This project seeks to optimize the number of fungicide applications (one versus two) and the interval between sequential fungicide applications for improved management of white mold in soybeans. The project will assess the profitability of a single versus two sequential fungicide applications targeting white mold in soybeans of early, mid and late 0-maturity; identify the optimal interval between sequential fungicide applications (7, 10, 12, or 14 days); and evaluate whether the low-cost, off-patent fungicide thiophanate-methyl (Topsin and generics) might be successfully utilized alone or in rotation with the fungicide boscalid (Endura) at the 40 fl oz/ac application rate permitted in a new supplemental label issued in May 2020. Applied at 40 fl oz/ac, Topsin is highly effective against white mold in dry beans. The standard recommendation of making sequential fungicide applications 10 to 14 days apart when targeting white mold in soybeans is not based on empirical data and differs from standard practice in other regions of the world. Fungicide residual activity declines with time, and a single fungicide application often does not confer satisfactory protection against white mold throughout the R2 to R4 growth stages when soybeans are most susceptible to white mold. Field trials will be conducted under overhead irrigation at sites with a history of white mold in Carrington and Oakes, ND. Testing will be conducted on each of three soybean varieties exhibiting intermediate susceptibility to white mold and differing in maturity (approx. 0.1, 0.5, and 1.0 maturity).

1. Quantify the profitability of making a single versus two sequential fungicide applications targeting white mold in soybeans of early, mid or late 0-maturity.
2. Optimize the length of the interval between successive fungicide applications (7, 10, 12 or 14 days).
3. Evaluate whether the low-cost, off-patent fungicide thiophanate-methyl (Topsin and generics) applied at 40 fl oz/ac can confer satisfactory white mold control in soybeans applied as a single application or with an optimized application interval when applied in rotation with Endura.

1. Development of recommendations on the number of fungicide applications (one versus two) that optimize soybean profitability under white mold pressure.
2. Optimization of the interval between successive fungicide applications when a second application is made.
3. Development of a low-cost alternative for effective management of white mold in soybeans.
4. Dissemination of results to North Dakota soybean growers, crop advisors, and extension personnel.

Update:
Completed work:
Soybeans were planted on May 27 in Carrington and on June 2 in Oakes; the relatively late planting date was due to the cold, wet spring. Because of concerns about delayed canopy closure associated with the relatively late planting date, soybeans were seeded to narrow (14-inch) rows. The experiments were established as a randomized complete block design with 8 replicates. White mold disease pressure is often highly variable over short distances, and a large number of experimental replicates was utilized to maximize the likelihood of each treatment being evaluated the same number of times in areas of high versus low disease pressure. Treatment plots were 25 feet long and 5 feet wide (consisting of four rows centered within the 5-foot width), with treatment plots established as pairs of 5-foot plots of different varieties. To facilitate overspray of treatments and capture any fungicide drift, pairs of treatment plots were separated by 5-foot wide non-harvested filler plots. Fungicides were applied with a 100-inch hand-held boom with six nozzles, each 20 inches apart (for a total spray width of 120 inches), and pressurized by compressed carbon dioxide. Applications were made by walking between each pair of 5-foot treatment plots. Fungicide spray volume was 15 gal/ac. Nozzles and application pressures were set such that the spray droplet size was calibrated relative to canopy characterstics. All applications were with TeeJet AIXR110015 flat-fan nozzles (Spraying Systems Company, Wheaton, IL) at 50 psi (coarse droplets); the soybean canopy was at or near closure at the first fungicide application and was closed in all subsequent applications. Parallel studies were established with each of four different soybean varieties at both study locations; this represented an increase from the three varieties per location proposed in the original grant proposal. Testing was conducted on the following varieties in Carrington: Xitavo ‘XO0602E’ (0.6 maturity), Asgrow ‘AG06X8’ (0.6 maturity), Xitavo ‘XO0731E’ (0.7 maturity), and Asgrow ‘AG09Xf0’ (0.9 maturity). Testing was conducted on the following varieties in Oakes: Asgrow ‘AG09Xf0’ (0.9 maturity), Asgrow ‘AG11X8’, Dairyland ‘DSR1120’ (1.1 maturity), and Peterson Farms ‘18X11N’ (1.1 maturity)’. The Asgrow and Peterson Farms varieties were Extend-type soybeans, the Xitavo varieties were Enlist-type soybeans, and the Dairyland variety was a Roundup-Ready 2 type variety. Fertility and weed management were conducted in accordance with best practices. In Carrington, fungicides were applied July 21 at the full R2 growth stage (98-100% of plants at the R2 growth stage, depending on the variety), July 28, July 31, August 2, and August 4. In Oakes, fungicides were applied July 24 at the full R2 growth stage (100% of plants at R2), July 31, August 3, August 5, and August 7. In Carrington, supplemental irrigation was applied via low-output rotating micro-sprinklers with a 20-foot spray radius established in a 20-foot offset grid pattern. Irrigation commenced at late vegetative growth and continued through the R4 growth stage, with irrigation delivered as needed to maintain the top half-inch of the soil moist (to facilitate production of apothecia and spores by the Sclerotinia pathogen) beginning at late vegetative growth and as needed to create conditions favor. In Oakes, supplemental overhead irrigation was applied via a linear irrigator as needed to optimize soybean agronomic performance. White mold was assessed on October 14, 17, 18 and 19 in Carrington and October 21 in Oakes when soybeans were at maturity. All plants in the second row (counted from the south) of each four-row plot were individually assessed for white mold severity on a 0 to 5 scale representing the percentage of the plant impacted by Sclerotinia stem rot: 0 = 0%, 1 = 1-25%, 2 = 26-50%, 3 = 51-75%, 4 = 76-99%, 5 = 100%. Soybeans were harvested October 20 in Carrington and October 21-22 in Oakes.

