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.

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

• Increased understanding of fungicide application at growth stage R3 under different growing conditions through out Iowa.
• Information to assist farmers with identifying the most effective fungicides for foliar disease management.
• An established fungicide-resistance plan and ability to test for possible fungicide-resistance.
• Management strategies will be analyzed within each state, field, and subfield to develop recommendations for specific subfield characteristics.
• Recommendations for specific targeted management subfields will be developed from information gathered from field trials to improve best management practices for soybean fields.

Update:
Objective 1: Compare new fungicide products with industry standards for foliar disease management and yield protection
A total of 15 fungicides containing single and multiple active ingredients were evaluated at each location. Fungicide treatments were laid out in randomized complete block design with four replications. Plot size ranged from 25.5 to 40 ft long and 10 to 15 ft wide (4-6 rows 30-inch inter-row spacing). Soybean cultivar NK 28-T3XF was planted in all locations. Corn was planted in previous year in all the locations. Planting dates across the locations and other field activity details are provided in Table 1. 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 (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. 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 (caused by Cercospora sojina) and Cercospora leaf blight (caused by Cercospora species) if present in 10 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.

Result summary
In 2022, the precipitation pattern during the season was different across the locations, Ames, Kanawah and Nashua received higher cumulative precipitation (May to September) (>15 inch) than the other locations. However, the total precipitation and precipitation occurred in August, the critical month for foliar disease development, was fairly low in most of the locations. Frogeye leaf spot and Septoria brown spot were the two diseases observed most frequently but both occurred at very low levels in all locations. Where frogeye leaf spot did show up, its development started extremely late in the growing season (end of August). Fungicide effect was not statistically significant at all locations for Septoria brown spot and 5 locations for frogeye leaf spot, perhaps because of the very low level of disease. There was significant effect of fungicides on severity of frogeye leaf spot in Ames and Nashua. Across the seven locations, Miravis Neo, Lucento, VGR90, Delaro Complete, Regev HBX, Quadris Top, Veltyma, Revytek and Acropolis significantly lowered (34-71%) frogeye leaf spot severity as compared to untreated control (UTC) where Delaro Complete, Veltyma and Lucento had lower severity (65-71%). No other foliar diseases were observed at significant level in any locations.
As compared to untreated control (UTC), yield was significantly higher in Miravis Neo at McNay and in Domark 230, Lucento, Quadris Top and Veltyma at Sutherland. However, overall yield response to fungicide across the seven locations was not statistically significant (Figure 1). In 2022, overall soybean yield was similar to 2021 but greater than 2020 (Mueller et al. 2021, 2020). Check plots, when all the locations were combined, yielded 64.6 bu/A. Overall, no fungicides produced statistically greater yield than check although yield in some fungicide treatment was numerically greater.

Objective 2: Establish a fungicide resistance management plan in Iowa
Isolation of Cercospora leaf blight/Purple seed stain (PSS), Frogeye leaf spot (FLS), and Septoria brown spot (SBS) diseases from IA was done on 2022 (N= 62 isolates from CLB/PSS, N= 35 isolates from FLS, N= 10 isolates from SBS) and more isolation still underway. Randomly selection was done of 30 isolates including the three diseases in origin from historical isolates from Mueller’s lab (2016–2019). Fungicide sensitivity (N= 10 CLB isolates) is being studied on azoxystrobin technical ingredient on conidia germination by previous established procedure using a serial dilution of 0, 0.001, 0.01, 0.03, 0.05, 0.1, 1 and 10 ppm. Fungicide sensitivity (N= 10 SBS isolates) is being studied on azoxystrobin technical ingredient on conidia germination by previous established procedure using a qualitative discriminatory concentration (0.5 ppm). Fungicide sensitivity (N= 10 FLS isolates) is being studied on azoxystrobin technical ingredient on conidia germination by previous established procedure using a qualitative discriminatory concentration (1 ppm). Then, we will use the qualitative discriminatory concentrations for the big collection of 2022. Next, we will identify them by phylogenetic analysis using specific primers on standard PCR. Likewise, isolates with low sensitivity will be subjected to molecular confirmation of target fungicide resistance of G143A mutation by previous established method. In addition, we are validating a a quick and less-effort labor fungicide test on conidia germination to ease the task of monitoring resistance of foliar diseases.

Management recommendations
Soybean fungicides can provide yield benefits when there are foliar diseases. Some reports show yield benefit of fungicides at low levels of disease or when disease was absent but this is not consistent. Cercospora sojina and Septoria glycines strains resistant to QoI fungicides were confirmed throughout Iowa a few years ago. Judicious use of fungicides and an integrated method of disease management that does not depend only on fungicides, should be used to manage the diseases and preserve the efficacy of existing fungicides. Disease management practices such as crop rotation, planting disease-resistant cultivars, and application of fungicides with multiple modes of action can all be used slow selecting for fungicide resistance.

