The soybean cyst nematode (SCN) is the most damaging pathogen of soybeans throughout Iowa, costing Iowa farmers tens to hundreds of millions of dollars in lost revenue each year through yield reductions. SCN was successfully managed into the decade of the 2000s by growing SCN-resistant soybean varieties in alternating years with corn, a nonhost crop. However, almost all (>95%) SCN-resistant soybean varieties available to Iowa soybean farmers were developed with a single breeding line named PI88788. SCN populations in fields throughout Iowa have developed high levels of reproduction on resistant soybean varieties due to exposure to resistance genes from PI88788 for two to three decades.

An additional challenge with SCN-resistant soybean varieties is that not all varieties provide the same level of SCN control, not even with SCN populations that are well controlled by the PI88788 SCN resistance. The genetic basis of SCN resistance is complex and still being elucidated by researchers. Some seed companies do not test the level of SCN control provided by their soybean varieties, and some obtain varieties from third-party genetics providers and do not do verify the SCN resistance of varieties they have licensed.

Greenhouse testing at ISU has shown that numerous varieties marketed as SCN-resistant allow moderate or higher levels of SCN reproduction and a few allow as much nematode reproduction as susceptible varieties. The situation exists because there is no legal definition or standard level of suppression of SCN reproduction required for a variety to be marketed as SCN-resistant, and no government testing program verifies the SCN resistance in soybean varieties sold to farmers.

New soybean varieties become available for Iowa farmers each year, and there is a recurring need to evaluate the SCN control provided by varieties that are marketed as resistant to SCN. These assessments are as important now as ever before because soybean varieties with new or improved herbicide resistance traits may have been rushed to market and may have diminished resistance to SCN. Also, there currently are a few new soybean varieties with SCN resistance from the non-PI88788 breeding lines Peking and PI89772, and it is likely (and hoped!) that more will become available each year. It is important to assess the SCN resistance and yield performance of these varieties with non-PI88788 resistance. Such results can confirm that the varieties are effective for use in managing SCN and are profitable to grow - information that will reassure farmers and encourage them to grow varieties with new sources of SCN resistance.

Beginning in the mid 2000s, nematode-protectant seed treatments became available that are marketed as providing protection against SCN. At least 10 such products currently are available on the market. Like SCN resistance claims made for soybean varieties, there is no standard of control and no official program or organization that verifies and evaluates the effects of nematode-protectant seed treatments on SCN.

Four to five different types of field experiments will be conducted in each of Iowa’s nine crop-reporting districts each year in the project described in this proposal. A resistant variety evaluation experiment will be conducted plus three or four experiments evaluating nematode-protectant seed treatments. The experiments will provide SCN control and yield data from hundreds of SCN-resistant soybean varieties and the most popular and newest nematode-protectant seed treatments. The results of the experiments will help Iowa farmers make the best SCN management decisions. Also, project personnel will create an annually updated extension publication listing the more than 800 SCN-resistant soybean varieties for use in Iowa, which will help farmers know what varieties are available for each growing season along with key agronomic characteristics of the varieties.

Update:
Project progress from 1 April 2022 through 30 September 2022 included:
- Received seed from seed and seed-treatment companies for field experiments in 2022, designed the 2022 field experiments, and packaged and organized the seeds for the various experiments.

- The company names and numbers of varieties (in parentheses) included in the 2022 SCN-resistant soybean variety trial experiments are as follows: Agrigold (12), Asgrow (13), Channel (8), Cornelius (3), Credenz (3), Dyna Gro (9), Federal Hybrids (3), Golden Harvest (8), Hoegemeyer (8), Jacobsen (4), Kruger (9), Latham (9), LG Seeds (15), Merschman (13), NK (18), Nutech (11), Pioneer (7), Stine (19), Xitavo (14), Don Mario (3), and Virtue (2). Most of the varieties tested in 2022 possessed SCN resistance from PI 88788 but the following companies had varieties with Peking SCN resistance (number of varieties in parentheses): Agrigold (1), Dyna Gro (1), Federal Hybrids (1), Hoegemeyer (6), Jacobsen (1), Merschman (1), NK (1), Nutech (5), Pioneer (2), Stine (7), Xitavo (1), and Don Mario (2).

- Set up, planted, soil sampled, maintained and prepared to harvest nine field experiments located throughout Iowa to evaluate the yield enhancement and nematode control provided by 191 different SCN-resistant soybean varieties in 2022.

- Set up, planted, soil sampled, maintained, and prepared to harvest nine field experiments located throughout Iowa to evaluate the agronomic performance and nematode control provided by three different nematode-protectant seed treatments - namely Ilevo (from BASF), Saltro (from Syngenta), and Trunemco (from NuFarm).

- Extracted SCN cysts (egg-filled dead SCN females), then eggs, from soil samples collected at planting from all 9 resistant variety evaluation experiments and the 27 seed treatment experiments to determine initial SCN egg population densities.

- In early September, the field research plots at the 36 total field experiments were end trimmed in preparation for harvesting, which began on October 5, 2022.

- In August 2022, we contacted 55 seed companies and gathered information about SCN-resistant soybean varieties to use to update the ISU Extension publication titled “Soybean Cyst Nematode-Resistant Soybean Varieties for Iowa”. This publication will be compiled and released in PDF format by early November 2022.

Currently there are three options for managing SCN: 1) growing nonhost crops such as corn, 2) growing SCN-resistant soybean varieties, and 3) using nematode-protectant seed treatments on soybean seeds. The results of the experiments conducted in this project help Iowa soybean farmers decide which resistant varieties and seed treatments to purchase for use in fields infested with SCN. The results of the experiments conducted in 2022 are not yet available because not all of the end-of-season SCN soil samples have been processed and counted, but all work on the 2022 experiments will be completed by the end of calendar year 2022, after which planning for 2023 will commence. The report of 2021 variety trial experimental results and the 2021 list of SCN-resistant soybean varieties for Iowa are attached to this final report.