Foliar diseases of soybean significantly impact soybean production and grain quality. Several foliar diseases (e.g., frogeye leaf spot, Cercospora leaf blight, Septoria brown spot, etc.) are commonly found on soybean throughout Iowa. In parts of Iowa the past few years, all three of these diseases have become more prevalent. We now know how some of these diseases may affect yield and when fungicides should be used for management. However, there is still much to learn about how fungicides will affect yield in the presence or absence of stresses besides foliar diseases. In addition, fungicides with new and multiple chemistries are available in the market and the fungicide program cost can increase with some of these new fungicides. Although yield increases can occur sometime with foliar fungicide treatments, current market prices and application costs may limit profitability when disease pressure is low.

Resistance in many soybean pathogens, mainly Cercospora sojina (frogeye leaf spot) and Septoria glycines (brown spot), have been reported in several states both in southern and northern regions and QoI fungicides have been found ineffective to manage frogeye leaf spot. We are identifying how widespread the resistant strains of these pathogens are in Iowa and what fungicides would best manage these resistance strains. In Iowa, QoI fungicides are showing decreasing efficacy in managing frogeye leaf spot and brown leaf spot as fungicide resistant populations expand in the state. We will continuously screen for resistant isolates. This work will address pressing economic and environmental concerns in Iowa. These findings are directly applicable to many farmers and agribusinesses. This work would have beneficial immediate and long-term effects on farm productivity and input use.

The knowledge gained from this research will benefit Iowa soybean farmers by enabling them to:
• Make more informed decisions regarding foliar disease management
• Increase understanding of fungicide application at growth stage R3 under different growing conditions.
• Further understand when and how fungicides will fit into an integrated pest management (IPM) strategy for disease management.

Objective 1: Compare new fungicide products with industry standards for foliar disease management and yield protection
New products continue to become available for soybean farmers. Unbiased evaluations by public institutes are needed for farmers to make informed fungicide selections. Since 2008, PI Mueller has evaluated various fungicides for efficacy against foliar diseases. To build on this research, replicated (four reps/treatment) field plots will be established at seven Iowa State University (ISU) Research Farms near Crawfordsville, Nashua, Kanawha, Sutherland, Armstrong, Chariton, and Ames, Iowa. The same soybean cultivar will be selected for all seven locations. These farms experience different stresses annually, including different disease pressure and different weather conditions. Plots with foliar fungicide application will be compared to no-spray control plots. Fungicides will be applied as recommended by university and industry scientists for penetration deep into the soybean canopy: 20 gal/acre, droplet size between 200-250 microns, greater than 50 psi application pressure, and the correct adjuvant for each fungicide. Treatments will be arranged in a randomized complete block design within four-row plots. Parameters that will be measured include foliar diseases, yield (bushels per acre), grain moisture and weather data.
We will also coordinate with the ISA on-farm network to help establish on-farm fungicide trials. We will coordinate protocols with ISA to make sure disease notes are taken in a similar fashion. We also will include fungicides used in the on-farm trials in our statewide trials.

Objective 2: Establish a fungicide resistance management plan in Iowa
Most foliar fungicides for soybeans fall into just three classes, Class 3 (DMI), Class 7 (SDHI), and Class 11 (QoI). When soybean pathogens become resistant to fungicides in any of these classes, this will dramatically reduce the number of effective options for soybean farmers to manage these pathogens. Monitoring and developing a resistance plan for resistant pathogens will ensure continued use of disease management tools.
We have been working closely with Dr. Carl Bradley’s, extension plant pathologist at the University of Kentucky and Dr. Ahmad Fakhoury, professor at Southern Illinois University, to establish a fungicide resistance monitoring system for Iowa soybean. We will sample leaves from fungicide trials across the state (both small plot and on-farm) as well as from commercial farmer’s fields that exhibit foliar diseases such as frogeye leaf spot, Cercospora leaf blight and Septoria brown spot. We will isolate these pathogens and test isolates using appropriate methods, including the “poison plate assays” and PCR technique to test for resistance to QoI and other fungicide classes.

