Good news and bad news on soybean cyst nematodes
[APRIL 10, 2000]  One of the below-ground pests of soybeans and other plants is the soybean cyst nematode (SCN).  This creature is actually a small, parasitic roundworm that attacks the roots.  Most nematodes can only be seen with magnification, but the adult females and cysts of SCN are about 1/32 of an inch and are visible to the human eye.  This pest is important to Logan County producers because of potential yield losses of up to 10 bushels per acre, and the entry wound can allow plant diseases to enter the plant.  This could mean losses in the millions of dollars for Logan County.

Soybean cyst nematodes were first confirmed in our county in the mid-1980s in the Hartsburg area.  Since then, fields in virtually every part of Logan County have been analyzed and found to have populations of SCN.  In early stages of field infection, no visible signs on the plants occur.  As populations build in the soil, the above-ground plant parts may start showing damage that is often taken for nutrient deficiencies, compaction damage, herbicide injury or other plant diseases.
     The life cycle of the cyst nematode has three major stages:  egg, juvenile and adult.  The entire life cycle can be completed in 24 to 30 days under good conditions.  When spring temperature and moisture conditions are right, the eggs hatch into worm-shaped juveniles.  These juveniles then infect the soybean roots where they begin feeding.  As the females continue to feed, they become so large they burst through the root and are exposed to the soil.  The males simply migrate out of the root and fertilize the females.  The swollen females then start to produce eggs. The eggs remain in the dead female, which is then called a cyst.  Each cyst may contain up to 400 eggs, which will usually hatch the next year, but may survive in the cyst for several years.

 

 SCN is spread with lots of help.  On its own, these nematodes could move a few inches per year.  Anything that moves soil infested with nematodes will also spread them.  Farm machinery, people, birds, animals and water are the main ways nematodes are distributed. Water and machinery probably have the greatest impact.  Flowing water can actually carry the cysts to other field areas.

       

 

 

 

There is good news and bad news when it comes to SCN.  The bad news is that you can never eliminate the pest once it is in your field.  The good news is that the nematode can be managed to minimize damage.  The main form of management is the use of rotating crops, but other management tools such as proper fertility, sanitation and nematicides can play a role.  The proper place to start is with a predictive soil sample test.  Soil samples are simply analyzed for the number of cysts and eggs that they contain.  Knowing these levels, management recommendations can be made.

The traditional rotation recommendation for most Logan County fields is to plant a non-host crop such as corn the first year.  The next year a resistant variety of soybeans would be planted, followed by another non-host crop in year three.  In year four, a susceptible variety of soybeans would be planted to allow for maintenance of race 3 of the nematodes (the main race in Logan County).  Race 3 is a good type of SCN to have, since there are many soybean varieties that are resistant to Race 3.  It is also a dominant race.  If allowed to build up in population, it will crowd out other races that would not have resistant soybean varieties.  With extremely high numbers from soil tests, non-host crops may have to be planted two or more consecutive years.

 

 

 

Logan County test results have been running about 80 percent positive fields over the last three years.  It has almost reached the point of saying every field in the county should be rotated under the assumption that you need to.  There are also several gardens that have been found to have SCN (from the green beans, pole beans or other ornamentals growing there).  Rotating crops is always a good idea.

For more information on SCN or soil analysis, contact me at the Extension office at 732-8289.

 

[John Fulton]

 

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