New Fungi Discovered Causing Phomopsis Seed Decay of Soybean

New Fungi Discovered Causing Phomopsis Seed Decay of Soybean

CPN 5007. Published August 12, 2021. DOI: doi.org/10.31274/cpn-20210820-0

Kristina Petrovic, Institute of Field and Vegetable Crops, Novi Sad; Demetra Skaltsas, Diné College; Lisa A. Castlebury, U.S. Department of Agriculture Agricultural Research Service; Brian Kontz, South Dakota State University; Tom W. Allen, Mississippi State University; Martin I. Chilvers, Michigan State University; Nancy Gregory, University of Delaware; Heather M. Kelly, University of Tennessee; Alyssa M. Koehler, University of Delaware; Nathan M. Kleczewski, University of Illinois, Urbana-Champaign; Daren S. Mueller, Iowa State University; Paul P. Price, III, Louisiana State University; Damon L. Smith, University of Wisconsin-Madison; and Febina M. Mathew, South Dakota State University.

Summary

  • Phomopsis seed decay can result in reduced soybean seed quality during crop maturation.
  • In this research, soybean seeds were collected from fields with Diaporthe diseases from eight states to test for Phomopsis seed decay. 
  • While Phomopsis longicolla has always been considered the primary cause of Phomopsis seed decay, multiple species of Phomopsis were found associated with soybean seed exhibiting Phomopsis seed decay.
  • The discovery of additional fungi causing Phomopsis seed decay presents the need to create a comprehensive survey across U.S. soybean production areas. 

Phomopsis is an important fungal pathogen impacting both yield and seed quality of soybean production systems in the US, with losses exceeding 81 million bushels in 2018 alone (Bradley et al. 2020). In the United States, Phomopsis seed decay causes yield losses and reduced seed quality during soybean crop maturation. This disease results in tiny, shriveled seeds, which may have a white discoloration or powdery residue (Figure 1). While seed infected with species of Phomopsis may often show no visible symptoms, wet and warm weather conditions greatly facilitate Phomopsis seed decay. Delayed harvest as a result of late season rain and warmer temperatures have increased disease prevalence in recent years. 

Figure 1. Soybean seeds with symptoms and signs of Phomopsis seed decay (right) compared to healthy soybean seed (left).

Image: Kristina Petrović

Prior research has suggested that other species of Phomopsis are involved in Phomopsis seed decay (Mengistu et al. 2009Batzer and Mueller 2020). However, few studies have noted or attempted to identify these possible causal species. This study seeks to identify species of Phomopsis gathered from soybean fields in the United States to improve understanding of species diversity and ability to cause disease.

The Research

Soybean grain samples were collected from 17 locations in a total of eight states. Twenty-five seed (symptomatic and asymptomatic) from each lot were surface disinfested and grown on media under laboratory conditions for seven days. Any white fungal growth resembling Phomopsis was further studied and characterized based on growth characteristics (Figure 2). Fungal mycelium of 45 isolates were collected and used to perform a DNA extraction. Ten species of Phomopsis were discovered based on sequencing a specific region of the DNA. 

Figure 2. White fungal growth resembling Diaporthe from soybean seed on laboratory media. 

Image: Kristina Petrović

One isolate representing each of the10 Phomopsis species was investigated for pathogenicity on soybean seed. Pathogenicity was evaluated seven days after infection to determine the percentage of seeds and seedlings exhibiting characteristics of decay and necrosis possible. It was observed that all species were capable of causing seeds to decay and seedling necrosis. While the isolates of P. bacilloidesP. longicolla, and P. ueckerae caused greater percentage of decayed seeds, the isolate of P. aspalathi caused the greatest seedling necrosis.

Previous research indicated P. longicolla as the most important fungus causing Phomopsis seed decay in the United States. However, this study revealed that of the 45 isolates, only 27 (60%) were classified as P. longicolla (Figure 3). The remaining isolates included nine other Phomopsis species (six as Pcaulivora, five as P. ueckerae, one as P. aspalathi, one as P. kongii, one as P. sojae, one as P. unshiuensis, and three isolates of undetermined Phomopsis species) as contributors to Phomopsis seed decay. The additional species of Phomopsis found alongside P. longicolla suggest that soybean seed decay may arise from separate species operating together to form a disease complex.

