Almost all of the soybean varieties available to Iowa farmers that are resistant to the soybean cyst nematode (SCN) were developed with a breeding line named PI 88788. The SCN control provided by these varieties can vary greatly despite having a common source of resistance. The SCN-resistant Soybean Variety Trial Program at Iowa State University assesses the agronomic performance and SCN control provided by hundreds of varieties each year with soybean checkoff support from the Iowa Soybean Association. Results of these experiments also illustrate the connection between yield and SCN egg population densities (numbers). This relationship is why effective SCN control is necessary to achieve high soybean yields in SCN-infested fields.
Variety trial SCN results are revealing
The variety trial experiments mentioned above are conducted in each of Iowa’s nine crop reporting districts annually. The 2021 experiments were harvested in October, and soil samples were collected from each replicated research plot at harvest. SCN eggs are extracted from the soil samples and counted to determine the final SCN population density in each plot. These SCN data allow for comparing the yields of the varieties with the SCN control they provide.
Processing of the soil samples from all of the experiments takes several weeks and will not be completed until early December, but SCN data became available from the first few experiments last week. The results of the experiment conducted in Fruitland, in southeast Iowa, provide a look at the extent of SCN reproduction that occurred in 2021. The data also illustrate how control of SCN reproduction, as reflected by egg population densities, affected soybean yields substantially. All of the varieties in the experiment had SCN resistance from PI 88788.
The overall mean SCN population density in the Fruitland experiment at the start of the growing season was 1,672 eggs per 100 cm3 of soil, a low level of infestation. End-of-season SCN egg population densities ranged from less than 5,000 eggs for some varieties (a moderate infestation) to nearly 39,000 eggs per 100 cm3 of soil for others (a very high level of infestation). The figure below shows the average yield and end-of-season SCN population density for the 69 resistant varieties studied in the experiment.
There is a negative relationship between yields of the varieties and end-of-season SCN egg population densities. The varieties with the lowest end-of-season egg population densities were the highest yielding, and yields tended to be lower with increasing egg population densities. The two highest-yielding varieties produced 60.6 and 60.4 bushels per acre and had end-of-season SCN egg population densities of 4,725 and 6,725 eggs per 100 cm3 of soil, respectively. These were among the lowest end-of-season egg population densities of all varieties in the experiment. The variety with the lowest average yield, 39.7 bushels per acre, had the greatest end-of-season SCN population density, nearly 39,000 eggs per 100 cm3 of soil.
Characteristics of the SCN-resistant soybean varieties and soil-related factors could have affected yields in this experiment. However, the relationship between SCN control (end-of-season SCN population densities) and yields of the soybean varieties is clear.
The results of all nine SCN-resistant soybean variety trial experiments will be compiled into an annual report, ISU Extension publication IPM 52, which will be available online at the ISU Extension Store in early January 2022. A printed copy of the report will be inserted in the January 8, 2022, issue of the Iowa Farmer Today.
Managing SCN long term
Farmers should seek out and grow soybean varieties with SCN resistance from the Peking source in rotation with varieties with good SCN resistance from PI 88788 because many Iowa populations of SCN reproduce well on varieties with PI 88788 resistance. This recommendation is most important for SCN-infested fields in which soybeans will be grown in 2022 following soybeans in 2021. Using effective nematode-protectant seed treatments ion resistant soybeans may offer additional yield protection.
For more information on SCN and the Iowa State SCN-resistant Soybean Variety Trial Program: www.isuscntrials.info
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