Research Highlights

Biology and Management of Soybean Stem Diseases

By Leonor Leandro, Iowa State University

Soybean stem diseases, such as sudden death syndrome (SDS), white mold, charcoal rot and stem canker, can severely limit crop yields across the U.S. In order to protect soybean yield, development of genetic resistance to these diseases, improved management practices and a deeper understanding of how the different pathogen strains affect soybean would all be valuable. This $347,543 project will deliver soybean varieties with improved resistance to stem diseases, and will advance knowledge of the biology of the pathogens that cause those diseases, allowing for improvements in management of soybean stem diseases.

The main goals of this project are to:

• Increase availability of genetic resistance to stem diseases in soybean varieties
• Understand the identity and diversity of soybean pathogens that cause stem disease and determine fungicide resistance present in those pathogens
• Advance ability to predict stem disease progression
• Improve stakeholder awareness of stem disease pathogens and best management practices for diseases.

Initial plant-breeding efforts focused on characterizing advanced breeding lines with resistance to SDS, charcoal rot, white mold and stem canker. Field trials were established in Iowa, Michigan and South Dakota. Successful development of disease symptoms allowed assessment of breeding lines for resistance, and a range of resistance levels were observed from replicated tests. These efforts will allow release of soybean varieties with disease ratings, so that growers can make decisions based on their needs. In addition, private seed companies will be able to utilize this information and, after licensing public germplasm, will be able to develop more disease-resistant cultivars. 

Next, genetic studies were conducted to determine the pathogenic variability, host preference and fungicide sensitivity among isolates of Macrophomina phaseolina, a seed and soilborne pathogen that causes many of these stem diseases. Genomic regions related to fungicide insensitivity were identified and are being analyzed from 96 isolates. Protocols were developed and optimized to conduct infection studies with Macrophomina phaseolina, charcoal rot pathogen and Fusarium virguliforme, the SDS pathogen.

In order to expand the white mold disease forecasting model nationally, the existing soybean Sclerotinia stem rot (SSR) advisory tool, Sporecaster, was revised to account for varietal resistance in soybean by including new thresholds determined by field trials in Wisconsin and Michigan. 

Finally, field trials were conducted in Wisconsin, Iowa and Mississippi to study the role of management strategies, including the use of genetic resistance on disease development. One goal was to identify soybean varieties with a high level of resistance to white mold that are stable across the North Central region. Several commercial varieties were identified that appear to have good physiological resistance in the greenhouse and acceptable field resistance in multiple environments. These varieties were tested in Wisconsin, Michigan and Iowa in the 2019 field season. Disease data was collected, and yield data collection is ongoing. 

The information obtained will be shared with stakeholders in multiple formats, through outreach materials designed to improve stakeholder awareness of stem disease pathogens and best management practices for diseases. 

As more is learned about the causes, progression and management of soybean stem diseases, more can be done to manage these devastating problems, allowing growers to increase yield and improve soybean profitability.

Published: May 14, 2020