We have also started prepping for the upcoming year - where we plan to establish fungicide trials at seven ISU research farms again. We also plan to collect more isolates of pathogens to test for fungicide resistance.

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 seven Iowa locations. Two new fungicides were included in 2023, which were Adastrio and Propulse. Fungicide treatments were laid out in randomized complete block design with four replications. Plot size ranged from 28 to 40 ft long and 10 to 15 ft wide (4–6 rows 30-inch inter-row spacing). Soybean cultivar P29A19E was planted in all locations. Corn was planted in previous year in all the 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. Other foliar fungal diseases were also assessed, including frogeye leaf spot (FLS caused by Cercospora sojina) and Cercospora leaf blight (CLB caused by Cercospora species) if present in 10 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% moisture and yield was calculated in bu/ac.

In 2023, the precipitation pattern during the season was different across the locations (https://mesonet.agron.iastate.edu/request/daily.phtml?network=ISUSM), for example, Ames(CRF), Armstrong (SWFR) , Sutherland (NWRF), and McNay (SCRF) received more cumulative precipitation (May to September) (>13.6 inch) than the other locations. However, a low percent of the total precipitation occurred in August, the critical month for foliar diseases. Then, foliar disease development, was fairly low in most locations. Frogeye leaf spot and SBS were the two diseases observed most frequently, but both occurred at very low levels in all locations. Where FLS did show up, its development started extremely late in the growing season (end of August). No statistical fungicide effect was found in FLS in any location compared with the untreated control. Also, no statistical difference was found in yield between treatments in any location. A complete report of disease levels and yield responses will be published in the ICM News in December.

Objective 2: Establish a fungicide resistance management plan in Iowa.

Leaves with symptoms of CLB, FLS, and SBS were collected from different locations across Iowa. Some leaves were from plots treated with fungicide (e.g. plots from statewide trial) and some were from fields with no fungicide treatment. We then isolated the pathogens that cause CLB, FLS, and SBS. In total, ~100 CLB isolates were obtained, ~50 FLS isolates, and ~50 SBS isolates were collected. For all the isolations, we disinfected the leaves, put the leaves in humid chambers and picked individual conidia (spores) from the leaves and transferred to petri dishes.

In order to best study if isolates of certain fungi are resistant to different classes of fungicides, we need to get as many conidia as possible to test. An experiment was set using CLB (purple seed stain) Cercospora species growing on five different media (DV8, PGM, SSLB, carrot, clarified V8) at 12 h light/12 h dark (25°C) for 14 days. The experiment was completed twice. In both experimental runs, we did not observe any differences or increases in sporulation of the fungi in any of the media tested. We then decided to test Cercopsora species for fungicides resistance to QoIs fungicides based on radial mycelial growth inhibition.

Fungicide sensitivity (N= 10 FLS isolates from IA 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). Out of the ten isolates, 6 showed to be phenotypically resistant to QoIs (IDs # 86, 87, 90, 92, 98, 99).

Fungicide sensitivity (N= 16 SBS isolates— historical isolates from 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). Interestingly, up to date, out of 11 historical isolates tested, five isolates (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) were resistance to QoI fungicides. We will confirm resistance to both the FLS and SBS isolates that show phenotypic resistance using molecular techniques, by identifying if they have the G143A mutation.

We are looking at several isolates of the fungi that cause CLB (formerly called Cercospora kikuchii) to properly identify what species causes CLB in Iowa. We sampled leaves from five different fields, including a variety trial, with increased levels of CLB near Newton, IA. Leaf symptoms were classified based on the advance of the disease in purple color, red brick color, and blight and from the fields they were sampled from (N=5). To encourage conidia sporulation, we placed the symptomatic leaves in a humid chamber using 12h dark/12h fluorescent light or 16h light (fluorescent + black-UV)/8h dark at 25°C.

Soybean fungicides can provide yield benefits when there are foliar diseases. Some reports show yield benefit of fungicides at low levels of disease or when disease was absent, but this is not consistent. Cercospora sojina and Septoria glycines strains resistant to QoI fungicides were confirmed throughout Iowa a few years ago. Judicious use of fungicides and an integrated method of disease management that does not depend only on fungicides, should be used to manage the diseases, and preserve the efficacy of existing fungicides. Disease management practices such as crop rotation, planting disease-resistant cultivars, and application of fungicides with multiple modes of action can all be used slow selecting for fungicide resistance.

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 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.