Sample for nematodes to protect yield potential

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Three species of nematodes–soybean cyst nematode (SCN), southern root-knot nematode (RKN), and reniform nematode (RN)–can cause significant yield losses in soybeans grown on infested fields in the Midsouth.

Even though the title of this article indicates that sampling for nematodes is the primary topic, there are some basic points about these three nematode species that should be considered, especially with the changes in cropping systems in the Midsouth.

  • SCN will infect soybeans, but corn, cotton, grain sorghum, rice, and wheat are non-host or poor host crops.  Therefore, rotation of soybeans with these crops is an effective management tool for SCN.

Varietal resistance to SCN, along with crop rotation, is the best defense against SCN.

A variety with resistance to a specific population or race of SCN should not be planted year after year because SCN adapts to varieties.

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

Soil texture affects movement of SCN in the soil and also may affect its reproduction and development.  Basically, major damage to soybeans by SCN infestation occurs when the crop is grown on medium- and coarse-textured soils.

  • RKN will infect not only soybeans, but also corn, cotton, and wheat.  Thus, rotation of soybeans with these crops is not a management option for this nematode.  Rotation of soybeans with flood-irrigated rice or grain sorghum will lower RKN numbers dramatically.

Varietal resistance is the most effective tool for management of RKN.  However, there are few current varieties that are resistant.  Resistance to RKN is more prevalent in MG 6 through 8 varieties than in MG 5 and earlier varieties.

RKN tends to be associated with sandier soils on sites that have previously been devoted to cotton production in the Midsouth.

  • RN will infect soybeans, but this nematode has not been a major threat to Midsouth soybean production.  However, in fields that are infested with RN, rotation of soybeans with corn, rice, grain sorghum, or wheat, which are poor hosts for RN, is an effective management tactic.  Rotation of soybeans with cotton, which is an excellent host for RN, should not be done on infested fields.

Varietal resistance to RN is an effective management tool, but there are few resistant varieties.

The detrimental effect of plant parasitic nematodes on soybeans is well documented in an article on this website.  Important points from that article are:

  • To assess potential damage from nematodes in soybean fields, growers must get a determination of which nematode or nematodes are present.
  • Accurate identification of the nematode species and population levels present in a field requires that soil samples be properly collected and sent to a diagnostic lab for evaluation (see below links).
  • The ideal time to sample for nematodes in soil is in the fall, either shortly before or soon after harvest when nematode numbers are highest.
  • Nematicides applied to seed or used in-furrow can reduce early-season root infection by nematodes, but do not provide season-long control and may not be economical.
  • Nematicides will not replace the use of resistant varieties and variety/crop rotation as primary nematode control practices.

The change in cropping systems in Mississippi in recent years has led to increased concerns about nematode infestations of soybeans.  The effect of these changes are:

  • Increased acreage of corn that may be rotated with soybeans has led to heightened concern about soybeans being infested with RKN.
  • Growing soybeans on sites once devoted to cotton has led to heightened concern about soybeans being infested with RN.

Because of cropping system changes, the need to sample for nematodes has become even more important because of the added risk of infestations from RKN and RN as well as SCN.

  • Why sample?  Properly collected and evaluated soil samples are the best tool for detecting the presence and species of nematodes in the soil.

Properly analyzed samples will indicate where control practices are not needed, and conversely will indicate where control practices are needed to protect yield potential.

The following management tenets should be followed depending on test results.

  • If test results indicate that the above nematode species are not present in a field, care should be taken to prevent their introduction since nematodes can be moved from field to field by soil that is transported on field equipment.
  • If test results indicate the presence of nematodes, the management goal is to keep the nematode population as low as possible since they are very difficult to eliminate.  This involves using management practices presented in the above-cited article for each nematode species.
  • Crop production practices that provide adequate nutrients and water and minimize stress due to insects, weeds, and diseases will enhance soybeans’ ability to withstand some nematode feeding damage, but will not prevent yield loss where infestations are severe.

Below is the important take-home message from the above discussion.

Sampling for nematodes should be considered as important as sampling for soil fertility.  This is especially true if there is no history of nematode sampling on either old or new soybean production sites.  Once documentation of the absence or presence of nematodes is established for given fields, then management options outlined for either case in the above-cited article can be adopted.

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