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Now is a Good Time to Look for SCN (07/09/20)

Soybean Cyst Nematode (SCN) is the most serious disease of soybeans in eastern North Dakota, and the first signs of SCN are now appearing in fields.

Soybean Cyst Nematode (SCN) is the most serious disease of soybeans in eastern North Dakota, and the first signs of SCN are now appearing in fields.

In the spring, juvenile nematodes (microscopic worms) emerge from eggs, and begin a new SCN life cycle.  The juvenile SCN females enter the roots of a host (soybean and dry edible bean), feed, and enlarge dramatically until they rupture the root. It typically takes four to six weeks before you can see the cysts. The cysts are white to cream colored, very small, but can be seen with an unaided eye or small magnifying glass. 

These photos were taken on July 6, 2020 near Casselton. The SCN cysts are relatively easy to see against black soil, and very small, white to cream colored, and appear lemon shaped with magnification (Figure 1). They are much smaller than the similarly colored nodule (Figure 2). Eventually these cysts will turn darker in color and be more difficult to see.  Each cyst contains 100-200 eggs, and the life cycle will likely repeat 1-2 more times this growing season.

There are two reasons you might consider taking a look for SCN in your fields right now.  First, if you do not know if you have SCN, you might be able to detect it now.  To be clear, fall soil sampling is a better way to detect SCN, but looking in your fields right now is fast and easy.  Secondly, if you are actively managing SCN in your field, you may be able to determine how well your management tools are working.  For example, if you planted a resistant variety and roots are profusely covered with cysts, the resistance is not as effective as it should be.

To scout for SCN, I recommend going to areas of a field most likely to be infected (or known to be infected). Because SCN moves with soil, this is likely the field entrance, low spots, along shelterbelts, etc. Also, an area with chronically low yields can be a good place to scout. Use a shovel to carefully dig up plants and gently remove soil from the roots; a bucket of water may help.  Examine the roots under good light and consider using a hand lens. 

SCN generally does not cause above-ground symptoms unless very significant yield loss is occurring, and when they do occur, they do not appear until later in the season (August-September).  Consequently, the yellowing soybean you may see now are more likely from other causes, and examining the roots is the best way to visually scout for SCN.

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Sam Markell

Extension Plant Pathologist, Broad-leaf Crops

 

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