Thursday, December 8, 2022

Know Thy Enemy

Sample Regularly To Identify Shifts In Nematode Populations, Species

• By Vicky Boyd,
Managing Editor •

arkansas nematode sampling
A technician collects nematode samples in a University of Arkansas nematicide field trial shortly after harvest. — photo by Travis Faske, University of Arkansas

In his book about military strategy, “The Art of War,” Chinese philosopher Sun Tzu talked of winning the battle by knowing your enemy.

The same can be said for plant parasitic nematodes that lurk in soils and may attack cotton roots, causing yield loss and even plant death under severe infestations.

With regular soil sampling, you’ll be able to gain knowledge about a possible nematode infestation, how severe it is, the species involved and species shifts.

“I think there’s no question that nematodes are on the rise,” says Steve Brown, Alabama Extension cotton specialist. “We have some options by which we can address both root-knot and reniform, but you have to know you have them by sampling at the right time. That would be a very valuable piece of information as you make variety choices in the future.”

He was referring to new and pending varieties that cottonseed companies have bred with resistance to Southern root-knot and/or reniform nematodes.

Although those two are the predominate plant parasitic nematodes found in Mid-South and Southern cotton fields, other species also may inhabit them and damage crops.

Columbia lance nematodes, for example, are common in North Carolina, causing considerable root damage. In Georgia, sting and lance join Southern root-knot and reniform as cotton pests.

And the newly introduced guava root-knot nematode, considered one of the most damaging species worldwide, is expanding its range as the result of infested sweet potato and ornamental plant materials. Currently, it has been found in Florida, North Carolina, South Carolina and Louisiana.

Sampling 101

There are two types of nematode samples, says University of Arkansas Extension plant pathologist Travis Faske. One is the diagnostic, which is pulled from a troublesome spot in a field to determine whether nematodes are causing the issue.

southern rootknot nematodes
Galling on young cotton roots from the Southern root-knot nematode — photo courtesy LSU AgCenter

The other is the predictive sample, which tells you how many nematodes you’ll have next year and the species breakdown, he says. This one is taken shortly before harvest or right afterward.

Typically, growers or consultants pull predictive samples every three years unless the cropping rotation has changed, soil has been moved as with leveling or the grower picks up a new piece of ground. Under these scenarios, samples may be pulled more frequently to predict shifts in nematode species or populations or to identify potential nematode issues with a new field.

How To Pull Samples

Take the samples within the plant row and root zone — nematodes want to be close to the food source and don’t move large distances on their own. In the Southeast, dry conditions may prevail during harvest, complicating sampling.

“You should take them late season in September and October, but that’s typically when we have our drier months,” Brown said. “We need to take samples concurrent with good moisture, and that’s a trade-off.”

Wait too long, and cooler fall and winter temperatures could cause sample results to be unreliable, and some species — such as root-knot and sting — could be missed entirely.

The University of Arkansas provides these simple sampling instructions for cotton:

• Use ¾-inch tube-type soil probe.

• Insert 6-8 inches deep in soil near root zone.

• Collect 10-20 cores in zig-zag pattern.

• Mix and place 1 pint of soil in plastic bag, secure with wire tag, label with pencil.

• For instructions on how to submit samples, contact your county Extension office.

Some may balk at the number of cores, but Faske says they are designed to provide a representation of the field.

“Sampling is only as good as what area it covers,” he says. “If you go out and dig one sample, you don’t want to use that to reference your entire field. Would you base your fertility program on one sample you pulled in the middle of your field?”

Faske also has seen people store samples on ice in a cooler, but he says that’s not needed. Instead, avoid extreme shifts in temperature by putting samples in an ice chest without ice, keep them out of direct sun and send them to the analytical lab as soon as possible.

Submitting The Sample

Submitting those samples also has become more economical in some states, thanks to the support of a number of commodity checkoff programs. In Arkansas, for example, the Arkansas Corn and Grain Sorghum Board and Arkansas Soybean Promotion Board underwrite the Arkansas Nematode Diagnostics Laboratory’s testing of samples from fields that have been in those crops or that are going to be rotated to those crops, Faske says.

Although he doesn’t have firm figures on how many samples are submitted annually to the Arkansas lab, Faske says they have increased significantly since the commodity groups began funding the program six or seven years ago.

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