Soybean Stem Zone Lines: Fact and Fiction

***Updated in 2025, this version replaces the previous 2016 Soybean Stem Zone Lines: Fact and Fiction publication.***

FACT: The presence of soybean stem zone lines is associated with stem disease in the Diaporthe-Phomopsis disease complex. This disease complex includes Diaporthe seed decay caused by D. longicolla and several other Diaporthe species, pod and stem blight caused by D. sojae and D. longicolla, and stem canker caused by D. caulivora and D. aspalathi.

FICTION: Soybean stem zone lines are associated with the stem disease charcoal rot, caused by the fungus Macrophomina phaseolina. However, this association of soybean stem zone lines and charcoal rot is incorrect and has persisted for several decades, leading to frequent misdiagnosis of the disorder.

Zone lines associated with Diaporthe species are visible inside the lower soybean stems and roots when split longitudinally or when the outer stem layer is scraped away (Figure 1). Lines are thin and dark, appearing in irregular patterns and small circular shapes in mature soybean plants.

Signs of charcoal rot are tiny, dark survival structures called microsclerotia within and on the surface of the lower stem and taproot. Microsclerotia cause light gray discoloration or a charcoal-like appearance of these plant parts (Figure 2).

However, signs and symptoms of charcoal rot and zone lines may appear in the same soybean stem, because simultaneous infection by M. phaseolina and Diaporthe species can occur in the same plant (Figure 3).

The key takeaway is that zone lines on the interior of soybean stems are associated with Diaporthe fungi and not the fungus that causes charcoal rot, as previously reported. This discovery provides a clearer picture of soybean diseases and the symptoms that result from pathogen infection. Knowing the difference is crucial for selecting the appropriate resistant varieties for breeding programs focused on disease resistance. Accurate disease identification ultimately leads to better disease management, improving farm profitability and stewardship.

Acknowledgements

Authors

Kiersten Wise, University of Kentucky; Daren Mueller, Iowa State University; Carl Bradley, University of Kentucky; Martin Chilvers, Michigan State University; Maira Duffeck, Oklahoma State University; Dylan Mangel, University of Nebraska-Lincoln; Adam Sisson, Iowa State University; Damon Smith, University of Wisconsin-Madison; and Albert Tenuta, Ontario Ministry of Agriculture, Food, and Agribusiness.

Reviewers

Mandy Bish, University of Missouri; Travis Faske, University of Arkansas; David Langston, Virginia Tech; Horacio Lopez-Nicora, The Ohio State University; Dean Malvick, University of Minnesota; John Mueller, Clemson University; Edward Sikora, Auburn University; Darcy Telenko, Purdue University; and Wade Webster, North Dakota State University.

Sponsors

The Soybean Disease Management series is a multi-state and international collaboration sponsored by the United Soybean Board. The authors thank the United States Department of Agriculture - National Institute of Food and Agriculture and the Grain Farmers of Ontario for their support.