Modern Integrated Management Practices for Controlling White Mold of Soybean

CPN 5009. Published March 11, 2022. DOI: doi.org/10.31274/cpn-20220314-1

Richard W. Webster, University of Wisconsin-Madison; Mitchell G. Roth, The Ohio State University; Brian Mueller, University of Wisconsin-Madison; Daren S. Mueller, Iowa State University; Martin I. Chilvers, Michigan State University; Darcy E. P. Telenko, Purdue University; Jaime F. Willbur, Michigan State University; Spyridon Mourtzinis, University of Wisconsin-Madison; Shawn Conley, University of Wisconsin-Madison; and Damon Smith, University of Wisconsin-Madison.

Summary

• Fungicide applications remain an effective tool for reducing white mold levels if applied between the R1 and R3 growth stages.

• Not all fungicide products are equally effective at controlling white mold, with Endura® remaining the most effective product if applied between the correct growth stages.

• If planting in row spacings of 15 inches or less and in fields with a history of white mold, use a seeding rate no greater than 110,000 seeds/ac.

• If planting in fields with a history of severe white mold, widen the row spacing to 30 inches and use a seeding rate no greater than 110,000 seeds/ac.

• Reducing seeding rates can reduce the need for fungicide inputs and balance partial profits, especially when soybean prices are lower.

Introduction

Soybean production in the Upper Midwest region of the United States and Canada is frequently threatened by the development of white mold (also known as Sclerotinia stem rot) (Roth et al. 2020). The causal agent of white mold, Sclerotinia sclerotiorum, is known to persist in the soil for up to five years as hard black structures called sclerotia, which resemble rodent droppings (Adam and Ayers 1979). Upon the occurrence of conducive weather conditions (prolonged temperatures between 46-70°F, high moisture, and soybean canopy closure), these sclerotia germinate to develop small, circular tan mushrooms (known as apothecia), which release infectious spores (known as ascospores) into the canopy of the soybean crop (Clarkson et al. 2004; Willbur et al. 2019a; Michael et al. 2020). If these spores then land on soybean flowering tissues, infection into the main stem of the soybean occurs. This indicates that the soybean crop is most susceptible to infection during the flowering periods, between the R1-R3 (beginning flower to beginning pod) growth stages (Fehr et al. 1971). After infection occurs, the fungus may fully restrict the main stem of the plant leading to the reduced ability for water transport. This ultimately results in premature plant death and poor grain yield. Towards the later stages of infection, the fungus begins to form new sclerotia on both the interior and exterior of the stem and pods. These sclerotia return to the soil and serve as overwintering structures and the next susceptible crop’s (dry bean, pea, potato, soybean, sunflower, etc.) inoculum source.

            Management practices for controlling the development of white mold have been studied extensively because white mold is such a detrimental disease. It has previously been shown that using a wide row spacing can lead to less white mold compared to a narrow row spacing due to a delay in the row canopy closure, which reduces moisture levels and modifies light penetration, ultimately leading to a reduction in sclerotia germination and development of apothecia (Grau & Radke, 1984; Fall 2018). Lowering seeding rates has also shown promising results for reducing white mold levels (Lee et al. 2005). Fungicide applications are an additional effective method for managing white mold, but differences are found among fungicide products (Willbur et al. 2019b). Research results have shown that the most effective time to spray fungicides is between the R1 and R3 growth stages when the soybean crop is most susceptible to infection (Willbur et al. 2019b). Once the disease has already been established within the soybean plants, fungicide applications have a minimal effect in reducing disease severity and protecting against yield losses. To improve upon the timing of fungicide applications, risk prediction models have been developed which give daily recommendations on whether an application should be made or not (Willbur et al. 2018a,b). These models, publicly available as Sporecaster® on smartphones, use site-specific weather data and calculate a risk level for the presence of the apothecia during the soybean flowering time. With proper use of Sporecaster®, unnecessary fungicide applications may be avoided, helping to increase profitability and decrease the risk for selection of fungicide resistant pathogen populations. Despite prior research on all of these practices individually, a modern integration of these practices together has not been examined.

