Research Highlights

Research Highlights
Fungicide Resistance in FLS and Use of Foliar Fungicides in Nebraska

Characteristic “frog eye” spots caused by the frogeye leaf spot (FLS) pathogen, Cercospora sojina, on a soybean leaf and pods. These samples were collected from Burt County in 2020. Fungicide resistance in this pathogen may reduce the efficacy of single-site QoI Group 11 fungicides.

By Asha G. Mane, Sydney E. Everhart, Tamra Jackson-Ziems

A foliar disease of soybean called frogeye leaf spot (FLS) is becoming more common in Nebraska. FLS is caused by the fungal pathogen, Cercospora sojina. The fungus overwinters on soybean plant residue and seeds, and during the growing season, it can be spread across fields by wind and rain. Long periods of wet weather during the growing season favor disease development.

Several management practices can combat this disease, including planting available FLS-resistant varieties, crop rotation with a non-host (like corn), tillage (only as necessary), and application of foliar fungicides. After scouting to determine whether a foliar fungicide application is needed, it is best to select products that use active ingredients with different modes of actions to delay development of fungicide resistance. Although fungicides with a single-site mode of action in the QoI (Strobilurin) group were historically the most effective for FLS management, fungicide resistance to these products now appears to be widespread in the soybean-growing region of eastern Nebraska.

Figure 1: In 2019, we collected FLS samples from 10 counties shaded in yellow. Among the 113 Cercospora sojina isolates tested, only two were identified as sensitive to QoI fungicides. (Neves 2020)

We collected 113 FLS samples in 2019 from 12 fields in 10 Nebraska counties and determined all but two of the FLS samples were resistant to QoI fungicides (Figure 1) (Neves 2020). To understand whether these were isolated cases, we expanded our sample collection in 2020 with support of the Nebraska Soybean Board and found all 375 samples collected from 48 counties (Figure 2), were resistant to the QoI fungicides, suggesting that QoI fungicide resistance is widespread in Nebraska

We confirmed these results using a DNA-based rapid assay where we screened one representative isolate from each county. In the DNA-based assay we used a single reaction to identify up to three mutations (G143A, F129L, and G137R) in the cyt b gene, allowing direct detection from an infected leaf sample in only two to three hours. Our next goal is to analyze a larger number of leaf samples per reaction to reduce the cost per sample and enable a broader survey at a single time. Thus, we propose pooling lesions from multiple leaf samples with known sensitivity into the same reaction and determination of the detection threshold. 

Figure 2: In 2020, we collected FLS samples from 128 fields in 48 counties shaded in yellow. All 375 Cercospora sojina isolates tested were resistant to QoI fungicides in our laboratory assay. The results were confirmed with a DNA-based assay in the lab and suggest widespread resistance to the QoI Group 11 fungicides in the soybean-growing region of Nebraska.

To improve our ability to communicate information about best practices for disease management to limit fungicide resistance, we recently developed an informational survey to find out how fungicide application decisions are made and identify sources of information used to make disease management decisions. This was a producer- and applicator-focused research survey to obtain information about foliar fungicide use in Nebraska. The survey included 10 questions with both multiple choice and short answer questions that took approximately 10 minutes to complete. Participants were recruited at row crop-related meetings.

Responses from 618 participants from 78 counties perceived that their most important soybean diseases were: sudden death syndrome (19.0%), white mold (16.5%), and frogeye leaf spot (12.0%).  Crop rotation (23.7%), herbicides (21.4%), and foliar fungicides (13.3%) were the most cited crop management strategies. More than half (64.3%) reported that they used or recommended foliar fungicides for soybean in the last five years.

Participants who did not apply or recommend fungicides considered soybean cyst nematode to be more important than frogeye leaf spot. Factors most reported as an influence on their disease management decisions were the amount of disease (23.2%), fungicide cost (21.6%), and crop market value (18.4%). The most cited source of information used to make disease management decisions was recommendations from the local agricultural cooperative service provider (14.9%).

Whether participants considered applying foliar fungicides or not, the ranking for factors and sources of information used remained the same. These results will be used to customize future educational outreach materials to inform farmers and agribusiness representatives about fungicide use.

This project was funded by the soybean checkoff. To find research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.