Nematodes aren’t visible to the naked eye but they still steal bushels
John Buck works year-round to fend off robbers who sneak around his farm. Not the bad guys who break into homes and shops to steal electronics and tools but the thieves who lurk underground. The microscopic worms nibble at his corn, soybeans and wheat, robbing yields and income when every bushel counts.
“Most farmers have some level of a nematode problem,” Buck says. “In Ohio, soybean cyst nematode (SCN) is a huge issue. Most of our researchers say it’s the No. 1 cause of yield loss. What makes it hard is we’re losing yield to something we can’t actually see.”
U.S. farmers lose an estimated $1 billion each year to SCN alone. While many farmers think planting a resistant variety is enough, that’s no longer the case in soybeans where resistance is building. In corn, there’s no natural plant resistance, so it’s important to diligently monitor nematode populations to ward off damage.
Nematodes typically don’t completely kill your crops; instead they slowly whittle away at yield. Since they’re parasitic they need their host, your crops, to be alive to survive.
Buck’s New Bloomington, Ohio, farm has a variety of soil types from sandy loam to dense muck that will sink a tractor. With diverse field conditions, he has to cater his nematode strategy down to the acre.
“We soil sample on a 1-acre grid pattern,” he says. “You learn a lot about what your soil is capable of and what nematodes are in the soil. Soil type and nematode populations change faster than your typical 2-acre grid.”
Nematode damage typically doesn’t visually appear above ground in corn or soybeans, but if it does it often mimics other problems. To identify nematode damage you have to get your hands dirty and dig soil samples.
Soil testing becomes critical when nematodes, or what you think are nematodes, become severe. “If it looks like water damage, yellowing or stunting, it can be a sign of nematodes,” says Greg Tylka, Iowa State Extension plant nematologist.
If you experience unexplained yield loss and can’t attribute it to another cause, check the plant roots. Send in the soil and roots to be tested five to ten weeks after planting.
When you send in a soil sample, make sure the lab can test for nematodes and ask for a nematode test that includes endoparasitic and ectoparasitic evaluations. A nematode test involves counting the number of nematodes present in 100 cc, a half-cup, of soil. If the number is below threshold, don’t let up on control measures. If the number is above threshold, especially in the most aggressive nematode species, switch up management practices to get a handle on the situation.
If the SCN population exceeds 16,000 per 100 cc following soybeans or 12,000 following corn, infestation levels are too high. Addressing nematode populations will require more intensive measures, such as rotation to a non-host crop for multiple years, sampling every fall and possibly a seed treatment or use of resistant varieties when planting soybeans.
“SCN is an introduced pest, so it has few natural enemies,” explains Terry Niblack, senior associate dean of the College of Food, Agriculture and Environmental Sciences at The Ohio State University and a nematologist by training. “This sets you up for an epidemic. It’s also a sedentary endoparasite; the worst kinds of nematodes are in this group.”
SCN is the major nematode in soybeans. It invades the roots and then tricks the plant into sending food to the roots where the parasite can feed on it instead of above ground where the food can benefit the plant. This can drastically reduce yield—up to 40% loss with no above-ground symptoms, says Palle Pedersen, Syngenta head of Seedcare product marketing.
Once SCN strikes a field, it’s there forever, and it can take years to lower high populations. SCN can lay dormant in the soil for more than a decade on host plants, such as weeds.
To address high SCN levels, it might be time to experiment with crop rotation or nematicide seed treatment. “Rotation can help. The first year you’ll see a reduction in population, but it won’t be so drastic in subsequent years,” Niblack adds.
Nematodes can cause yield loss even when damage isn’t visible to the naked eye.
Numerous types of nematodes strike corn, but four, in particular, merit attention. Needle nematodes are one of the most damaging to yield. Identifying needle nematode damage can be tricky since the symptoms mimic water damage. If five to 25 needle nematodes are present per 100 cc of soil you need to step up your game, according to action levels from the University of Illinois.
“Invasion can result in clubbed roots and a lack of fine roots, resulting in a stunted root system,” Tylka says. “Needle nematodes are big and bad on their own, but they only occur in sandy soils, which tend to be dry when not irrigated, and that compounds the matter, resulting in severe damage.”
Sting nematodes are also damaging but are only found in southern Illinois and the South. They stick to sandy soils because of their larger size, making it easier for them to move around. Below-ground symptoms of sting nematodes are similar to needle: clubbed roots and a lack of fine roots.
Kansas State University says if you reach 10 sting nematodes per 100 cc it’s time to take action (that number varies for other states). “Sting and needle are the problems I call the 800-lb. gorillas,” Tylka says. “Take precautions. Try not to move them from field to field with dirty tillage equipment.”
Lesion nematodes invade the roots and cause black lesions where cells die, Niblack says. “The worst thing is they go in and out of the roots, making more holes and opening it up to more pathogens like root rot.”
Lesion nematodes are not picky when it comes to soil type and can be found just about anywhere. They don’t typically cause as much damage as needle and sting, and thresholds range from 50 to 200 lesion nematodes per 100 cc of soil, depending on the state.
Dagger nematodes are occasionally found in soils. While they prefer sandy soil they’ll live just about anywhere. They kill root tips and interrupt the roots from bringing water and nutrients to the growing corn plant. In severe cases, dagger can cause stunting and chlorosis. However, severe cases are not common—dagger is still not as damaging as sting and needle, with a threshold range of 50 to 250 nematodes per 100 cc of soil.
At the end of the season, there’s nothing more devastating than yields that don’t meet your expectations. Microscopic pests can rob you of hard- earned yields and dollars.
“As farmers, we need to be more proactive and see what’s under the ground and what it’s doing to our crops. I switch genetics every year to keep the nematodes guessing,” Buck says. “If we don’t do what we can to mitigate problems we can’t see, we’re shooting ourselves in the foot.”