Have our SCN sorrows been drowned?

Those looking for any bit of good news among all the rain-soaked suffering we have endured this growing season have asked if the overabundance of moisture has drowned SCN. Unfortunately, the answer is no.

Nematodes are worms (animals) that require oxygen. And they absorb oxygen through their body wall or cuticle, which is made almost exclusively of proteins. Waterlogged soils may have greatly reduced levels of oxygen, but many plant-parasitic nematodes, including the soybean cyst nematode (SCN), can survive long periods of time with little oxygen.

In the early 1970s, scientists in Arkansas conducted experiments to determine whether SCN could survive in flooded conditions. They found that hatched SCN juveniles (see figure below) survived in water up to 630 days (probably longer, but the experiment ended after 630 days)! They also tested survival of SCN in flooded soils, and juveniles survived 7 to 19 months depending on soil texture. The research paper is available online here.

SCN eggs can survive in a dormant state for many years in the absence of soybeans, particularly the eggs that occur within the body of the dead female or cyst (see figure). Typically, the eggs are more tolerant of environmental stresses than hatched juveniles. So it is likely that SCN eggs in infested fields are not adversely affected by waterlogged soils either.

More bad news...

A bit of additional bad news is that soil moved by erosion due to heavy rains and flood waters may spread SCN to new places. It is not possible to quantify the magnitude or frequency of this happening. Considering how widespread SCN already is in Iowa and the Midwest, perhaps the movement of SCN in soil moved by rainfall and flood waters will not have a great impact. Nonetheless, it is quite possible that some fields may have had SCN introduced in soil from other fields through flooding. Consequently, soil samples should be collected this fall to test for SCN in fields where soybeans will be grown in 2025. Guidelines for collecting SCN soil samples can be found online.

A possible silver lining to the storm clouds?

Multiple SCN generations occur (likely four or more) throughout a normal growing season in Iowa. And it takes about 30 days for SCN to complete a single generation once soils warm up in late spring and summer. If soybean planting is delayed by several weeks, as in 2024, there likely will be one or two fewer generations of SCN occurring during the season. And that means less of an increase in SCN numbers simply because there are fewer weeks (less time) for SCN to reproduce on soybeans in 2024.

But beware! The potential for large increases in numbers and for severe damage always exists with SCN, especially if the weather turns hot and dry–ideal conditions for SCN reproduction. The numbers of SCN eggs in soil can build up quickly over multiple generations. A few hundred eggs can increase to nearly 40,000 in just three generations, as shown in the infographic from the SCN Coalition below.

Manage SCN for the long term

Successful, long-term management of SCN requires an active, integrated approach of growing nonhost crops such as corn in rotation with SCN-resistant soybean varieties. Farmers should seek out and grow soybean varieties with the Peking and PI 88788 sources of resistance to grow in alternating soybean years. And nematode-protectant seed treatments are available to bolster the performance of SCN-resistant soybean varieties. For more information about the biology and management of SCN, visit soybeancyst.info, soybeanresearchinfo.com and thescncoalition.com.