Soybean Stem Zone Lines: Fact and Fiction
FACT: The presence of soybean stem zone lines is associated with stem disease in the Diaporthe-Phomopsis disease complex. This disease complex includes Phomopsis seed decay caused by D. longicolla, pod and stem blight caused by D. sojae, 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 years, resulting in misdiagnosis of the disease.
Zone lines associated with Diaporthe species appear on the inside of lower soybean stems and roots when split longitudinally, or if the outside layer of the stem is scraped away (Figure 1). Lines are thin and dark, appearing in irregular patterns and small circular shapes in mature soybean plants.
Figure 1. Split soybean stem or scrape away outside layer to show “zone lines” caused by Diaporthe fungi
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).
Figure 2. Microsclerotia within and on the surface of soybean stem or root tissue
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 bottom line 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 thought. This discovery provides a clearer picture of soybean diseases and the symptoms that result from pathogen infection. Accurate disease identification is the first step to making better disease management decisions, which improves farm profitability and stewardship.
Figure 3. Soybean stem showing symptoms of infection by M. phaseolina and Diaporthe species
Kiersten Wise, Purdue University; Daren Mueller, Iowa State University; Carl Bradley, University of Kentucky; Martin Chilvers, Michigan State University; Anna Freije, Purdue University; Loren Giesler, University of Nebraska-Lincoln; Adam Sisson, Iowa State University; Damon Smith, University of Wisconsin; and Albert Tenuta, Ontario Ministry of Agriculture, Food, and Rural Affairs
All photos were provided by the authors except Figure 3, which is the property of and was provided by Teresa Hughes.
The Soybean Disease Management series is a multi-state and international collaboration sponsored by the United Soybean Board and the North Central Soybean Research Program (NCSRP). This project was funded in part through Growing Forward 2 (GF2), a federal-provincial territorial initiative. The Agricultural Adaption Council assists in the delivery of GF2 in Ontario. The authors thank the United States Department of Agriculture - National Institute of Food and Agriculture and the Grain Farmers of Ontario for their support. Contributors to this series come from land-grant universities in the North Central states and Canada.
This information is provided only as a guide, and the authors assume no liability for practices implemented based on this information.
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