3 years, although Kelly says if previous screening results show higher nematode levels, you’ll want to sample on a yearly basis. If nothing else, you’ll want to sample to see if your management practices are reducing your nematode number. 

Nematode management options 

“Every field will be different and need its own plan, but in general the best management practice is to switch to a non-host crop,” Kelly. “For SCN that can be corn or cotton. You’re not going to eliminate the nematodes, but you can drop those populations way down by simply getting a non-host crop in the rotation.” 

For those going back with soybeans, Kelly said look for varieties that have built-in resistance and consider pairing them with a nematode protectant seed treatment. Most soybean varieties on the market do have resistance to SCN. The downside is the majority have the exact same source of resistance, PI 88788. 

“Unfortunately, because that source of resistance has been widely used for many years now, we have found populations of SCN that can reproduce on cultivars that have that source of resistance. So, you can no longer just rely on planting a resistant variety for SCN management.” 

More SCN resources are available at the SCN Coalition website. For submitting samples to the University of Tennessee see the Nematode Sampling Guide and Submission Form.  

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