• 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.
• Identify fungicide-resistance to several foliar pathogens across Iowa.

Update:
Objective 1: Compare new fungicide products with industry standards for foliar disease management and yield protection.
A total of 18 fungicides containing single and multiple active ingredients were evaluated at each location. Two new fungicide commercial products were included compared to 2022: Adastrio, and Propulse. Fungicide treatments were laid out in a randomized complete block design with four replications. Plot size ranged from 25.5 to 36 ft long and 10 to 15 ft wide (4–6 rows 30-inch inter-row spacing). Soybean cultivar Pioneer P31T64E was planted at the McNay Research farm and NK28-T3XF was planted in all other locations. Corn was planted in the previous year in all locations. All fungicides were sprayed using a self-propelled research sprayer at recommended rates at the beginning of pod (R3 growth stage) with nonionic surfactant (Induce at 0.3% v/v). Foliar diseases were assessed when soybeans reached the R6 (full seed) growth stage. Septoria brown spot (SBS, caused by Septoria glycines) progression was assessed by measuring the height of the highest infected leaf in the canopy, and the disease severity was estimated as the percent of leaf area covered by the disease on the highest infected leaves which was converted to Septoria brown severity index (BSX). A total of two assessments, one in each row (a couple plants per row), were collected and averaged for a plot. Other foliar fungal diseases were also assessed, including frogeye leaf spot (FLS, caused by Cercospora sojina) and Cercospora leaf blight (CLB, caused by several Cercospora species) if present in 10 random leaves in the upper canopy of each plot. Total seed weight/plot and moisture were measured with a 2009 Almaco SPC20 research plot combine. Seed weight was adjusted to 13 percent moisture and yield was calculated in bushels per acre.

In 2023, the precipitation pattern during the season was different across the locations. Ames received the most cumulative precipitation between May and September (>15”) among the seven locations, whereas Nashua received the least amount of precipitation (9”). Precipitation in August, the critical month for foliar disease development, was low in most locations.

Frogeye leaf spot (FLS) and Septoria brown spot (SBS) were the two diseases observed most frequently, but both occurred at very low levels in all locations (overall mean of less than 1% severity for both diseases). FLS did show up late in late August. The effect of fungicide applications was statistically significant for FLS severity and SBS severity index. Across the seven locations, Veltyma, Affiance, Miravis Top, Revytek, Miravis Neo, Lucento, and Propulse had significantly less FLS severity as compared to the UTC, Priaxor, and Aproach Prima. Similarly, Quadris Top, Adastrio, Delaro, Delaro Complete, and Regev HBX also had significantly less FLS than the UTC. For SBS, none of the fungicides showed a reduction of the severity index (BSX) compared to the UTC. Treatments did not show a significant effect on overall yield response across the seven locations. In 2023, overall soybean yield was less than in 2022 and 2021 (Nieto et al. 2023, Kandel et al. 2021). The application of foliar fungicides can provide yield benefits when there is significant foliar disease pressure. Some reports show yield benefit of fungicides at low levels of disease or even when disease was absent, but these results are not consistent. Cercospora sojina and Septoria glycines strains resistant to QoI fungicides were confirmed throughout Iowa a few years ago; thus, judicious use of fungicides is crucial. Moreover, an integrated method of disease management that does not rely only on fungicides should be incorporated to manage crop diseases and preserve the efficacy of existing fungicides. Disease management practices such as crop rotation, planting disease-resistant cultivars, and applying fungicides with multiple modes of action can all be adopted to prevent fungicide resistance development.

The results from this research objective were summarized in the Iowa State ICM Blog in early 2024. Data were also presented at several farmer/agronomists meetings this past winter - more than 15 presentations and 1,000 participants.

