Sustainable Soybean Production—Part IV

Soybean producers in the Midsouth must contend with nematode pests that include soybean cyst nematode (SCN), southern root-knot nematode (RKN), and reniform nematode (RN).

A recent summary of nematode management in Midsouthern soybeans was recently posted on this website.

What We Know

 

• Accurate identification of the nematode species and population levels present in a field requires that soil samples be collected and sent to a diagnostic lab for evaluation. The ideal time to sample is in the fall, either shortly before or soon after harvest when nematode numbers are highest.


• SCN is the most damaging pest to soybeans in the US.


• RN has not been a major threat to growing soybeans in the Midsouth.


• Using resistant varieties is the best tactic to prevent yield-reducing damage from all three nematode species.


 

The number of current varieties that are resistant to RKN colonization is low. Using varieties that are only moderately resistant will allow RKN populations to be maintained or increased.

Resistance to RKN is more prevalent in Maturity Group (MG) 6 through MG 8 varieties than in MG 5 and earlier varieties.

Breakdown of resistance to RN in soybean has not been reported.

• Major damage to soybean by SCN infestation occurs when the crop is grown on medium- and coarse-textured soils. RKN tends to be associated with sandy soils.


• Crop rotation, an effective tool for managing all three nematode species when growing soybeans, should be considered along the following lines.


 

SCN: Growing nonhost crops such as corn, cotton, and grain sorghum successfully reduces SCN populations.

RKN: Growing soybeans in a rotation with rice that is flood-irrigated or grain sorghum will lower RKN numbers dramatically. The common rotational crops of corn, cotton, and wheat all serve as hosts for RKN, so growing soybeans in rotation with these crops is not a control measure.

RN: Growing soybeans in a biennial rotation with rice, grain sorghum, or corn, which are poor hosts for RN, is an effective management tactic. Rotating soybeans with cotton, which is an excellent host for RN, should not be done on infested fields.

• Continuous planting of a soybean variety with resistance to a specific population of a race of SCN could lead to the development of a different SCN race that damages the crop, making that variety useless for SCN control.


• It is important to determine the race of SCN in a field and the race-specificity of the resistance gene of a previously planted soybean variety when planning to use a new resistant variety for SCN management.


• Resistant varieties are more reliable and cost-effective than nematicides for managing/reducing nematode populations.


• Nematicides applied to seed or used in-furrow can reduce early-season root infection by nematodes, but do not provide season-long control.


• Nematicides will not replace the use of resistant varieties and variety/crop rotation as primary nematode control practices.


• New nematicide products (Votivo, Poncho/Vitovo, Activa Complete Beans) are available, but little is known about their effectiveness in situations with known populations of nematodes. Therefore, at this time there is no supposition that they will replace the accepted practices for nematode control and/or management.


 

 

What We Need to Know/Have for Sustainability

 

• Research to determine the damage potential of nematodes and to establish action thresholds for injury or control when growing soybeans is lacking in much of the southern US. Damage thresholds for recent “newcomers” such as RN to the soybean arena have not been well established.


• Widespread use of MG 4 and earlier varieties in the Midsouth has become common. Therefore, early-maturing varieties with RKN resistance are needed for use in Midsouth rotation systems that may include corn, cotton, and wheat on coarse-textured soils that have threshold levels of this nematode.


• SCN resistance in the majority of varieties is derived pirmarily from one genetic source of resistance. This has led to the adaptation (race shift) of some SCN populations. To sustain soybean production on SCN-infested soils, more genes for resistance need to be incorporated into the public germplasm pool so that resistant varieties continue to become available for use as the most effective management tool against future SCN populations.


• Research is needed in soil environments with known levels of nematode infestations to determine the effectiveness of and economic return to new nematicides available for use on soybeans.


 

larryheatherly@bellsouth.net