By Katie Sullivan and Terry Kirkpatrick, Arkansas Nematode Diagnostic Laboratory
Look at your soybeans. No, really look at your soybeans. Give that plant a good inspection—and dig it up and look at the roots. The field in the background of the first photo below is a healthy looking field: nice and green, even in height, looks like a pretty good crop.
But those galls on the roots tell another story. This plant, and many others in this field are infected by the root-knot nematode. And with galls like these comes less functional roots, meaning less water and fertilizer gets from the soil to the pods, and lower yield.
The Arkansas Nematode Diagnostic Laboratory would like to raise awareness among soybean growers of the potential for the root-knot nematode to affect soybeans in virtually every soybean-producing county in Arkansas. In the past two years, thanks to a survey sponsored by the Arkansas Soybean Promotion Board, we have received soil samples from all soybean-producing counties in the state (with the exception of Independence, Lee, Monroe, Hot Spring, Crawford, and Sebastian counties). We found root-knot nematodes in almost a fourth of the 1,635 fields that were sampled. In other words, one out of four fields in our state is infested with this nematode.
Don’t be fooled! You need to dig up plants and look at the roots to diagnose root-knot in the field. Soybean cyst nematodes (SCN), charcoal rot, and several other issues are capable of causing similar above ground symptoms. Guessing is dangerous and planting a SCN-resistant variety or providing extra irrigation won’t help you with a root-knot problem. Nor will planting a cultivar that is rated as moderately susceptible (a rating of 5.0 in the Arkansas Soybean Update) where the nematode is severe. That is why it’s important to check a suspicious field to confirm the identity of the culprit. While some fields don’t show severe damage (yet), as with the field/picture mentioned earlier, severe root-knot infestations generally are pretty evident at this time in the season because they show up as stunted spots with poor soybean growth or even some dead plants.
Granted, most fields in Arkansas that have root-knot are somewhere between asymptomatic (no aboveground symptoms) and dead. But now is the time to look. If you suspect root-knot, simply dig up some of the plants and look for galls. Don’t confuse galls with nitrogen nodules, however. Galls are swellings of the root itself while nodules are attached to the root. If you find root-knot you still may want to send in a field-wide sample to get some idea how high the nematode population is in the whole field – and to see if there are any other economic nematodes also present. Nematodes are hidden and their damage is hidden, but with root-knot the roots don’t lie! Pretty on the outside can be pretty ugly underneath!
2016 is the last year of the free soybean assays that are being provided by the Arkansas Soybean Board. So, to confirm any suspicions of root-knot, dig up a few plants and then send a soil sample to the Arkansas Nematode Diagnostic Lab. An excellent “tutorial” for how to take and package nematode soil samples can be found at: http://courses.uaex.edu/course/index.php?categoryid=63