Soybean Seed Treatments and Oomycete Fungicide Resistance Testing

Martin Chilvers, Michigan State University; Austin McCoy, Michigan State University; Zachary Noel, Auburn University; Alejandro Rojas, University of Arkansas; Alison Robertson, Iowa State University; Travis Faske, University of Arkansas; Daren Mueller, Iowa State University; Damon Smith, University of Wisconsin-Madison; Albert Tenuta, Ontario Ministry of Agriculture, Food and Rural Affairs; Kiersten Wise, University of Kentucky.

Fungicide seed treatments are commonly used to manage oomycete (water molds) seedling diseases caused by Pythium and Phytophthora species on soybean. A variety of fungicides have activity against oomycetes including metalaxyl, mefenoxam, azoxystrobin, pyraclostrobin, and newer products such as ethaboxam, oxathiapiprolin and picarbutrazox. These fungicides vary in efficacy against these diseases and are often added in combinations for optimum efficacy. Product ratings for fungicide active ingredients can be found at CPN-1020-W: Fungicide Efficacy for Control of Soybean Seedling Diseases

Metalaxyl is a mixture containing equal amounts of “R” and “S” enantiomers (molecules that are mirror images of each other). The R-enantiomer has more fungicidal activity than the S-enantiomer. Mefenoxam, which is also known as metalaxyl-M, contains primarily the R-enantiomer. Mefenoxam and metalaxyl have been used as a soybean seed treatment for many years, and repeated use of fungicide active ingredients can lead to fungicide resistance. There have been concerns of mefenoxam and metalaxyl resistance in both Pythium and Phytophthora. Ethaboxam, oxathiapiprolin, and picarbutrazox are newer fungicides that have been promoted to manage oomycete diseases in addition to mefenoxam or metalaxyl. 

Research goals 

• Develop a tool that can quickly screen oomycetes for fungicide resistance

• Assess fungicide resistance within oomycete species 

• Evaluate the effect of temperature on oomycete fungicide resistance

• Improve management of seedling diseases

The research 

Performing fungicide resistance testing on many individual oomycete strains (isolates) is time consuming and costly. To more quickly screen these isolates, a high-throughput fungicide resistance assay capable of screening up to 30 oomycete isolates at a time was developed (Noel et al. 2019a). This assay makes it possible to perform fungicide resistance testing on hundreds of isolates, saving time and resources compared to traditional methods (Figure 1). This will allow researchers to screen more oomycete isolates to better judge if resistance is developing to our fungicide seed treatments. 

In a survey of soybean seedlings from across the Midwest, 84 oomycete species were identified (see CPN 5003: Detection and Prevalence of Oomycete Seedling Diseases on Soybean and Rojas et al. 2017). Representatives of these 84 oomycete species were tested for resistance to mefenoxam (Noel et al. 2019a; Noel et al. 2019b). Results indicated that mefenoxam resistance is rare, and not widespread. Another short-term (two year) field study was conducted using soybean seed treated with mefenoxam and ethaboxam. Pieces of soybean roots from treated seeds were collected and different oomycete species growing from these root pieces were identified using molecular methods, while fungicide resistance was also tested. No evidence of a shift in oomycete species or fungicide resistance was observed due to mefenoxam over these two years.

In contrast, there was substantial evidence that the variation in response to ethaboxam is related to differences between oomycete species (Noel et al. 2019b; Figure 2). These data are important when choosing seed treatment active ingredients and for understanding the breadth of fungicide activity. Ethaboxam is active against the most abundant oomycete species associated with soybean, and species that were not affected by ethaboxam were sensitive to mefenoxam. Therefore, if a history of oomycete seedling disease is suspected in a given field, seed treatments containing more than one fungicide active ingredient are recommended. However, it will be important to continue to monitor for fungicide resistance over time (Figure 3). Current research into the breadth of activity with other seed treatment fungicides like pyraclastrobin, oxathiapiprolin, and picarbutrazox are underway. 

In addition, recent research has highlighted how different oomycete species that attack soybean and corn seedlings are adapted to different temperatures. Although cool temperatures that impose stress on the plant contribute to seedling disease, the recent work has highlighted that some oomycete species prefer cool temperatures while others favor warm temperatures. These recent findings also demonstrate that the disease temperature optima for the various Pythium species corresponded to higher fungicide EC50 values. These findings have implications in efforts to tailor seed treatment blends and the screening of soybean varieties for seedling and root rot resistance (Matthiesen et al. 2016, Rojas et al. 2017c).  