Objective 2: Establish a fungicide resistance management plan in Iowa.
Single spore isolation of foliar Cercospora leaf blight (CLB), Frogeye leaf spot (FLS), and Septoria brown spot (SBS) fungi soybean pathogens from Iowa was done in 2023 from different locations among fungicide and variety trials. In total, 170 CLB isolates were obtained, 25 FLS isolates, and 80 SBS isolates. The best approach to isolate these fungal diseases was to take direct conidia from the humid chamber with leaves.

A study of fungal CLB diversity is being completed from variety-trial fields in Newton, IA, which was a hotspot for CLB in 2023. Leaf symptoms were classified based on symptoms development in purple color, red brick color, and blight, and the evaluation is still underway. Also, isolates were classified by the fields they were coming from (N=5 fields). Lastly, it was observed in which humid chamber was able to get more conidia sporulation: 12h dark/12h fluorescent lightbulbs of 5,000 lumens or in 16h light (fluorescent lightbulbs of 5,000 lumens + black-UV)/12h dark, both at 25°C. In both light regimes, the fungus sporulated at the same rate. Preliminary data from the 93 random samples showed that 83% of the samples were Cercospora cf. flagellaris. The identification of the remaining samples is still underway. These results are similar to our lab’s results of PSS endophytes from 2021, where 66% were Cercospora cf. flagellaris, 16 % C. cf. sigesbeckiae, and 16 % C. kikuchii. In addition, of the same 93 samples, 67% had the G143A mutation for QoI fungicides, indicating they are resistant to this class of fungicides. Corroboration of some isolates by qualitative discriminatory concentration (1 ppm; if = 50 % germination is shown, it is considered as resistant) and evaluation of fitness traits is still underway. Likewise, these results are similar to the results of PSS endophytes 2021, where 70 % of the isolates were identified to have the same mutation indicating resistance to QoI fungicides.

Fungicide sensitivity (N= 10 representative FLS isolates from comprehensive IA of 2019) was studied on technical azoxystrobin by conidia germination with a previously established procedure using a qualitative discriminatory concentration (1 ppm; if = 50 % germination is shown, it is considered as resistant). Six of the ten isolates were phenotypically resistant to QoIs. However, all the isolates showed molecularly target-fungicide resistance, carrying the G143A mutation —these confirm results with a publication of 2018 showing widespread FLS resistance in the USA.

Fungicide sensitivity (N= 12 SBS isolates— historical isolates from comprehensive IA and SD) is being studied on azoxystrobin technical ingredient on conidia germination by both standard serial dilution of 0, 0.001, 0.01, 0.05, 0.1, 1, 5 and 10 ppm; and by a previously developed qualitative discriminatory concentration (0.5 ppm; if = 50 % germination is shown, it is considered as resistant). Also, radial growth from conidia germination was measured on the standard serial dilution. Interestingly, radial growth works well to detect resistance 30 days after inoculation, although timing can be a limitation. Interestingly, up to date, out of 12 historical isolates tested, five isolates (41 %) (ID # S_8, S-10, S_11, S_44 from IA; ID # S_24 from SD) from 2016 (two years before the official first report in IA) shown to be resistant to QoIs. These FLS and SBS that show phenotypic resistance will be confirmed using molecular protocols of target-fungicide resistance of G143A mutation by a previously established method. This phenotype resistance ratio matches a publication of 2022 showing widespread SBS resistance in the USA. These results showed a high prevalence of QoIs fungicide resistance in FLS and CLB/PSS in Iowa.

