Nematodes are common pests in many soybean fields in the state. Sometimes called the “hidden enemy,” root-knot, soybean cyst, and reniform nematodes can be costly if they are ignored. Because they are not visible, damage caused by nematodes is often overlooked.
Nematode are microscopic root-parasites, and the symptoms they cause, including plant stunting, yellowing of the foliage, and low yield (Figure 1) are a direct result of the damage they cause to the soybean root. Right now in Arkansas, root-knot nematodes pose the greatest threat to our soybean crop. As the name implies, infection results in severe damage to soybean root systems because the nematodes cause galls to form in response to their feeding (Figure 2).
When roots are severely galled, water and nutrients are not absorbed or translocated in the plant efficiently. Soybean cyst and reniform nematodes can also become severe enough to cause yield loss, but they are not quite as harsh or as widespread in the state as root-knot.
Both cotton and corn are also good hosts for root-knot, so fields that were in either corn, cotton, or soybean last year could have a significant problem this year. Some of the most severe root-knot problem fields are fields that had a long history of cotton production and have recently been converted to soybeans. Soybean is the only major row crop that is a host for soybean cyst nematodes, so fields that were in any other crop last year are likely low risk for a soybean cyst nematode problem this year.
We have only limited information on the importance of reniform nematodes as a pest of soybean, but we know that the nematode will infect the crop. Corn is not a host for reniform nematodes, so fields that were in corn last year are also “low risk” fields from a reniform nematode perspective. Fields that were in either cotton or soybean last year, however, may have a significant reniform nematode population.
Managing nematodes begins with an awareness of where they are on the farm and which nematodes are present. Since nematodes are not very mobile on their own, they tend to be field-specific in that certain fields may have a significant issue and other fields adjacent or nearby may not.
Grower-experience is an excellent way to identify fields with nematode problems, particularly with root-knot. If severe galling was present on last year’s crop in the field, then the risk of a nematode problem this season is very high. For those who did not check roots last fall, soil sampling is the only sure way to detect and quantify nematodes in a field. Unfortunately, spring samples give a much less accurate picture of the root-knot nematode status of a field — summer or fall samples are the most accurate. Samples will, however, be useful if you suspect soybean cyst or reniform nematodes may be present.
The most effective and cost-effective way to manage any nematode problem is with resistant cultivars — where they exist. Unfortunately, there are very few soybean cultivars in the MG IV – early MG V maturity range with high levels of resistance to any of the major nematodes.
An excellent source of information on resistant cultivars is the 2014 Soybean Update, available through local county offices and in the Publications section of Arkansas Crops. Additionally, the University of Arkansas Soybean Cultivar Disease Screening Program, sponsored by the Arkansas Soybean Promotion Board and the Division of Agriculture evaluates all cultivars that are entered into the Official Soybean Variety Trials (OVT) each year for resistance to nematodes. Nematode resistance ratings for all cultivars entered into the 2013 can be found at http://arkansasvarietytesting.com/home/soybean/.
Although only marginally effective under severe nematode pressure, seed treatments that include a nematicidal component can help in nematode management under less severe pressure or in combination with resistant or moderately resistant cultivars.