Figure 3. Species composition of Phomopsis isolates taken from soybean seed samples in the northern and southern United States.  

This research revealed meaningful connections between Phomopsis seed decay, geographical locations, and environmental conditions where soybean seed samples were collected. Areas in the southern United States (Delaware, Louisiana, Mississippi, and Tennessee) have wider species diversity than areas in the northern United States (Iowa, Michigan, South Dakota, and Wisconsin) (Figure 3). While the weather conditions in southern United States is generally more favorable for disease development than the northern areas, more humid and wet weather conditions during the study (2017) resulted in the majority of seeds showing symptoms of Phomopsis seed decay derived from the northern United States.

Conclusion

This goal of this study was to characterize species of Phomopsis from soybean seed in the United States. The research isolated ten fungi from soybean seed, suggesting that P. longicolla may not be the sole cause of Phomopsis seed decay. Several species may contribute to Phomopsis seed decay, and are influenced by the geography, environment, and host physiology, forming a disease complex. The identification of new fungi and the expansion of previously known fungi reveal new perspectives on Phomopsis species diversity. With the discovery of multiple Diaporthe species infecting soybean seeds, and the international effort to merge fungal names to one genus (Wingfield et al. 2012), we suggest renaming Phomopsis seed decay to Diaporthe seed decay. Future research should develop a comprehensive survey to explore cases of Diaporthe seed decay in U.S. soybean production areas.

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This research was based on the following manuscript

Petrović, K., Skaltsas, D., Castlebury, L. A., Kontz, B., Allen, T. W., Chilvers, M. I., Gregory, N., Kelly, H. M., Koehler, A. M., Kleczewski, N. M., Mueller, D. S., Price, III, P. P., Smith, D. L., and Mathew, F. M. 2021. Diaporthe seed decay of soybean [Glycine max (L.) Merr.] is endemic in the United States, but new fungi are involved. Plant Disease. ArticleGoogle Scholar

References

Batzer, J. C. and Mueller, D. S. 2020. Soybean fungal endophytes Alternaria and Diaporthe spp. are differentially impacted by fungicide application. Plant Disease 104:52-59. Article / Google Scholar

Bradley, C., Allen, T., Tenuta, A., Mehl, K., and Sisson, A. 2020. Soybean Disease Management – Soybean Disease Loss Estimates From the United States and Ontario, Canada – 2018. Crop Protection Network. CPN 1018-18. Article

Mengistu, A., Castlebury, L., Smith, R., Ray, J., and Bellaloui, N. 2009. Seasonal progress of Phomopsis longicollainfection on soybean plant parts and its relationship to seed quality. Plant Disease 93:1009-1018. Article / Google Scholar

Wingfield, M. J., De Beer, Z. W., Slippers, B., Wingfield, B. D., Groenewald, J. Z., Lombard, L., and Crous, P. W. 2012. One fungus, one name promotes progressive plant pathology. Molecular Plant Pathology 13:604-613. Article / Google Scholar

Acknowledgements

Authors

Kristina Petrović, Institute of Field and Vegetable Crops, Novi Sad; Demetra Skaltsas, Diné College; Lisa A. Castlebury, U.S. Department of Agriculture Agricultural Research Service; Brian Kontz, South Dakota State University; Tom W. Allen, Mississippi State University; Martin I. Chilvers, Michigan State University; Nancy Gregory, University of Delaware; Heather M. Kelly, University of Tennessee; Alyssa M. Koehler, University of Delaware; Nathan M. Kleczewski, University of Illinois, Urbana-Champaign; Daren S. Mueller, Iowa State University; Paul P. Price, III, Louisiana State University; Damon L. Smith, University of Wisconsin, Madison; and Febina M. Mathew, South Dakota State University.

Reviewers

Martin Chilvers, Michigan State University; Andrew Friskop, North Dakota State University; Alyssa Koehler, University of Delaware; and Kiersten Wise, University of Kentucky.

Inititial draft and chart development by Brittany Eide.

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New Fungi Discovered Causing Phomopsis Seed Decay of Soybean

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