Updated October 25, 2024:
Objective 1: Fungicide Trials for Foliar Disease Management in Soybean
In both 2023 and 2024, we conducted comprehensive evaluations of 18 fungicides across seven Iowa locations, with treatments applied at the R3 growth stage and disease assessments conducted at R6. In 2023, Septoria brown spot (SBS) and frogeye leaf spot (FLS) were observed at low severity levels. Fungicides such as Veltyma, Affiance, and Miravis Top significantly reduced FLS compared to untreated plots, but no fungicide substantially impacted SBS severity. Yield response across locations was minimal, likely due to low disease pressure and reduced August precipitation. These findings were shared on the Iowa State ICM Blog and presented in over 15 events, reaching more than 1,000 attendees and highlighting the importance of integrated disease management strategies. In 2024, trials continued with the same fungicides at the same locations, with disease observations made throughout the season. While frogeye leaf spot and Septoria brown spot severity remained low, additional pathogens—possibly including Colletotrichum (Anthracnose) and other uncommon fungal species—were detected in foliar lesions. Pathogenicity testing is currently underway to assess the role of these fungi in foliar disease.

Objective 2: Fungicide Resistance Management Plan
In 2023, we collected isolates of Cercospora leaf blight (CLB), FLS, and SBS pathogens to evaluate resistance. Of the isolates tested, 67% had the G143A mutation, indicating QoI fungicide resistance. Preliminary diversity studies in Newton, IA, identified Cercospora cf. flagellaris as the predominant species. Sensitivity tests on azoxystrobin revealed widespread resistance, with six out of ten FLS isolates and five out of twelve SBS isolates from historical samples showing resistance. Notably, historical Septoria isolates were detected molecularly as Septoria sp., and some exhibited phenotypic QoI resistance two years before the first official report (Neves et al., 2022). EC50 estimates from conidial germination did not correlate with those from mycelial growth, though mycelial growth could serve as an alternative method for estimating EC50, albeit with a longer evaluation period (about one month post-inoculation). Historical Septoria isolates also demonstrated a higher fungicide resistance frequency than previously reported. Among confirmed QoI-resistant CLB, PSS, and seed endophyte isolates, 82.3% showed resistance, with C. cf. flagellaris being the predominant species in resistant/sensitive profiles (84%), followed by Cercospora sp. (7.6%), Cercospora kikuchii (2.5%), and Cercospora cf. sigesbeckiae (2.5%). In 2024, we isolated 18 Septoria and 12 FLS samples from different Iowa locations, which are currently being tested for molecular fungicide resistance.

In both 2023 and 2024, we tested 18 different fungicides across seven locations in Iowa to see how well they could manage soybean leaf diseases and help protect yields. In 2023, while the diseases Septoria brown spot and frogeye leaf spot were present, they were not very severe. Some fungicides, like Veltyma, Affiance, and Miravis Top, were effective in reducing frogeye leaf spot, but none had a strong impact on Septoria brown spot, and overall, the treatments did not boost yield much, likely due to low disease levels and dry weather in August. We shared these findings with farmers and agronomists through blog posts and presentations to stress the importance of using multiple methods to manage these diseases. In 2024, we continued the trials and again found low levels of these diseases, but we also detected some new fungi that might play a role in leaf disease. Meanwhile, we collected samples of common soybean leaf diseases like Cercospora to test for resistance to fungicides. Most of these disease samples showed some resistance to commonly used fungicides, confirming earlier findings. We’re also finding that some historical samples of Septoria brown spot, from as early as 2016, show fungicide resistance, which seems more common than previously thought. This highlights the need for diverse disease management approaches to help slow the spread of resistance in Iowa fields.

These findings are directly applicable to many soybean farmers and agribusinesses. This work would have beneficial immediate and long-term effects on farm productivity and input use. The knowledge gained from this research will benefit Iowa soybean farmer. Specific outputs from this project:
• Increased understanding of fungicide application at growth stage R3 under different growing conditions throughout Iowa.
• Information to assist farmers with identifying the most effective fungicides for foliar disease management.
• An ongoing effort to test fungi for possible fungicide-resistance.
• Increase understanding of fungicide application at growth stage R3 under different growing conditions. This was most clear in years with more rain and will continue to be important as fungicides loss their effectiveness.
• Further understand when and how fungicides will fit into an integrated pest management (IPM) strategy for disease management.
• We communicate our fungicide-resistance findings with chemical companies and have talked at Extension events about fungicide resistance.