USA - Soybean yields excellent, but fungicide resistance concerning

By Gary Lesoing

While soybean prices and yields have generally been excellent last year, we still have many challenges to be aware of in Nebrask. Probably one of the biggest concerns is the resistance of Frogeye Leaf Spot to QoI (strobilurin, FRAC group 11) fungicides. In 2020 Extension Educators across Nebraska sampled fields for frogeye leaf spot to determine how widespread resistance is to QoI or group 11 fungicides.

Out of 375 samples from 128 fields in 48 counties, they all were resistant to the group 11 fungicides. This is major resistance to a fungicide. This doesn’t been that this fungicide can not be used, but it should be used in combination with other fungicide groups if you want tot control frogeye leaf spot.

Recommendations for control of frogeye leaf spot include: 1) plant resist varieties, 2) use longer crop rotations and 3) use fungicides with active ingredients from 2-3 effective classes and modes of action. The Crop Protection Network has a good publication on the efficacy of various fungicides on control of different soybean diseases, including frogeye leaf spot. You can find this publication at this site; https://cropprotectionnetwork.s3.amazonaws.com/CPN1019_FungicideEfficacyControlSoybean_2022.pdf

It shows still good control of frogeye leaf spot, with combination of fungicides. Fungicides such as; Fortix SC, Preemptor SC, Delaro325 SC, Lucento 4.17 SC, and Revytek had good to very good control of frogeye leaf spot. In trials conducted in 2020 -2021 at (HAL) the Haskel Ag Lab at Concord, NE and SCAL, South Central Ag Lab, at Clay Center, combinations of the different classes of fungicides decreased to incidence of frogeye compared to group 11 alone.

Group 11 fungicide treatment still had significantly less disease than no fungicide treatment. In 2021 at HAL, there were no significant differences in soybean yields between untreated and fungicide treated soybeans, all yielding between 65-70 bu/ac. At SCAL there was a significantly higher yield from soybeans treated with the fungicide Delaro Complete, 88 bu/ac vs 81 to 84 bu/ac for the other fungicide treatments, with the untreated soybeans yielding 82 bu/ac.

Soybean Cyst Nematode (SCN) is still an issue in several counties across Nebraska. Almost half, 47% of the SCN population can infect soybeans from PI 88788, which is > 95% of soybeans planted in Nebraska. A new source of resistance is available from PI 89772.

Only 25% of Soybean Cyst Nematode populations can produce on this source of resistance and 30% of Nebraska SCN can affect soybeans from Peking. So there is some progress happening in this area of SCN resistance. If you are concerned about SCN, sample your fields for SCN. Your Nebraska Soybean Board has a program that will pay for it. You are paying for it as a soybean producer. If interested in sampling your fields, contact Gary Lesoing at (402) 274-4755 or glesoing2@unl.edu.

In regard to Sudden Death Syndrome (SDS), ILeVo and Saltro are a couple of seed treatments that have provided some protection from significant SDS yield reductions.

Some areas in southeast Nebraska have been impacted by the soybean gall midge. Most of the infestations are to the north of Nemaha County, but several people in Otoe and Cass County have been impacted by this pest. The University of Nebraska and other universities are putting a lot of effort into treatment and strategies to best control this best.

In the southern areas and even in Otoe County, farmers have been impacted by the Dectes Stem Borer in soybeans. Researchers are still trying to find best management practices to reduce the impact of this pest as well. Important to scout fields throughout the season to check for diseases and insects that could impact your crop yield and quality.