Monthly Archives: September 2012

It’s Time to Sample for Nematodes

Nematodes are unsegmented roundworms, some of which feed in or on roots of plants.  More than 100 species of plant-parasitic nematodes feed on soybean roots, but only a few are economically important.  In Virginia, most nematode species are found in the sandier Coastal Plain soils.  However, some nematode species can also develop and reproduce on the heavier-textured soils of the Piedmont and Shenandoah Valley.  This guide will focus on those that can cause damage to soybean in Virginia.

Many soybean growers do not realize that nematodes may be reducing yields by 7-8%.  Therefore management of these pests begins with sampling and determination of the species and number present.  If nematodes are determined to be a threat, certain management practices are available to help prevent further spread and reduce the economic losses that they cause.

During 2007-2010, over 1000 soil samples were taken and analyzed for nematodes in problematic corn and soybean fields in eastern Virginia.  These fields were not chosen at random, but were selected because of low productivity or were showing symptoms that were typical of nematode damage.  Of the “problem” soybean fields, 98% contained nematodes and 71% of the fields were at moderate to high risk of nematodes causing a significant yield loss.

Sampling and Thresholds.  Virginia Tech’s Nematode Advisory Program depends on the cooperation of the agricultural community, Extension Agents, and the Nematode Assay Laboratory. Proper sampling, completion of appropriate forms, and careful laboratory analysis are all necessary to provide the grower with appropriate recommendations on nematode management. The Nematode Advisory Program can help growers avoid costly yield loss due to plant-parasitic nematodes if the steps outlined below are followed.

The Virginia Tech Nematode Assay Laboratory currently performs assays for two different purposes:

  1. Predictive: The predictive assay determines if nematode populations at harvest are likely to affect next year’s crop.  There is a fee for predictive samples.  Routine assays are $11 per sample and routine plus cysts are $19 per sample.
  2. Diagnostic: The diagnostic assay determines if poor growth in the current year’s crop is caused by nematodes.  There is no fee for diagnostic samples.

When to Sample.  The most appropriate time to sample depends on the crop and the purpose of the sample.

Predictive Assays: Fall sampling provides the most reliable information for predicting nematode problems for a future crop. Nematode populations are highest at the end of the growing season and decline as the soil temperatures drop.  Sample at or immediately after harvest of previous crop, September 15 to November 15.

Diagnostic Assays: Sample at the onset of symptoms, during the growing season. Nematodes feed only on living plants; therefore, sample soil around live plants showing symptoms. Some nematodes spend part of their life cycle inside the roots and more accurate diagnosis of nematode damage can be made from samples including roots.  Also, send another sample from a healthy plant to compare population densities.

How to Sample.  Always sample within the feeder-root zone; this varies for each crop. Avoid collecting samples when the soil is extremely dry or extremely wet. DO NOT add water to the soil after sampling. Sample areas of common crop history. For example, if one half of the field is planted to corn and the other half to soybean, sample each area separately.

  1. Collect vertical core subsamples of soil with a soil sampling core or shovel within the feeder-root zone (see figure at right). A 6″ depth should be adequate. Nematodes do not occur uniformly throughout a field; thus, more than one subsample must be taken from the same field. The number of subsamples needed depends on the size of the field:
    1. For small fields (less than four acres), collect at least 20 subsamples.
    2. For large fields (more than four acres), divide the field into four-acre sections. If the field consists of several soil types, divide the field into as many sections as there are soil types. Collect at least 20 subsamples from each section.
  2. Mix the subsamples in a clean bucket.
  3. Place at least one pint (500 cc) of the soil mixture into a nematode soil sample bag or plastic bag. LABEL COMPLETELY with the grower’s name, address, county, agent, crop information, and field or sample number.
  4. Complete the appropriate form to send with the samples. The forms are available at Virginia Cooperative Extension offices at no charge.  Soil sample bags may be available at these offices, as well, however quart-size, sealable, plastic bags are also suitable.
  5. Store samples in a cooler or refrigerator until shipping.  It is best to ship samples on Monday or Tuesday to avoid them sitting in a hot mailroom or truck.
  6. Mail samples with the appropriate form, and a check for predictive assays, immediately to the Nematode Assay Laboratory, 115 Price Hall , Virginia Tech, Blacksburg, VA 24061-0331

Interpreting Predictive Assays.  Predictive nematode sampling use nematode risk thresholds to determine whether to take action against nematodes.  These thresholds are based on results of on-farm tests over several locations and years.  The table below lists three levels of risk for yield loss according to population densities in a 500 cc sample of soil.  Risk thresholds apply to soil samples collect in late summer or early fall.  Soil samples collected during winter or spring always contain reduced levels of nematodes due to unfavorable temperatures and the absence of a host crop.  Note that if more than one nematode is present at the borderline level, the likelihood of a profitable response to a control measure increases.

