By Emmanuel Byamukama, Connie Strunk and Connie Tande, SDSU Extension

Soybean harvest is nearing as most, if not all, soybeans have turned color or dropped leaves. Fall time is the best time of year to sample and test the soil for soybean cyst nematode (SCN), the number one silent yield robber of soybean.

Soybean cyst nematode is estimated to cause over $1 billion annually in the U.S. soybean crop. As of 2017, SCN has been detected in 30 counties in South Dakota. Some fields have been found to have very high SCN population densities (>60,000 eggs per 100 cc of soil) and therefore the yield loss caused by SCN in such fields is high.

The difficulty in managing SCN is the apparent lack of obvious above ground symptoms on soybean when soybean plants are infected with SCN. A plant can look “healthy” but upon examining the roots, hundreds of cysts can be seen (Figure 1). Yet, SCN can cause up to 30% yield loss without showing obvious above ground symptoms.

SCN Soil Testing

The best way to determine if a field is infested with SCN is to have the soil tested. To take a soil sample for SCN testing, use a soil probe or spade to obtain 15-20 soil cores 0-6 inches deep targeting the field entrances, low spot areas, consistently low yielding areas, and along the fence line. Soil underneath soybean rows (root zone) should be targeted.

Bigger fields should be divided into 15-20 acre parcels and each parcel sampled separately. Fields that are not currently in soybeans this year but will be planted into soybeans the next growing season can also be tested for SCN so that an appropriate SCN resistant cultivar can be selected. SCN soil testing is free of charge to South Dakota producers, thanks to a grant from the South Dakota Soybean Research and Promotion Council. Soil samples for SCN can be sent or dropped at:

Why is soil testing important?

There are two reasons to test soil for SCN; first, to determine whether a field is infested with SCN management practices can be implemented, and second, to determine if current management practices being used are reducing the SCN numbers or if a change in management tactics is needed.

For instance, increased SCN numbers in a field where resistant cultivars have been used may indicate that the SCN population has adapted to the source of resistance being used. This would therefore require the use of soybean cultivars with a different source of SCN resistance.

Our recent research funded by South Dakota Soybean Research and Promotion Council indicated that so far HG (Heterodera glycines) type 0 is the most common. This indicates that any SCN resistance source used can prevent this SCN population type from reproducing on such SCN resistant cultivars.

The same research also indicated a few HG type 2 SCN populations that can reproduce on PI #2 (PI88788). Although most SCN resistant cultivars have resistant genes derived from PI88788, a few cultivars that have resistance derived from Peking (PI #1) are becoming available. Growers are encouraged to rotate cultivars with different SCN resistance genes in order to delay development of SCN populations which can overcome deployed resistance genes.

Management

• Practice crop rotation in order to avoid SCN populations from build-up. For fields with high SCN population (>10,000 eggs/100cc of soil), longer rotations away from soybeans are encouraged.
   
• Plant SCN resistant cultivars. SCN resistant cultivars curtail SCN build-up in soil while also yielding well.
   
• Rotate within SCN resistant cultivars to avoid SCN populations from adapting to a single source of resistance.
   
• Control weeds proactively. Some weeds such as field pennycress and purple deadnettle are good hosts for SCN, therefore controlling these weeds in fall can help prevent the continued reproduction of SCN in the field when the soybean is not present.
   
• For fields with high SCN population, use of nematicide seed treatments may prevent yield loss. A few nematicide products are available.