Soybean plants in South Dakota are at or just past the R3 (beginning pod) growth stage. Now would be a good time to scout for soybean cyst nematode (SCN) on soybean roots as symptoms are often seen around this time of the year. Other symptoms that mimic SCN (e.g. mineral deficiency, herbicide injury, water logging) are usually minimal by this time. Soybean cyst nematode is the number one soybean production constraint in South Dakota and scouting to determine soybean field status is the first step for management. One of the challenges in managing SCN is the apparent lack of visual above ground symptoms. By the time symptoms are visible, up to 30% yield loss is already occurring in the field. It is therefore important to scout for SCN even when no symptoms are present. Soybean cyst nematode symptoms, if present, include stunted plant growth, yellowing plants, and soybean rows with uncovered and uneven canopy.
Scouting to determine SCN can be done by gently uprooting soybean plants and carefully examining the roots for female cysts (Figure 1). Use a shovel to dig-up soybean plants and then gently remove the soil around the roots being careful to avoid stripping the cysts off the roots. However, if the SCN population is low, cysts on roots may be few and hence not easily seen. Cysts on roots are very small - the size of a period at the end of a sentence. Their color ranges from white to yellow to brown, depending on the age of the cyst. The most reliable way to determine if SCN exists in the soil is to do a soil test.
Figure 1. (Above) Female soybean cyst nematode on a soybean root. Color of the cyst on the root will depend on the age of the cyst.
When sampling for SCN, attention should be paid to these areas: field entrance, along fence lines, low spots, previously flooded areas, waterfowl activity areas, high pH areas, and low yielding/stunted portions of the field. Collect 20 soil cores 6” deep from these areas in a zigzag pattern using a soil probe or a spade. The soil cores should be thoroughly mixed and put in a soil sample bag or a zip-top plastic bag. Larger fields should be divided into smaller portions between 10-20 acres and each portion sampled separately. Soil samples should not be collected when the soil is frozen or too wet. Soil samples should be kept at room temperature or in a cooler until shipped to the SDSU Plant Disease Diagnostic Clinic (Box 2108, SPSB 153, Plant Science Building, Brookings, SD 57007). There is no cost for SCN testing for South Dakota growers. The testing is sponsored by the South Dakota Soybean Research and Promotion Council.
Soybean cyst nematode is spread by anything that moves soil including tillage equipment, water erosion, wind erosion, shoes, and wildlife (deer, migratory birds). Once detected in a field, it is practically impossible for SCN to be eradicated, partly because cysts can survive in the soil up to 10 years and SCN is also highly prolific. This nematode completes its life cycle every 24-30 days hence completing 3 cycles in a single growing season. One cyst can have up to 300 eggs and in one season, can give rise to 27 million eggs! Therefore one cyst is the threshold to start managing SCN.
Once SCN is detected in the soil, an integrated approach should be employed to keep numbers from building up. The first approach is crop rotation to non-SCN hosts like corn, alfalfa, small grains, sunflowers, flax, and canola. For highly infested soils, longer rotations out of soybeans for several years may be necessary to bring the numbers down. The second approach to SCN management is to use resistant cultivars. Resistant cultivars play two roles: they are able to give high yield in SCN-infested soils, and they also prevent SCN numbers from increasing. The third approach is to rotate within soybean resistant cultivars. This ensures that SCN populations that can overcome a given resistance do not develop because of growing the same cultivar over and over. Other practices that promote plant health like maintaining optimum fertility, weed control (especially SCN weed hosts like pennycress and henbit), and proper drainage may increase soybean yield and limit damage by SCN. Several seed treatment products for managing SCN are available on the market but their effectiveness is still being evaluated.