In future studies we would like to understand how the use of seed treatments could contribute to oomycete fungicide resistance development. We would also like to better understand seed treatment and oomycete interactions with other fungal seedling pathogens like Rhizoctonia and Fusarium.

Earn Certified Crop Advisor CEUs

Successfully complete a quiz for this publication to earn 0.5 CCA CEUs here.

This research summary was based on the following manuscripts

Matthiesen, R., L., Ahmad, A. A., and Robertson, A. E. 2016. Temperature affects aggressiveness and fungicide sensitivity of four Pythium spp. that cause soybean and corn damping off in Iowa. Plant Disease, 100(3):583-591. Article / Google Scholar

Noel, Z. A., Rojas, A. J., Jacobs, J. L., & Chilvers, M. I. 2019a. A High-Throughput Microtiter-Based Fungicide Sensitivity Assay for Oomycetes Using Z ′-Factor Statistic. Phytopathology, 109(9):1628-1637. Article / Google Scholar

Noel, Z. A., Sang, H., Roth, M. G., & Chilvers, M. I. 2019b. Convergent evolution of C239S mutation in Pythium spp. β-tubulin coincides with inherent insensitivity to ethaboxam and implications for other Peronosporalean oomycetes. Phytopathology, 109(12):2087-2095. Article / Google Scholar

Rojas, J. A., Jacobs, J. L., Napieralski, S., Karaj, B., Bradley, C. A., Chase, T., Esker, P. D., Giesler, L. J., Jardine, D. J., Malvick, D. K., Markell, S. G., Nelson, B. D., Robertson, A. E., Rupe, J. C., Smith, D. L., Sweets, L. E., Tenuta, A. U., Wise, K. A., and Chilvers, M. I. 2017a. Oomycete Species Associated with Soybean Seedlings in North America—Part I: Identification and Pathogenicity Characterization. Phytopathology, 107(3):280-292. Article / Google Scholar   

Rojas, J. A., Jacobs, J. L., Napieralski, S., Karaj, B., Bradley, C. A., Chase, T., Esker, P. D., Giesler, L. J., Jardine, D. J., Malvick, D. K., Markell, S. G., Nelson, B. D., Robertson, A. E., Rupe, J. C., Smith, D. L., Sweets, L. E., Tenuta, A. U., Wise, K. A., and Chilvers, M. I. 2017b. Oomycete Species Associated with Soybean Seedlings in North America—Part II: Diversity and Ecology in Relation to Environmental and Edaphic Factors. Phytopathology, 107(3):293-304. Article / Google Scholar

Rojas, J. A., Miles, T. D., Coffey, M. D., Martin, F. N., and Chilvers, M. I. 2017c. Development and Application of qPCR and RPA Genus- and Species-Specific Detection of Phytophthora sojae and P. sansomeana Root Rot Pathogens of Soybean. Plant Disease, 101:1171-1181. Article / Google Scholar

Acknowledgments 

Authors

Martin Chilvers, Michigan State University; Austin McCoy, Michigan State University; Zachary Noel, Auburn University; Alejandro Rojas, University of Arkansas; Alison Robertson, Iowa State University; Travis Faske, University of Arkansas; Daren Mueller, Iowa State University; Damon Smith, University of Wisconsin-Madison; Albert Tenuta, Ontario Ministry of Agriculture, Food and Rural Affairs; Kiersten Wise, University of Kentucky.

All photos were provided by and are the property of the authors and reviewers.

The authors wish to acknowledge the Michigan Soybean Promotion Committee, North Central Soybean Research Program, the United Soybean Board, and the United States Department of Agriculture - National Institute of Food and Agriculture (USDA-NIFA) for generous support of this research. 

This publication was developed by the Crop Protection Network, a multi-state and international collaboration of university/provincial extension specialists and public/ private professionals that provides unbiased, research-based information to farmers and agricultural personnel.

This information in this publication is only a guide, and the authors assume no liability for practices implemented based on this information. Reference to products in this publication is not intended to be an endorsement to the exclusion of others that may be similar. Individuals using such products assume responsibility for their use in accordance with current directions of the manufacturer.