To establish and maintain a good soybean program, many southern farmers may need to nullify nematodes. Left untreated, the minute critters can stop a crop in its tracks.
Terry Kirkpatrick, University of Arkansas Extension plant pathologist at Hope, says knowing which nematodes you have and having the right crop rotation are important in managing the microscopic worm parasites.
His major focus is management of the Arkansas Nematode Diagnostic Laboratory and coordinating the annual Soybean Cultivar Disease Screening Program, both partially funded by the Arkansas Soybean Promotion Board. And with the expansion of soybean acres in Arkansas in recent years, he’s been busier than ever.
“There are about 4 million soybean acres in Arkansas, which like the overall Mid-South, has seen an increase in acres at the expense of cotton and rice,” Kirkpatrick says. “A lot of cotton fields were monoculture before shifting to soybeans. Farmers managed them primarily by using nematicides, and were able to hold the pests at bay.
“When growers pulled out of cotton and switched to soybeans, many of them didn’t try to match up resistant varieties with fields where they knew they had nematode problems.
Nematode populations had been building up for years in cotton, and when farmers planted a susceptible crop like soybeans, some had a major problem.”
There are three main nematode types that can impact soybeans: root-knot nematode, soybean cyst nematode and reniform nematode.
“The jury is still out on the economic impact of reniform nematodes,” Kirkpatrick says. “But we know soybean cyst and root-knot are problems; root-knot is by far the most significant in Arkansas, as well as parts of the Midwest.”
About the only way to identify which nematodes are present is to have soil from the field tested at a laboratory. Several samples may be needed per farm. “Testing cost is about $10 per sample, and samples should be taken to represent no larger than 40-acre portions of a field,” Kirkpatrick says.
Late summer and fall are the best times for sampling. That’s when nematode populations will be at their highest level. Areas with the most injury should be evident at this time.
“Sampling at the end of the growing season is in preparation for next year’s crop and not all fields will need to be sampled every year,” he says. “Sampling one out of every three years may be sufficient.”
University of Georgia Extension plant pathologists point out that root-knot larvae enter roots of host plants near root tips and remain at one location inside the root throughout their life.
As larvae feed, the root cells enlarge rapidly near the nematode’s head. This rapid cell enlargement causes the swelling or knots on roots that are characteristic of root-knot nematodes.
The female nematode undergoes three additional molts and changes shape to become a pear-shaped adult. The adult female may live two to three months or longer and lay 500 to 1,000 eggs.
Eggs may hatch if temperature and moisture are favorable. However, hatching is delayed by extremely dry soil conditions or low temperatures.
If temperatures are high enough and a large number of unhatched eggs are present, Georgia pathologists note that rain following prolonged dry soil conditions may cause many eggs to hatch at the same time.
Above-ground symptoms of root-knot injury include stunting, yellowing, excessive wilting, reduced yield and premature death of plants.
Below-ground symptoms are swollen, knotted roots. Georgia pathologists say the most severe damage usually occurs if the plants are infected as seedlings, but significant damage may also result from the infection of older plants.
Root-knot galls should not be confused with nitrogen-fixing nodules that develop on roots of legumes. These nodules are attached to the side of roots. Root-knot nematode galls are formed by enlarged root tissue, which will not flick off like nitrogen nodules.
Kirkpatrick says damage from nematodes is sometimes incorrectly diagnosed as a nutrient deficiency. “Although nutrients may be plentiful, root damage caused by nematodes may prevent absorption essential for plant growth and good economic yield,” he says.
Growers typically plant soybean Maturity Group 4 and 5 varieties in much of the Mid-South. “You need late 5s and 6s, which have more resistant varieties and are more appropriate for production in the Deep South and Southeast,” Kirkpatrick says.
Once farmers have identified the nematode species through testing at a nematode laboratory, they can establish a crop rotation program to help manage the pest.
“If root-knot nematodes are present, if possible, soybeans should not be rotated with corn,” Kirkpatrick says.
“Like cotton, corn is also a good host for root-knot, and you’re going to aggravate the problem and increase the population. Farmers are learning this — but they’re learning it the hard way.
“A rotation with sorghum helps. Sorghum is less susceptible to root-knot and can benefit soybeans both agronomically and through grassy weed control. Rice is also a good rotation, but less land is suitable for both rice and soybean production.
“If a field has soybean cyst nematodes or reniform nematodes, a corn-soybean rotation can be used.”
For more on nematode control, contact your regional Extension plant pathologist. Also, consult with your seed dealer for information on nematode-resistant varieties that are right for your growing area.