Preliminary results:
In Carrington, white mold pressure was high, and strong statistical separation in soybean yields was observed across treatments (Table 1). Results parallel findings from the first two years of this project. In the 0.6-maturity, 0.7-maturity and 0.9-maturity varieties assessed in Carrington, soybean yields were maximized when sequential fungicide applications were made 7 days apart. Whether applied alone or as the first application in a two-fungicide sequence, no statistical separation was observed between the fungicides Topsin (40 fl oz/ac) or Endura (5.5 oz/ac), but, on average, Endura was generally associated with slightly higher yields than Topsin.
In Oakes, white mold pressure was low, and no statistical separation and very little numerical separation in soybean yields was observed across treatments (Table 1). Disease pressure differed sharply across the study: While there was virtually no white mold on the south end of the study, white mold pressure was moderate on the north end of the study.

Work to be completed:
Disease assessments were taken by evaluating individual plants and reading disease ratings into a voice recorder, and the audio recordings of the disease assessments have not yet been transcribed. Transcription of the audio recordings should be completed by the end of December 2022, and disease data will be analyzed immediately thereafter. If the disease data from Oakes show that the area with moderate disease pressure coincided with the blocking of the experimental replicates, the data will be re-analyzed to exclude those replicates in which there was no white mold disease pressure.
This project represented the final year of a 3-year research effort. A multi-year data summary and final conclusions will be prepared in January 2023, and a user-friendly summary of results will be posted to the NDSU Carrington webpage by March 2023. Major results will be disseminated to stakeholders at winter crop meetings and summer plot tours from January to September 2023. Results from this project and a previous project evaluating the returns to one versus two fungicide applications targeting white mold will be combined for an academic publication, with work on that manuscript anticipated in winter 2023-2024.