Research goals

• Evaluate the effect of foliar fungicide applications on the development of white mold under field conditions

• Evaluate the effect of integrating row spacing, seeding rates, and fungicide applications on the development of white mold

• Examine the partial profits associated with seeding rates and fungicide applications

The research

A set of five fungicide trials were performed in 2021. One trial was performed in Indiana, Michigan, and Wisconsin, and two trials were performed in Iowa. These small-plot trials examined 15 unique pesticide programs (Table 1) and a non-treated control. In each of these trials, the development of white mold was assessed at the R6 growth stage (Fehr et al. 1971), and yield was recorded at harvest. From this study, pesticide programs differed in their development of white mold levels in a measurement called disease severity index (disease severity index P < 0.01; Fig. 1). The disease severity index score is a combination of both disease incidence (percentage of plants infected) and the severity of disease (degree of disease within sections of the plot). Applications with Endura® following either the recommendations of the Sporecaster® tool or at the R1 and R3 growth stages resulted in the lowest disease severity index levels. Applications of Omega® at the R3 growth stage by spraying below the canopy (Wisconsin: TeeJet drop line with a double swivel nozzle body; Iowa: TeeJet drop line with a 360 nozzle body) also resulted in low disease severity index levels, which demonstrated the potential of applying fungicide products directly to sites of infection. Yield across the fungicide programs did not differ (P = 0.13; Fig. 2), but the greatest yield was observed with a single application of Endura at R1.

Table 1. List of pesticide programs used for foliar fungicide trials in 2021 across the Upper Midwest (N=5).

^x Growth stage at fungicide application: R1- one open flower present on plant, R2-one open flower on one of two uppermost nodes, R3-pod is  inch long at one of four uppermost nodes

^y Types of applications made: Foliar – applications were made over the top of the crop; Under canopy – applications were made under the crop canopy using a TeeJet drop line attached with either a double swivel adjustable nozzle body or a 360 nozzle body.

^z Fungicide applications were made as recommended by Sporecaster tool for each respective environment. The growth stage of application may differ as each environment was subjected to different weather conditions.

Another set of trials examined the integration of row spacing (15 and 30 inches), seeding rates (110,000, 140,000, 170,000, and 200,000 seeds/ac), and two fungicide programs (Standard = growth stage-dependent program with applications of Aproach® at R1+R3 and Model = dependent program with applications of Aproach® as recommended by the Sporecaster tool) and a non-treated control. A total of 18 site-years across the Upper Midwest region were performed from 2017 to 2019, with seven site-years in Wisconsin, five site-years in Minnesota, four site-years in Iowa, one site-year in Illinois, and one site-year in Michigan.

            In these trials, wide row spacing of 30-in led to decreased white mold levels compared to a narrow row spacing of 15-in (P < 0.01, Fig. 3A). The use of the standard fungicide program decreased white mold further, to the lowest levels observed in this study (Fig. 3A). A seeding rate of 110,000 seeds/ac resulted in the lowest white mold levels with all three higher seeding rates resulting in similarly high levels of disease severity index (P < 0.01, Fig. 3B). The 15-in row spacing yielded greater than the 30-in row spacing (P < 0.01, Fig. 4A). Yield was highly responsive to increases in seeding rates within a narrow row spacing. Conversely, yields were less responsive to increases in seeding rates within a wide row spacing (Fig. 4A). The use of the standard fungicide program with two applications of Aproach® yielded higher than the model program and the non-treated check (P < 0.01, Fig. 4B).

