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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.
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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.
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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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