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.

Updated December 2, 2022:
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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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.