Soybean Growers
Encouraged to Test for Soybean Cyst Nematode
Mike Rankin
Crops and Soils Agent - Fond du Lac County
University of Wisconsin - Extension
The Wisconsin Soybean Marketing Board and Soybean
Growers Association is encouraging all soybean producers to test at least one of their
fields for the presence of soybean cyst nematode (SCN). To help in this effort, the cost
of testing one field per soybean producer is being picked-up by check-off dollars from
WSMB. Sample bags for submitting a soil sample and directions on how it should be done can
be obtained from the Fond du Lac County
UW-Extension Office.
What is soybean cyst nematode?
SCN is a small plant-parasitic roundworm that attacks the roots
of soybeans. The infective stage in the disease cycle caused by the nematode is the
second-stage juvenile (it molts once in the egg). At this stage the nematode is too small
to be seen with the naked eye, but it is able to penetrate soybean roots and cause the
formation of specialized feeding cells in the root's vascular system. If the juvenile
becomes a male, it leaves the root and moves through the soil and probably does not
contribute further to plant damage. If the juvenile becomes a female, it remains in the
roots and swells to a lemon-shape as it matures.
Plant damage is mainly due to the feeding of females. The mature females are barely
visible, a white dot about the size of a period at the end of this sentence. The white
females become yellow as they age and then brown after they die. The brown stage is the
cyst for which the soybean cyst nematode is named. Each cyst contains hundreds of eggs,
which are protected by the cyst. Symptoms of SCN infection are patches of stunted,
yellowed, less vigorous plants. There is often a sharp dividing line between stunted and
healthy plants, indicating the presence or absence of the nematode in the soil. Roots
infected with SCN are dwarfed or stunted, with a decrease in the number of nitrogen-fixing
nodules on the roots. The only definitive symptom of SCN infection is the presence of
adult female nematodes and cysts on the soybean roots.
Short soybean rotations increase the risk of yield loss from the soybean cyst nematode
(SCN). The greatest yield loss occurs on sandy soils, but significant loss can occur on
heavier soil types. However, the nematode is difficult to detect in loam to clay loam
soils because symptoms are not as apparent compared to SCN activity in sandy loam soils.
SCN can not reproduce without a host plant, and can infect other crops such as edible
beans. The soybean cyst nematode is moved every way that soil moves: water, wind and farm
equipment.
Assessment of SCN risk
The soybean cyst nematode (SCN) is found in 28 counties in Wisconsin including the
neighboring counties of Sheboygan and Washington. Historically, SCN in most states has
gone from essentially undetected for years to a widespread problem leaving people
wondering where it came from. The truth is that SCN was simmering for years in a stealth
mode even though people could have detected it years before it became a major production
problem. It is not a case of if, but when SCN damage will be more evident and widespread
in Wisconsin. Even in states with a long history of SCN, the pest does not cause dramatic
symptoms and yield loss, but rather essentially undetectable symptoms and yield loss in
the 3-5 bu/a range. This situation is often referred to as "hidden SCN damage".
Management of SCN
Crop rotation: Crop rotation and resistant varieties are the
foundation of a SCN management plan. SCN populations are suppressed in fields where 2 or
more years of a nonhost crop is planted. Good non-host crops are corn, small grains,
alfalfa and red clover. Edible beans are a host for SCN.
Soybean variety selection: The number of SCN varieties available is
limited, but more are released
each year. Races of SCN exist, but failure of an SCN resistant variety has not been
documented in Wisconsin. Difference in yield between SCN-resistant and SCN-susceptible
varieties is greatest in high yield-potential environments. Some SCN resistant varieties
are also resistant to brown stem rot, white mold and Phytophthora root rot. SCN does not
appear to break down brown stem rot resistance.
Tillage: There is some evidence that SCN is not as damaging in no-till
systems.
Weed control: Broadleaf weeds common to Wisconsin soybean fields are
not hosts to the SCN. SCN resistant soybean varieties withstand more weed pressure than
susceptible varieties.
Other SCN Links:
Mangement of SCN in
Wisconsin
Detecting SCN
1999 Soybean Varieties
Resistant to Soybean Cyst Nematode
Soybean Cyst
Nematode (Iowa State Cooperative Extension)
Manage Soybean Cyst
Nematode (United Soybean Board)
Scouting for SCN
(Iowa State University)
SCN
Coalition web site
For
more information contact Mike Rankin