Soybean diseases such as soybean rust and sudden death syndrome are getting the press, but soybean cyst nematode continues to be the dominating yield robber in Minnesota soybean fields.
SCN is a small, plant-parasitic roundworm that attacks the roots of soybeans. Most nematodes can be observed only with magnification, but the adult females and cysts of SCN are about 1/32-inch long and visible to the unaided eye.
The best way to monitor SCN field populations is through a good soil sample. After the crop is off and before field work is an ideal time to sample fields slated for soybeans in the upcoming growing season.
Here are a few key points about soil sampling for SCN:
-A soil probe is the ideal sampling tool, although you can use a trowel
-Limit the area sampled to about 20 acres
-Samples should consist of about 20 soil cores, taken to a depth of 6 to 8 inches, in a zigzag pattern throughout the field
-Combine and mix samples thoroughly, placing one pint of soil into a soil sample or plastic bag
-Properly label the sample bag
-If the sample cannot be sent immediately, store it in a refrigerator
-Don't expose samples to direct sunlight for an extended time
The importance of a representative sample can't be overemphasized when detecting and tracking SCN field levels. A sampling demonstration conducted in 2006 by the University of Minnesota Extension Service at four locations - Slayton, Windom, Kasson and Sibley County - illustrates this point.
In this demonstration, a composite sample of 20 soil cores to a depth of 6 to 8 inches was taken with a soil probe at each location. The composite sample was mixed well, then split and sent in for analysis (except for the Sibley County site where the composite sample was not split). For comparison, shallow, single-point samples were taken at a specific location in each respective field.
The widest variability between sampling methods was found at the Slayton site. Here the composite sample indicated an average population of 14,150 eggs per 100 cubic centimeters of soil, while single-point samples taken from different spots in the field resulted in populations of 1,125 and 34,450 eggs per 100 cubic centimeters of soil.
The University of Minnesota recommends planting a non-host crop when SCN levels are greater than 10,000 eggs per 100 cubic centimeters of soil. In our example, management recommendations would have differed greatly depending on which sample was used. This illustrates the importance of a good, representative soil sample.
Variety selection and crop rotation are key elements in managing SCN, but a good soil sample is where it all starts. Check out www.soybeans.umn.edu for the latest U of M SCN variety trial results.
Lizabeth Stahl and Ryan Miller are crops educators with the University of Minnesota Extension Service.