Nematode Risk Thresholds for Soybean (per 500 cm3 soil)

Risk Level





Soybean Cyst larvae



















Root Knot











Stubby Root







Recommendation Codes:Low = nematodes are no likely to cause crop damageModerate = borderline populations in which crop damage may occur if other factors stress the cropHigh = populations are likely to cause crop damage and significant yield loss

For more information on soybean nematodes and their management, see the VCE publication AREC-9, Soybean Nematode Management Guide.  You can access the guide on the web at or obtain a hard copy at your County Extension office.


Now is a Good Time to Evaluate Your Varieties for Foliar Diseases

September is a great time to evaluate your crop and the performance of varieties that you chose.  In addition to general growth and health of the crop, take some time to determine if you have any of the below diseases.  If so, you could be losing some yield.  If you sprayed with a fungicide and still have disease, reconsider the product and rate used and the time that the fungicide was applied.  Keep in mind the weather conditions when the application was made and the conditions 2 to 3 weeks after or before the product was applied.  Cool temperatures (70’s) and high relative humidity (>95% for 12 hours or more) will usually increase disease incidence.

Another caution is to never diagnose a specific disease on the plant without verifying it with a person trained to identify plant pathogens.  Only when the reproductive structures are found on the leaf can a disease be confirmed.  Many things will cause look-alike symptoms.  Be sure before you cast the blame.  There are more diseases than just the ones shown below, but these are the most common.  Brown spot is normally found in the lower part of the crop canopy (the lower leaves), Cercospora blight and leaf spot will be found throughout the canopy, and the frogeye leaf spot and downy mildew tend to be found in the upper part of the canopy.


Corn earworm AVT results, BMSB and Kudzu bug update….nearing the end (Dr. Ames Herbert)

Our final batch of corn earworm moths showed only 26% survivorship, down from last week. This season results showed a 37% survival rate for the seasonal total which exceeds all previous years. We are now up to 33 counties where brown marmorated stink bugs were/are present in soybean fields. Most are at pretty low levels compared with last year, but they are much more widespread. This week n some have been found in soybean fields in north central North Carolina. We are taking as much data as we can to help with answers for next season. And, the first kudzu bug nymphs were found in Charlotte County. We are nearing the end of this season in terms of insect pests and advisories…a long summer for sure.

Brown Marmorated Stink Bugs Infesting Soybean Fields – Not Playing By the Rules (Ames Herbert, Extension Entomologist)

With our full complement of field scouts in place, more soybean fields are being found with brown marmorated stink bug (BMSB) infestations. We are up to 20 counties in Virginia spread over a very large area of the state. Infestations were a little slow to develop compared with last year but we are predicting that by the end of the season, many more fields will be infested compared with last year. So far, most infestations are at low to moderate levels (from 2-3 to 4-6 per 15 sweeps) but a few are in the high infestation range 8-10 or more/15 sweeps. We know that sweep netting is not the ideal way to sample for these insects, but there is no other method other than simply easing into the field edge and counting what you see. Basically, if you can stand in one spot, do a 360 degree turn and count more than 5 or 6, that area needs protection. So, how are they not playing by the rules?  Last season, our first with treatable levels, bugs stayed on field edges and a single insecticide edge treatment provided season long control. This is true for many fields this year but in a few we are finding them deeper into fields, and, there are some cases where re-infestations are occurring in fields previously treated. The good news is that 1) based on our insecticide trials it is not difficult to kill them with several common products, and 2) many fields, especially the full season crop fields, are rapidly approaching the safe stage which based on previous field cage studies happens after R6 (full seed) when pod walls begin to toughen. Many growers are treating field edges this year and we are going to follow as many as possible to determine if those treatments worked and held. More on this as we progress.

This report was taken directed from Dr. Herbert’s Weekly Virginia AG Pest Advisory.  To access this advisory, go to


Soybean Crop is Still at Risk to Drought & Pests

Our soybean crop is moving right along.  Much of our May-planted crop has hit or it approaching the R6 stage.  Our double-cropped soybeans are in the R4-R5 stage depending on their maturity, planting date, and whether or not they experienced early-season drought.  There’s a tendency for us to believe that the crop is made after the seed have met in the pod.  However, as shown below, only 50% of our yield has been made at the R6 stage. 

Dry weight will continue to accumulate in the seed for the next 3 weeks until the crop reaches the R7 stage (physiological maturity), which is defined as one pod on the plant that has reached its final mature color.  Much of this seed weight is due to translocation from other plant parts, especially the leaves and petioles (remobilization).  Sometimes soybean is called a self-destructing plant.  This is largely true.  As photosynthate is moved from the leaves to the seed, the leaves will turn yellow and drop from the plant.  If the leaves are yellowing and dropping due to this natural process, then we shouldn’t be concerned.  However, if the leaves are dropping due to another reason such as dry weather or disease, we should take notice.  It is important to keep the leaves green as long as possible to maximize yield.

It takes about 2 weeks to move from the R5 to R6 stage and another 3 weeks for to move from R6 to R7.  This is a long time and much can happen during this period.  First, we must protect the developing seed from insects such as corn earworm and stink bugs.  After R6, these two pests are not as much of a concern, but defoliators such as soybean looper can still remove green leaf area, which I’ve already stressed, is vital for maximum yield.  We also need to be concerned about foliar disease, which can also rob us of green leaf area