Soybean producers across Iowa will benefit from fall soil sampling for soybean cyst nematode (SCN) eggs. The nematodes, which are microscopic worms about 1/100 inch long, feed on soybean roots and significantly reduce yields--often with little or no aboveground symptoms showing.

Although soybeans are essentially harvested across Iowa, now is an ideal time to sample fields for nematode eggs, which allows producers planning time over the winter to manage problem fields. During midsummer, females and cysts--the swollen bodies of dead female nematodes--can be seen as tiny, white or lemon-yellow structures attached to soybean roots. These cysts are filled with eggs, some of which can lie dormant for nearly a decade before hatching.

Females and cysts of SCN are almost never found on roots pulled in the fall because (1) pulling the plants instead of digging them will cause most of the females and cysts to become dislodged from the roots, and (2) most of the females and cysts are dark brown in the fall and are very difficult to see (plus, they are naturally beginning to fall off of the roots).

A good overview on how to approach mid- to late-fall sampling for SCN resulted from several questions asked recently by a Harrison County soybean farmer.

Is it too late to sample now? It is definitely NOT TOO LATE to collect soil samples for SCN analysis. Samples can be taken until the time that the upper 8 inches of soil is frozen solid.

Should producers ask for egg counts or a complete count? You would want to have a SCN egg count analysis done. The complete nematode count would give an idea of the number of hatched SCN juveniles in the soil, but these juveniles likely will not survive the winter. It is the eggs that survive long-term.

Incidentally, when dealing with SCN, you always want a SCN egg count. The complete nematode count is for diagnosing problems on corn, alfalfa, or garden crops. ISU Extension will change the name of the complete test in the future to clear up some of the confusion.

Can a sample represent more that 10 acres? Yes, but the larger the area a sample represents, the more inaccurate the sample results become. Ideally, growers limit their sampling to 20 acres or so, but many growers may sample larger areas because they do not have the time and funds to sample every 10 acres.

There are two other strategies that a grower might take concerning sampling to determine if their fields are infested with SCN:

Collect soil cores from "high-risk" areas of fields where SCN might have been introduced. Such areas include near field entry points (where equipment may have introduced soil from another field), along fence lines (where wind-blown soil accumulates), in low spots of the field (where surface water accumulates), or in areas of the field where unthrifty soybean growth has been observed in the past. This concept of a "targeted" collection of soil cores is diagrammed in ISU extension publication IPM-47-s, Scouting for Soybean Cyst Nematode.

If soil samples are already being collected for soil fertility analyses, simply pull off a subsample of soil and send it to the ISU Plant Disease Clinic (515-294-0581) or some other private testing laboratory that does SCN soil analyses. Many farmers are surprised at what they find after they actually begin testing for SCN. The ISU fee for SCN soil analysis is $15 per sample, and using subsamples of soil fertility samples is ideal. For more information, contact your county extension office.

If producers don't get samples taken now, when would be the next best time? Soil samples can be collected again in the spring, once the soil has thawed and drained properly. The results of soil samples collected in the spring probably are a bit more accurate than those from fall soil samples, but the grower has much less time to decide how to manage the problem if a SCN infestation is discovered.

My soybean yields have been good, and I have not seen areas in fields that are yellow and stunted, a classical symptom of SCN infestations. So why should I sample? Is it true that as many as 70 percent of the soybean fields in Iowa have SCN? That is correct. Nearly 70 percent of 400 randomly selected Iowa fields tested in 1995 and 1996 had SCN in them at some density. It would be wise for soybean producers statewide to test now for SCN because by the time that sick-looking beans appear, SCN has been in the field for 5-10 years and egg numbers will be very high. Early detection before the appearance of any sick-looking plants is the key to effective SCN management.