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Update:
For all tables and figures referenced in this progress report, please see the attached report in the 'uploads section'.
Methods:
Testing was conducted from 2020 to 2022 on fields with a history of white mold at the NDSU Carrington Research Extension Center, located approx. 3.5 miles north of Carrington, and at the NDSU Robert Titus Research Farm, located approx. 3 miles south of Oakes. The Oakes site has sandy soils characteristic of commercial irrigated fields in that geography, and the Carrington site has sandy loam soils. In Oakes, supplemental overhead irrigation was applied via a linear irrigator as needed to maximize yield potential. In Carrington, supplemental irrigation was applied via rotating micro-sprinklers established in a 20-foot offset, overlapping pattern, with irrigation applied as needed to simulate a wet summer: Irrigation was applied as needed to keep the top inch of the soil moist during bloom and to facilitate 24-hour periods of canopy wetness two to three times a week from the R2 through the R4 growth stages. At each location each year, testing was conducted on multiple soybean varieties differing in soybean maturity (Table 1). Over the course of the 3-year study, varieties with all major herbicide traits (Roundup-Ready 2, Enlist, and Extend) were evaluated. Soybeans were seeded in early, mid or late May or early June, depending on conditions (Table 1). Plots consisted of four rows, each 14 inches apart, or three rows, 21 or 22.5 inches apart, planted 5 feet center-on-center; plot length was 25 feet at planting and 16 to 21 feet at harvest (Table 1). To prevent edge effects, alleys were not cut on the ends of plots until soybean maturity. In 2020 and 2021, every plot assessed for yield was separated by a non-harvested plot established to facilitate overspray of fungicide treatments and to capture any fungicide drift. In 2022, soybean varieties were planted adjacent to each other as pairs of 5-foot wide plots, and each pair of plots was separated by a non-harvested plot established to capture herbicide drift. Fungicides were applied at early to full R2 growth stage and 7, 10, 12, or 14 days later using a hand-held boom equipped with flat-fan TeeJet AIXR110015 nozzles. In 2020 and 2021, applications were made with a 57-inch hand-held boom equipped with four nozzles, each 19 inches apart, and applications were centered over the treatment plot with overspray extending into the adjacent non-harvested plots. In 2022, applications were made with a 100-inch hand-held boom equipped with six nozzles, each 20 inches apart, with applications made to a pair of varieties concurrently by walking in the middle of each pair of 5-foot plots. Applications were made at 70 psi (medium droplets) when canopy closure averaged <80%, at 60 psi (coarser medium droplets) when canopy closure averaged 80-90%, and at 40 or 50 psi (coarse droplets) when canopy closure averaged > 90% (Table 1). Fungicide spray volume was 15 gal/ac, and applications were pressurized with compressed carbon dioxide. In 2020, Endura (5.5 oz/ac) was applied once at the R2 growth stage or twice sequentially at R2 and 7, 10, or 14 days later. The efficacy of Endura was compared to Topsin (20 fl oz/ac) applied once at R2 or twice sequentially at R2 and 7, 10, or 14 days later. In 2021 and 2022, the efficacy of one vs. two applications of Endura was compared to a single application of Topsin at 40 fl oz/ac and Topsin at 40 fl oz/ac followed by Endura at 5.5 oz/ac 7, 10, 12, or 14 days later. Soybeans were assessed for white mold severity at the R7 or R8 growth stage by evaluating every plant in the middle row of three-row plots and every plant in the second or third row of four-row plots for the percent of the plant impacted by white mold (diseased or exhibiting unfilled pods due to a lesion girdling the stem below the pods). A 0 to 5 scale was utilized for disease assessments: 0 = 0%, 1 = 1-25%, 2 = 26-50%, 3 = 51-75%, 4 = 76-99%, 5 = 100% of the plant impacted by white mold.
Testing was conducted on each soybean variety at each location in a randomized complete block design with 6 or 8 replicates (Table 1). White mold disease pressure is often highly variable over short distances, and a large number of experimental replicates was utilized to maximize the likelihood of each treatment being evaluated the same number of times in areas of high versus low disease pressure.
Data from individual studies and combined analyses across studies were evaluated with analysis of variance. Soybean varieties were established in a consistent (non-randomized) pattern within each treatment block, and data from each soybean variety were analyzed as separate, parallel studies. In combined analyses across studies, the treatment averages in individual studies were treated as experimental replicates. The assumption of constant variance was assessed with Levene's test for homogeneity of variances and visually confirmed by plotting residuals against predicted values. The assumption of normality was assessed the Shapiro-Wilk test and visually confirmed with a normal probability plot. The assumption of additivity of main-factor effects across replicates (no replicate-by-treatment interaction) was evaluated with Tukey's test for nonadditivity. If data did not meet model assumptions, a systematic natural-log or cube-root transformation was applied to correct deviations from normality and/or homoskedasticity, and analysis of variance was conducted on the transformed data. Data subjected to a systematic transformation are identified in Tables 2-4 by the symbol '‡' (natural-log) or ‘‡‡’ (cube-root) placed at the top of the applicable column of data in the summary table. For ease of interpretation, non-transformed treatment averages are presented in the summary table. Analyses were conducted with replicate and treatment x replicate interaction in the model. F-tests were conducted utilizing replicate-by-main-factor interaction for the error term. Single-degree-of-freedom contrasts were performed for all pairwise comparisons of isolates; to control the Type I error rate at the level of the experiment, the Tukey multiple comparison procedure was employed. Analyses were conducted with replicate and treatment as main factor effects, and they were implemented in PROC UNIVARIATE and PROC GLM of SAS (version 9.4; SAS Institute, Cary, NC).