            Additionally, the quantity of sclerotia produced at the end of the season was predicted based on previous research demonstrating that 0.9 pounds of sclerotia are produced per acre for each 10% of disease incidence of white mold (Lehner et al. 2017). From these predictions, planting at 110,000 seeds/ac resulted in the lowest predicted levels of sclerotial production while the highest seeding rate, 200,000 seeds/ac, had the greatest levels of sclerotia returned to the soil (P < 0.01, Table 2). The standard fungicide program also reduced the predicted quantity of sclerotia produced (P < 0.01, Table 3).

Table 2. Effects of soybean seeding rates on partial profits and estimated Sclerotinia sclerotiorum inoculum production when white mold was either absent or present.

^z Sclerotial production (SP) was calculated following the equation determined by Lehner et al. (2017): SP = Disease Incidence(%) x 0.9.

Table 3. Effects of fungicide programs on partial profits and estimated Sclerotinia sclerotiorum inoculum production when white mold was either absent or present.

^z Sclerotial production (SP) was calculated following the equation determined by Lehner et al. (2017): SP = Disease Incidence(%) x 0.9.            

When performing economic analysis, site-years examined were sub-grouped depending on the development of white mold. If a site-year had a mean non-treated disease severity index level greater than 0.5%, that site-year was considered to have white mold present. Conversely, if the mean non-treated disease severity index level was less than 0.5%, that site-year was considered absent of white mold. In the site-years with white mold absent, the 110,000 seeds/ac resulted in the lowest economic return, while 200,000 seeds/ac resulted in the highest economic return regardless of the grain sale price (Table 2). However, with white mold present, 200,000 seeds/ac resulted in the lowest economic return, and 170,000 seeds/ac resulted in the greatest economic return (Table 2). The use of fungicides profited similarly for all treatments at the highest grain sale price regardless of the presence or absence of white mold (Table 3). Yet, in site-years with white mold absent, fungicide applications lowered the partial profits for both grain sale prices of $9/bu and $12/bu (Table 3). With white mold present, partial profits at $9/bu were lower due to fungicide use, but at $12/bu, partial profits were similar for all treatments (Table 3).

Conclusion

These results improve our understanding of the effectiveness of integrated management practices for controlling white mold. The use of fungicides is an effective tool for reducing white mold levels, with some products performing better than others. The performance of these programs can be improved by optimizing application timing between the R1 and R3 growth stages. Endura® applications advised by the Sporecaster® tool or at both the R1 and R3 growth stages resulted in the lowest white mold levels. With the high cost of fungicide applications, the potential for reducing one or two spray applications by following the Sporecaster application could lead to economic savings. Management of white mold can be further improved by considering row spacing and seeding rate in conjunction with fungicide applications. It is recommended that use of wide row spacings and low seeding rates (< 140,000 seeds/ac) should be used in fields with a history of severe white mold. In fields with a history of moderate levels of white mold, the use of low seeding rates or prescription seeding rate technology regardless of row spacing should be used to minimize white mold and subsequent loss of yield. Finally, the quantity of sclerotia produced due to different management practices should be considered as this will help to protect yields for future production of soybean and other susceptible crops.

This research update is based on the work described in the following peer-reviewed research articles

Webster, R. W., Roth, M. G., Mueller, B. D., Mueller, D. S., Chilvers, M. I., Willbur, J. F., Mourtzinis, S., Conley, S. P., and Smith, D. L. 2022. Integration of row spacing, seeding rates, and fungicide applications for control of Sclerotinia stem rot in Glycine max. Plant Dis. Https://doi.org/10.1094/PDIS-09-21-1931-RE

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Acknowledgements

Authors

Richard W. Webster, University of Wisconsin-Madison; Mitchell G. Roth, The Ohio State University; Brian Mueller, University of Wisconsin-Madison; Daren S. Mueller, Iowa State University; Martin I. Chilvers, Michigan State University; Darcy E. P. Telenko, Purdue University; Jaime F. Willbur, Michigan State University; Spyridon Mourtzinis, University of Wisconsin-Madison; Shawn Conley, University of Wisconsin-Madison; and Damon Smith, University of Wisconsin-Madison.

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