Soybeans Today January 1998
Rotation May Help Control Soybean Cyst Nematodes
By Howell Medders
U of A nematologist Dr. Robert Riggs and research associate
Dr. Patty Timper are among the scientists trying to reduce yield losses from cyst
nematodes. |
A four-year rotation system is now recommended for soybeans in fields infested by
soybean cyst nematodes (SCN).
The new recommendation, which provides an option to the three-year rotation, is based
on the results of seven years of field tests supported by Arkansas Soybean Promotion Board
research grants.
University of Arkansas Division of Agriculture research on SCN control is conducted by
Dr. R.D. Riggs, Dr. R.T. Robbins and Dr. J.C. Rupe, all plant pathologists, and Dr. Lucas
Parsch, an agricultural economist.
The four-year rotation includes:
- one year of a soybean variety that is resistant to the prevalent SCN races identified in
the field,
- one year of an SCN-resistant variety with a source of resistant genes that is different
from the variety planted the first year,
- one year of a crop such as corn or grain sorghum that is a non-host to SCN, and one year
of a soybean variety that is susceptible to all SCN races.
- Riggs said data from the study show that, with this rotation system, growers can expect
yields from susceptible SCN varieties to equal or exceed those of resistant varieties.
"Over a period of time, soybean yields should increase with this combination,"
he said.
- He said the SCN population will decline in the three years when resistant varieties and
a non-host crop are planted. In the fourth year, with the susceptible variety, "the
nematode population will start out low and build to a high level, but not until after the
crop is made."
- The susceptible variety reduces the "selection pressure" on the nematode
population in a field to change races and overcome the resistance of a particular
resistant variety, Riggs said.
- A key to a successful rotation plan is having soil tested to identify the most prevalent
SCN races in each field, Riggs said. An estimated 65 percent of the soybean acreage in
Arkansas is infested by SCN.
Producers should contact the county Extension office for information on submitting soil
samples for a nematode assay.
The previously recommended three-year rotation of a resistant variety, susceptible
variety and a non-host crop remains a viable option, Riggs said.
He said the four-year rotation does not provide better SCN control than the three-year
rotation, but test results indicate that it is just as good.
A five-year rotation, with and without a non-host crop, is being field tested.
Crop rotation is much more effective for SCN control than chemical nematicides, which
have proven to be uneconomical and/or erratic in the control they provide, Riggs said.
Using SCN-resistant varieties for continuous soybeans on the same land is not
recommended because the nematode population will adapt to the resistance. A race to which
the variety lacks resistance, which may be present in small numbers in the soil, will
become the dominant SCN race.
Rotation considerations in addition to SCN control include broadleaf weeds and soybean
disease pathogens. Sudden death syndrome and other diseases appear to be more of a problem
in fields with high SCN populations, Riggs said.
In another long-term project supported by the Soybean Promotion Board, Riggs is testing
a fungus as a biological control agent against SCN. The fungus occurs naturally in most
soybean fields, and Riggs is working with a private company that has developed a
formulation for applying the fungus to the soil.
One year of tests in micro-plots was very encouraging, Riggs said. Tests were repeated
in 1997 and results were not available at press time.
Soybeans Today January 1998
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