Results:
(1) Profitability of one versus two sequential fungicide applications
Two sequential fungicide applications targeting white mold were more profitable than a single application when white mold pressure was high. When white mold incidence exceeded 15% in non-treated soybeans at the end of the season, making two applications conferred a higher total financial return versus a single application (Table 2). The frequency with which fungicide applications were profitable was also higher for two applications versus a single application (Table 2). The choice of Endura versus Topsin in the first application of a two-application sequence had little or no impact on profitability; when two applications were made and disease pressure was high, the profitability of applying Endura (5.5 oz/ac) twice versus Topsin (40 fl oz/ac) first followed by Endura was very similar (Table 3).
When white mold incidence was below 15% in non-treated soybeans at the end of the season, the most profitable fungicide treatment was a single application of Topsin (40 fl oz/ac) at early to full R2 growth stage (Tables 2, 3). Making a single application of Endura or two sequential fungicide applications (Topsin followed by Endura or Endura followed by Endura) was less profitable. The frequency with which fungicide applications were profitable was higher when Topsin was applied once versus Endura applied once or fungicides applied twice. Assuming a $24/ac cost for Topsin (product plus application cost) and a $28/ac cost for Endura (product plus application cost), average profitability was also highest for a single application of Topsin versus a single application of Endura or two fungicide applications.
The level of white mold pressure at which two fungicide applications became profitable differed by soybean maturity and by the interval between applications (Figure 1). In 0.5 to 0.7-maturity soybeans, a second fungicide application became profitable, on average, when applications were made 7 to 10 days apart and end-of-season white mold incidence in non-treated soybeans was 41-44%. In 0.9-maturity soybeans, a second fungicide application became profitable, on average, when applications were made 7-10 days apart and when end-of-season white mold incidence in non-treated soybeans was 23-27%. In 1.1-maturity soybeans, a second fungicide application became profitable, on average, when applications were made 12-14 days apart and when end-of-season white mold incidence in non-treated soybeans was 17-19%.

(2) Optimum interval between fungicide applications
In soybean varieties of 0.6 or 0.7 maturity, sequential applications of fungicides were most effective against white mold when applied 7 days apart (Figure 2). When Endura (5.5 oz/ac) was applied twice sequentially, white mold severity was minimized and soybean yield was maximized in 5 of 5 studies with an application interval of 7 days (Figure 4, Tables 4-6). When Topsin (40 fl oz/ac) was applied first and Endura (5.5 oz/ac) was applied second, white mold severity was minimized and soybean yield was maximized in 4 of 5 studies with an application interval of 7 days (Figure 4, Tables 4-6).
In soybean varieties of 0.9 maturity, sequential applications of fungicides were most effective against white mold when applied 7 to 10 days apart. On average, a 7-day interval optimized white mold management, but the difference between 7- and 10-day intervals was less than in 0.6- or 0.7-maturity varieties (Figure 2). Soybean yield was maximized in 6 of 7 assessments with applications 7 days apart, but when temperatures were highly favorable for white mold (average daytime highs in the mid-70 to low 80°F range) 10 to 12 days after the first fungicide application, white mold severity was minimized and soybean yield was either maximized or unchanged with an application interval of 10 days (Figure 4, Tables 4-6).
In soybean varieties of 1.1 maturity, sequential applications of fungicides were most effective against white mold when applied 12 to 14 days apart. On average, a 14-day interval optimized white mold management (Figures 2, 3), but results were strongly influenced by testing conducted on a single variety at one location and one year (‘AG11X8’ soybeans; Oakes, 2021). Across individual studies, soybean yield was maximized with either a 12-day or a 14-day interval between fungicide applications (Figure 4, Tables 4-6).
(3) Efficacy of Topsin/generics versus Endura
When white mold incidence was less than 15% in non-treated soybeans at the end of the season, Topsin (40 fl oz/ac) and Endura (5.5 oz/ac) performed equivalently (Figure 5). A single fungicide application was sufficient; a second fungicide application (Endura, 5.5 oz/ac) 7 to 14 days after the first did not improve white mold management. Because the price of Topsin/generics is lower than Endura, applying Topsin was more profitable than Endura when disease pressure was low (Tables 2, 3).
When white mold pressure exceeded 15% in non-treated soybeans at the end of the season, Endura (5.5 oz/ac) showed slightly higher efficacy against white mold than Topsin (40 fl oz/ac). Statistical separation between Endura and Topsin was not observed, but a consistent trend of slightly reduced disease and slightly higher yield was observed for Endura (Figure 5). Because the price of Topsin/generics is lower than Endura, the profitability of applying Topsin and Endura was similar when disease pressure was high (Tables 2, 3).

