By Rod Swoboda

Iowa State University annually compiles a list of soybean varieties available to Iowa farmers with resistance to soybean cyst nematode. ISU personnel contact more than 40 seed companies to collect details about the SCN-resistant varieties.

The information is in an extension publication, Soybean Cyst Nematode-resistant Soybean Varieties for Iowa, available to farmers and those who advise them. The recently updated list is available online here (download of a 25-page document). This work is financially supported by the Iowa Soybean Association.

The publication includes information on varieties in maturity groups (MG) 0, 1, 2 and 3. These are maturity groups for planting in Iowa. These varieties are likely available for use in other states, too. In addition to indicating the genetic source of SCN resistance, the publication has information on relative maturity, iron deficiency chlorosis tolerance and herbicide resistant traits of the varieties.

Available for 2024 planting

An underground parasite, SCN attacks the roots of soybean plants, reducing yield.   

“Planting an SCN-resistant soybean variety is a grower’s first line of defense,” says Greg Tylka, ISU Extension nematologist. “But in many fields, SCN has become resistant to the resistance trait because the same source of SCN resistance, known as PI 88788, has been used in about 95% of SCN-resistant soybean varieties for decades.”

However, the recently updated SCN-resistant soybean variety list has more varieties with the Peking resistance trait than ever before, notes Tylka. This provides soybean growers with more options for rotating their choice of SCN resistance traits in the varieties they plant.

The list of SCN-resistant soybean varieties available for planting in 2024 shows a total of 866 varieties and five blends. That’s 48 more varieties that have an SCN resistance trait than were available for planting in 2023.

Only two sources of SCN resistance are currently being used to breed into commercially available soybean lines. They are the PI 88788 trait and the Peking trait. Thus, all soybean varieties that possess SCN resistance are getting it from PI 88788 or from the Peking resistance trait.

Other highlights from the updated ISU publication:

Use multipronged approach

“Managing SCN requires an active, multiple-prong approach,” says Tylka. “In short, farmers should grow SCN-resistant soybean varieties with PI 88788 and with Peking resistance in alternating soybean crops and in annual rotation with corn, which is an SCN nonhost. Also, there are nematode-protectant soybean seed treatments that may provide added yield protection.”

The weeks immediately after corn and soybean harvest are prime time to collect soil samples from fields to determine if SCN is present and, if so, at what numbers. “Fields infested with SCN may not show stunting and foliar yellowing when nematode numbers are low or moderate,” Tylka notes. “In fact, the only ‘symptom’ a farmer often observes prior to confirming that a field is infested with SCN is unexplained, lower-than-expected soybean yields.”

Sampling and testing

Why sample fields for SCN if no symptoms are observed? Independent random surveys for SCN have been conducted by Tylka’s research group over the years. In these random surveys, SCN was detected in approximately 70% of the sampled fields. Given the fact that the fields that were sampled were randomly selected, there’s a 70% likelihood that any soybean field in Iowa may be infested with SCN.

“Also, SCN spreads in windblown soil,” Tylka says. “We know this because we have grown soybeans in soil collected from snowdrifts in late winter and observed adult SCN females on roots. So even if fields were sampled in the past and SCN was not detected, sampling again is warranted.”

The ISU Plant and Insect Diagnostic Clinic processes SCN soil samples for farmers and agronomists. Many private soil-testing labs also test samples for SCN, and you can get more information on sampling and submission online.more information on sampling and submission online

In the end

It is estimated that SCN is the most damaging pathogen of soybeans in Iowa and the entire U.S., costing farmers $1.5 billion annually. “This pathogen warrants more attention and effort to detect and manage than often is given,” Tylka says.

The yields of varieties with PI 88788 resistance can be significantly reduced by SCN. SCN populations in many fields have elevated levels of feeding and reproduction on these varieties. Soybean varieties with Peking SCN resistance can yield as much 22 bushels per acre more than varieties with PI 88788 resistance in these fields, because Peking resistance still is effective at keeping SCN feeding and reproduction in check.

Tylka strongly encourages farmers who are growing soybeans in Iowa fields infested with SCN to use soybean varieties with Peking SCN resistance in rotation with varieties with PI 88788 SCN resistance to achieve high soybean yields and to keep soil population densities of SCN in check. Peking varieties should not be used repeatedly without rotating with PI 88788, because SCN populations will overcome Peking resistance just as they overcame PI 887888 resistance.

ISU offers a quick guide to soybean varieties available for Iowa that have the Peking trait for SCN resistance.

Swoboda is a retired Wallaces Farmer editor.