Young soybeans with standing water

Editor’s note: The following was written by Greg Tylka, Iowa State University professor in the Department of Plant Pathology and Microbiology, for the university’s Integrated Crop Management News website June 23.

Those looking for any bit of good news in all of the rain-soaked suffering we have endured this spring have asked if the extreme overabundance of moisture has drowned soybean cyst nematode (SCN). Unfortunately, the answer is no.

SCN soybean cyst nematodes

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 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 SCN juveniles 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 the juveniles survived 7 to 19 months depending on soil texture.

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. Typically, eggs are more tolerant of environmental stresses than hatched juveniles.

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 movement. And considering how widespread SCN already is in Iowa and the Midwest, perhaps the spread 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 this spring. Consequently, soil samples should be collected this fall to test for SCN in fields where soybeans will be grown in 2020.

A possible silver lining to the storm clouds?

Multiple SCN generations occur (likely four to six 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 2019, there likely will be one or two fewer generations of SCN during the season. This means less of an increase in SCN numbers simply because there are fewer weeks for SCN to reproduce on soybeans in 2019.

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.

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