Nematode infestations are not as obvious as armyworms, thrips, or fleahoppers, but the damage they cause to numerous crops can be devastating. Farm Press called on Extension and Research pathologists From Texas to Georgia to ascertain the damage nematodes can cause, the symptoms to look for in-season, sampling recommendations, and management options. Also, new research seeks to find more varieties resistant to nematode damage.

The first and most crucial part of managing nematode infection is identifying the problem, including determining level of pressure and specific species present.

Timing and collection of representative samples are critical to identifying infection levels and creating a viable management plan.

“Sampling should wait until at least 60 days after planting,” Terry Wheeler, Texas A&M AgriLife Research Pathologist, Lubbock, said. 

“Soil sampling for a nematode analysis is fine from 60 days to the end of the season, as long as there is good soil moisture.”

She said soil tends to dry out in the fall, so sampling during the growing season “is usually better.”

“Taking soil samples is the only way to determine the population density of root-knot or reniform nematode,” said Travis Faske, University of Arkansas Division of Agriculture, Lonoke.

Related:Unmanaged nematode damage can exceed 50%

He added that other nematodes, including lesion nematode or lance nematode, could be a problem in some states. “Lance is generally not a problem here.”

He said the best time to sample in his area is just after harvest.

Sampling is crucial

“Sampling is the only way to detect problems of nematodes other than root-knot nematodes, the only one if you were scouting you could see stunted plants showing nutrition deficiencies in mid-season and could dig up and see galling.

“That’s where I come in, sampling and getting farmers to sample. I've talked with some who say they have sampled. I ask, how long ago? ‘Well, about 15, 20 years ago’ they say. A lot of things change within their production system in that time.”

“Fall is the best time,” said University of Georgia Extension Plant Pathologist Bob Kemerait, Tifton.

But Kemerait said some Georgia farmers were sampling this winter. “We were seeing a number of samples come back positive for high levels of nematodes, which is unusual for samples collected during cooler winter months,” he said.

He attributes those high populations to several La Niña winters, winters that are warmer than average and that allow nematodes to continue to feed.

And during the winter of 2022-2023 they had a buffet. “In warmer winters, some cover crops or winter wheat could be a host,” he said. “Some common winter weeds like henbit and chickweed also might be a host in a warm winter. In a winter like we’ve had this year, nematodes never really went to bed and may want to go out and get themselves a snack.”

Related:Resistant varieties, rotation suppress reniform nematodes

He said plants typically not a good host in colder winters could become fair hosts in warmer winters.

Does it make sense to sample in the winter?

“That's a great question,” Kemerait said. “I would say under most circumstances no. It's too late. But we've had growers across Georgia who, against my better judgment initially, took samples. They wanted to know what was in their fields. Surprisingly, we’ve found some very high levels of nematodes.”

He cited two reasons to avoid taking samples during winter months. “The first is if you come back with nothing in the sample, you don't know if that's because there's nothing there or because you sampled in the winter when nematode populations are suppressed. Also, our threshold levels are based upon fall counts, not spring. That makes interpretation of winter numbers a bit difficult.

“On the other hand, because it's been so warm this winter, we find successful growers who have nematode pressures at elevated levels, which we normally wouldn't see this time of year.”Kemerait said some growers are making good decisions based on what they find in late nematode samples.“Still, I wish they had sampled in the fall.”

Related:Nematode symptoms may resemble nutrient deficiency

He added that if growers get samples with high numbers, they can make management decisions. “But if they don't get low numbers back, they can't know for sure if they have nothing or the timing is off. I prefer that growers sample in the fall, just after harvest, before soils get cold.”

Population changes

Faske said producers should consider that nematode population density could change over time and sometimes in spite of resistant varieties.

He said producers may say, “’I don’t have a nematode problem,’ referring to root-knot but don’t think about reniform. Without a soil sample they don't know.”

Faske explained that nematode samples can be predictive or diagnostic.

“If growers see a problem, stunted plants, for instance, they take a diagnostic sample to determine why those plants are stunted. They would take a sample in sick-looking plants, not the dead plants. Nematode populations will be low in dead plants.”

He recommended comparing the sample area to a lush and green area in cotton, soybeans, or peanuts. “Send those two samples off and compare nematode numbers to determine if the symptom you're seeing is related to nematode density.”

He said producers take predictive samples in the fall.

“Then they should ask if they can predict damage based on nematode density or are they at a level that would cause damage for a subsequent crop, cotton, soybeans or peanuts.”

Representative sample

He said samples should represent the field as accurately as possible.

Samples do not have to be large, Faske said. “We don’t need a gallon of soil.”

Wheeler recommends taking a composite soil sample (10 to 20 spots per sample) to a depth of 8 to 12 inches.  “You can be close to the surface, 6 to 8 inches, nearer to midseason, deeper as you get close to harvest. 

“Take the sample near the taproot. Mix the soil well in a bucket and put it in a plastic bag,” she said.  “Keep the sample from getting hot, or from freezing.  Nematodes are very sensitive to extreme temperatures.”

Faske recommends producers stay with the same laboratory from year to year.“Although the process is generally the same, sometimes the way labs report findings are a little different.” He said Arkansas reports as per 100 cubic centimeters of soil; Mississippi reports as 500 cubic centimeters of soil; Missouri reports as 250 cubic centimeters of soil.

Thresholds

He said thresholds for nematode populations, unlike for insect pests, are difficult to establish.

“If samples show more than 100 per 100 cubic centimeters of soil per root, that’s a serious problem going into soybeans or cotton and growers should think about a management tactic.”

Faske said Arkansas commodity boards see nematode sampling as a critical tool in crop management and have “put their money where their mouth is, providing free assays to farmers. For the past few years, commodity boards have been paying for nematode assays.”

All three agree, growers have to know what’s there before they can develop a management plan.

8 key tips for nematode sampling

Timing and technique are critical aspects of pulling, protecting, and transporting nematode samples.

Morgan McCulloch, Texas A&M AgriLife, San Angelo, and Travis Faske, University of Arkansas, Division of Agriculture, Lonoke, offer tips to collect nematode samples.