Integrated Crop Management

Wet-weather issues relating to SCN

In the June 29 issue of the ICM newsletter, XB Yang discussed the effects of the wet 1998 growing season on fungal soybean diseases [1]. Growers and agribusiness also should be aware of two issues concerning the soybean cyst nematode (SCN) and the wet weather that we have experienced so far this growing season. First, SCN is moved by anything that moves soil. Consequently, any field or area of a field that has flooded may have had the nematode introduced into the field. Random survey results from 1995-1996 indicate that approximately two-thirds of the corn-soybean fields in Iowa are infested with SCN. Consequently, many of the fields that recently have flooded probably were infested with the pest, and the pest probably has been spread with soil moved by floodwater. Soil samples should be collected from flooded fields and submitted to one of several private laboratories throughout the state that offer nematode testing or to the ISU Plant Disease Clinic [2] (515-294-0581) for extraction and counting of SCN eggs. Samples sent to the ISU Plant Disease Clinic should be accompanied by a completed Plant Nematode Sample Submission Form (ISU publication PD 32 [3]), and there is currently a $15 per sample charge for this analysis. Detailed instructions on how to collect a representative soil sample for detection of SCN can be found on the back of the Plant Nematode Sample Submission Form.

The second issue concerning the wet growing season and SCN is that many Iowa growers who have not finished planting yet are faced with the prospect of planting soybeans in fields originally slated for corn production in 1998. Soybeans were grown in most of these fields in 1997, and growers may be tempted to grow soybeans in these fields again in 1999 to get the fields back into a corn-soybean rotation. Unfortunately, two or three successive years of soybean production can lead to the rapid buildup of high population densities of SCN. Consequently, growers who are faced with planting soybeans in 1998 in fields that had soybeans grown in 1997 should consider having the fields tested for SCN. And if an SCN infestation is discovered, the grower should plant a short-season, SCN-resistant soybean variety if possible. Also, if soybeans are grown for a second successive year in a field that is discovered to be infested with SCN in 1998, corn must be grown in the field in 1999 to avoid further increase in the SCN population densities.

The biology, life cycle, and recommended management of SCN are described in ISU publication Pm 879 [4], Soybean Cyst Nematode. ISU publication Pm 1649 [5], Disease-resistant Soybean Varieties for Iowa, lists soybean varieties with resistance or tolerance to four major Iowa soybean diseases, including SCN. Single copies of these publications are available free of charge from county extension offices or from the Extension Distribution Center [6] (515-294-5247).

This article originally appeared on page 134 of the IC-480(17) -- July 6, 1998 issue.

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