As Louisiana farmers shift acreage from cotton to soybeans and other crops, nematodes are likely to become an increasing threat.

Reniform, Southern root-knot and soybean cyst nematodes (SCN) are the most important nematodes on soybeans in Louisiana. These pests can cause serious soybean damage, including stunting, yellowing, early death and low yields. Variety selection and crop rotation are your best tools to combat these damaging plant parasites.

The reniform nematode is so widespread throughout Louisiana that substantial problems are likely to be encountered from it as soybean acreage increases. The pest favors sandy to silt loam soils, but symptoms of reniform nematode in soybeans are not always clear. Although you may get stunting, the most common signs are some yellowing and loss of yield. This nematode causes much greater damage during drought stress; in years with adequate moisture, you may not see much damage.

Unfortunately, little emphasis has been placed on breeding for resistance against reniform nematode. Deltapine DP 5634RR is the only resistant variety recommended for Louisiana.

The Southern root-knot nematode has a more limited range than reniform, and is confined mainly to sandy soils. But where it occurs, it can cause devastating losses, especially during droughts. Look for seriously stunted plants early on, or early death during bloom and pod fill. The galls are a distinctive feature of this nematode and make it easy to recognize.

There are only a few soybean varieties with resistance against Southern root-knot nematode. Cotton, corn, sugarcane and sweet potatoes are also excellent hosts for this pest, complicating management. If you are rotating this year behind one of these crops and the soil type is sandy — or if Southern root-knot nematode has been identified previously — be sure to plant a resistant soybean variety.

Nationwide, SCN is probably the most important nematode on soybeans. But in Louisiana this pest has declined over the past 20 years and is now relatively scarce. Still, it does have serious potential for damage and should be monitored for outbreaks or resurgence.

Problems with SCN can occur when soybeans are grown continuously and when varieties with the same type of resistance are planted. Rotating soybeans with corn, cotton, milo, sugarcane or rice has been very effective in breaking up the life cycle of SCN. And most of the soybean varieties that are recommended for Louisiana have some resistance against this nematode, which has several races.

Although SCN causes only minor problems in Louisiana, growers should be on the lookout for problem areas in the future. Because many of the current varieties seem to show only slight foliar symptoms, watch for yield reductions or areas not performing like they should.

Careful variety selection is your best defense against nematodes in soybeans. Unfortunately, most soybean varieties don't have multiple resistances against several nematode species. So it's important to know what kind of nematodes you have. Fall is the best time to collect nematode samples. Avoid sampling for root-knot nematodes in the spring.

Charles Overstreet is an authority on the management of plant-parasitic nematodes on many different crops, including soybeans and cotton.