Discussion:
The results suggest that two fungicide applications are needed for satisfactory white mold management in soybeans when disease pressure is high and that the optimal interval between sequential applications is contingent on soybean maturity length. In soybeans of 0.5 to 0.7 maturity, making two fungicide applications 7 days apart was profit-maximizing when white mold incidence exceeded 41-42% in non-treated soybeans at the end of the season. In soybeans of 0.9 maturity, making two fungicide applications 7 to 10 days apart was profit-maximizing when white mold incidence exceeded 23-25% at the end of the season. In soybeans of 1.1 maturity, making two fungicide applications 12-14 days apart was profit-maximizing when white mold incidence exceeded 17-19% at the end of the season. Under high disease pressure (>15% white mold incidence in non-treated soybeans at the end of the season), applying Endura (5.5 oz/ac) twice sequentially provided slightly better white mold management than Topsin/generics (40 fl oz/ac) followed by Endura (5.5 oz/ac), but profitability of both fungicide programs was similar. Under low disease pressure (<15% white mold incidence in non-treated soybeans at the end of the season), a single application of Topsin/generics (40 fl oz/ac) was profit-maximizing. Topsin (40 fl oz/ac) and Endura (5.5 oz/ac) exhibited similar efficacy against white mold, but Topsin was more profitable due to its lower cost.

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A single fungicide application targeting white mold is often insufficient in irrigated soybean production. This project sought to determine when a second fungicide application is profitable, the optimal interval between applications, and the efficacy of the lower-cost fungicide Topsin.
Research conducted:
Field trials were conducted under irrigation in Carrington and Oakes in 2020-2022. Three to four varieties were tested in each location each year, with testing focused primarily on 0.6 to 1.1 maturity varieties. Row spacing was 14, 21 or 22.5 inches. Endura (5.5 oz/ac) or Topsin (40 fl oz/ac) was applied at the early to full R2 growth stage (66 to 100% of plants at R2) either as a single application or prior to Endura (5.5 oz/ac) applied 7, 10, 12, or 14 days later. Applications were made in 15 gal/ac with a hand-held boom equipped with TeeJet AIXR11015 nozzles. Application pressure was 70 psi (medium droplets) when the canopy was open (< 80% closure), 60 psi (medium droplets) when canopy closure averaged 80-90%, and 40 or 50 psi (coarse droplets) when the canopy averaged > 90% closure.
Why the research is important:
Determining when a second fungicide application targeting white mold is likely to be needed, when the second application should be made, and whether a low-cost generic fungicide can be utilized facilitates profit-maximizing decision making.
Findings:
The optimal interval between sequential fungicide applications increased with soybean maturity (Figure 1). In 0.6 and 0.7-maturity soybeans, a 7-day interval between applications optimized white mold management. In 0.9-maturity soybeans, a 7- to 10-day interval was optimal. In 1.1-maturity soybeans, a 12- to 14-day interval was optimal. On average, a second fungicide application was profitable when end-of-season white mold incidence in non-treated plots reached 41-42% in 0.5 to 0.7-maturity varieties; 23-25% in 0.9-maturity varieties; and 17-19% in 1.1-maturity varieties. Under low white mold pressure (<15% incidence in non-treated soybeans at maturity), Topsin (40 fl oz/ac) and Endura (5.5 oz/ac) exhibited equivalent efficacy, and a single application of Topsin was profit-maximizing. Under high white mold pressure, a second fungicide application was profit-maximizing. Endura (5.5 oz/ac) applied twice provided slightly better white mold management than Topsin (40 fl oz/ac) followed by Endura (5.5 oz/ac), but it was not more profitable.
Benefits/recommendations:
In 0.5 to 0.7-maturity soybeans, two fungicide applications should be made 7 days apart if there is risk of severe white mold (>40% incidence). In 0.9-maturity soybeans, two fungicide applications should be made 7 to 10 days apart if there is risk of elevated white mold (>22% incidence). In 1.1-maturity soybeans, two fungicide applications should be made 12 to 14 days apart if there is risk of moderate to high white mold (>16% incidence). Topsin and generics (40 fl oz/ac) are an acceptable alternative to Endura (5.5 oz/ac) either as a single application or as the first application in a two-application sequence targeting white mold. When conditions favor white mold, previous research indicates the first fungicide application should be made at early to full R2 growth stage unless the canopy closes earlier.

FIGURE 1. Impact of fungicide application frequency and the interval between sequential applications on white mold management in soybeans; Carrington and Oakes, ND (2020-2022). The fungicide Endura (5.5 oz/ac) or Topsin (40 fl oz/ac) was applied at early to full R2 as a single application or prior to an application of Endura (5.5 oz/ac) 7, 10, 12, or 14 days later. Treatment averages followed by different letters are significantly different (P < 0.05 or P < 0.10 if an asterisk follows the letter).

This project will improve the profitability of soybean production in fields where Sclerotinia is a problem by identifying profit-maximizing strategies to improve soybean agronomic performance and profitability under white mold disease pressure.