Foliar diseases such as frogeye leaf spot, Cercospora leaf blight, and Septoria brown spot are becoming more prevalent in Iowa soybean fields, threatening yield and profitability. This project will evaluate the effectiveness of fungicides, including new and multiple-mode-of-action chemistries, and biological fungicides under various growing conditions. Additionally, it will assess fungicide resistance in key pathogens and identify management strategies for resistant populations. By generating data on fungicide performance and resistance trends, this research will provide Iowa soybean farmers with practical recommendations to improve disease management, optimize input costs, and integrate biofungicides into long-term sustainable production systems.

Objective 1: Compare new fungicide products with industry standards for foliar disease management and yield protection.
Objective 2: Compare fungicides with biofungicides in multiple Iowa locations.
Objective 3: Monitor for fungicide resistance in major soybean pathogens in Iowa.

· Increased understanding of fungicide application at growth stage R3 under different growing conditions.
· Information to assist farmers with identifying the most effective fungicides for foliar disease management.
· Information to assist farmers in understanding the short- and long-term benefits of biofungicide use in Iowa.
· Monitor for fungicide resistance to several foliar pathogens across Iowa.

Updated September 5, 2025:
Fungicide statewide trial
In 2024, Iowa State University researchers evaluated the effectiveness of 18 commercial foliar fungicides on soybean disease control and yield at seven research farms across the state. Septoria brown spot was the only disease observed consistently, but at low levels, and frogeye leaf spot was not detected. Fungicides were applied at the R3 growth stage, and data on disease severity and yield were collected. Results showed no statistically significant differences in disease severity or yield between treated and non-treated plots, likely due to low disease pressure and variable precipitation across sites. The study emphasizes the importance of integrated disease management strategies, including crop rotation, resistant cultivars, and responsible fungicide use to preserve efficacy and reduce the risk of resistance.

Frogeye leaf spot model
For soybeans, a weather-based predictive model for frogeye leaf spot was tested across 10 sites in eight states in 2023. The model successfully reduced the number of fungicide applications, with application at the R3 growth stage lowering disease severity (measured as AUDPC), though it did not impact yield at any site. In 2024, a new version of the model was developed and validated at 20 sites across 10 states, with data analysis currently underway. These efforts aim to support better decision-making for fungicide use as part of an IPM program.

Cercospora Isolate Pathogenicity and Resistance
Recent pathogenicity tests have been conducted on Cercospora leaf blight (CLB) and Purple seed stain (PSS) isolates, including Cercospora cf. flagellaris, C. cf. sigesbeckiae, and C. kikuchii. These isolates tested positive when inoculated on detached leaves but did not produce symptoms in planta. This discrepancy highlights the complexity of host-pathogen interactions and suggests that environmental or host factors may influence disease expression in the plant. A small number of PSS isolates collected in 2024 have undergone fungicide resistance screening. All tested isolates showed resistance to QoI fungicides, raising concern about effective chemical management options. However, these isolates have not yet been identified to species level, limiting our ability to draw firm conclusions about resistance patterns within specific Cercospora species.

Septoria Identification, Pathogenicity and Resistance
Isolates previously labeled as putative Septoria from 2016 have now been identified molecularly as Septoria glycines. Both these older isolates and newer ones collected in 2024 have been subjected to pathogenicity tests in planta and on detached leaves. In both settings, the isolates consistently produced positive results, confirming their pathogenic potential. To better understand disease severity, aggressiveness was evaluated for both contemporary and historical Septoria brown spot isolates. Tests were conducted in planta and on detached leaves, providing insight into potential differences in virulence between isolates from different time periods. A preliminary effort was made to validate EC50 estimations for azoxystrobin (a QoI fungicide) sensitivity in Septoria glycines. The method compares conidial germination with results from a high-throughput microtiter absorbance assay. However, inconsistent conidia production in certain isolates has made it difficult to reliably complete this comparison. This variability is currently a limiting factor in developing a robust sensitivity assay.

This project directly benefits Iowa soybean farmers by improving foliar disease management strategies, optimizing input costs, and increasing profitability. By evaluating traditional fungicides and biofungicides under various conditions, farmers will gain insights into the most effective disease control options. The research will also provide critical data on fungicide resistance, helping farmers select products that remain effective against evolving pathogen populations. Additionally, building a long-term database on biological fungicides will support sustainable production practices. The findings will enable farmers to make informed decisions, integrate biofungicides into disease management plans, and enhance overall soybean yield and quality while reducing unnecessary fungicide applications.