This proposal addresses the priority area of Soybean Pest Management. It focuses on research into managing against yield and quality limiting effects of stem diseases. Soybean stem diseases are widespread and damaging across Minnesota. They can kill plants or may be undetected and yet cause yield losses. A better understanding of when and where stem diseases occur and how to manage them more should result in yield benefits in many regions. This proposal is directed at brown stem rot (BSR) and pod and stem blight, two common stem diseases. The goals are to identify resistance to these diseases in breeding lines and varieties adapted to Minnesota, determine prospects for disease management with fungicides and crop rotation, and understand the distribution and risks of soybean stem diseases. The occurrence and types of stem diseases (e.g., BSR, stem canker, pod and stem blight, charcoal rot) may to be changing in response to cropping and environmental factors, suggesting that soybean growers and their advisors better understand these risks to plan for appropriate management. This proposed research builds on a related set of projects conducted in the previous year and addresses short- and long-term goals to manage important soybean stem diseases.

1. Assess and improve strategies to manage brown stem rot of soybean.
2. Advance management of pod and stem blight of soybean
3. Understand the distribution and prevalence of important soybean stem and root diseases/pathogens in Minnesota.

o Identify resistance to both prevalent types of the BSR pathogen in an expanded selection of soybean breeding lines and varieties from the U of MN soybean-breeding program.
o Determine if promising new methods to evaluate soybean for resistance to BSR are more consistent and have higher throughput than current methods.
o Determine the effects of seed treatment fungicides and potential alternative hosts of the BSR pathogen on risk and management of BSR.
o Identify the levels of resistance to the pod and stem blight pathogens in soybean breeding lines from the U of MN soybean-breeding program and select commercial varieties.
o Develop and extend updated information on the management, distribution, and risks of key soybean stem and root diseases for Minnesota soybean growers.

Updated September 5, 2024:
Minnesota Soybean Research and Promotion Council

Research Progress Report
August 30, 2024

Project Title: Addressing Management Challenges with Soybean Stem Diseases in Minnesota

Principal Investigator(s): Dean Malvick

Project Period Ending August 31, 2024
Research Question/Objectives: This project focuses on the priority area of soybean pest management. It centers on research into managing against yield and quality limiting effects of stem rots. Stem diseases are widespread and problematic across Minnesota. They often clearly damage plants and yield, but also go undetected and cause yield losses. Understanding how to manage soybean stem diseases more effectively could potentially result in yield increases in many areas. This project targets management strategies for the diseases brown stem rot (BSR) and pod and stem blight (PSB). BSR is a common and important soybean disease in Minnesota. Crop rotations and resistant varieties can suppress BSR, however, neither method is a complete solution to managing the disease. Because BSR is a continuing problem for soybean production in Minnesota, more information is needed to understand risk factors and disease management options. The BSR pathogen (Cadophora gregata) is reported to only infect soybean, adzuki bean, and mung bean, but the host range is poorly documented and understood; and the efficacy of seed treatment fungicides for managing BSR is unknown. Pod and stem blight is also a common and problematic disease in Minnesota soybean fields and causes more damage and yield loss than is recognized. Soybean varieties vary in resistance to PSB and crop rotation may be beneficial, but this disease and the best methods to manage it are poorly understood. In summary, this project is addressing short and long-term goals to manage two common and damaging soybean stem diseases: brown stem rot (BSR) and pod and stem blight (PSB).

Project Updates for this period:
Objective one, focused on brown stem rot.
• Field and greenhouse studies are underway to determine if any of the common seed treatments are effective against BSR or the causal pathogen. Field studies were established in Waseca and Rosemount with different seed treatments in May 2024. The studies have been progressing well throughout the summer. Crop development is 1-2 weeks behind normal in those studies, and we expect BSR to develop and the plants to be ready for evaluation in the first 2-3 weeks in September. Thus, we do not have results yet from the field studies. Lab and growth chamber studies for fungicide efficacy have been proceeding throughout the summer, and the preliminary results from suggest that some fungicides have suppressive effects on the BSR pathogen, although the effective concentration may be higher than the typical doses used to treat seeds. More testing and analysis will be done in the next project period to develop more solid and conclusive results. The studies in the growth chamber are also underway and will be completed later in the fall.
•The host range of the BSR pathogens is unknown, which could have implications for management of BSR with crop rotation. Twenty-seven crop and weed species were evaluated in greenhouse experiments to determine if they can serve as hosts of the BSR pathogen. Although the final analysis of those plants and results is continuing, initial results confirm that mung bean is a host, and suggest that some edible bean (Phaseolus vulgaris) varieties may also be hosts of C. gregata. Final testing and analysis of the plant samples will be completed this fall to confirm these results and to determine if other plants are hosts of the BSR pathogen.
•Our work to develop and validate an improved and higher throughput method to evaluate soybean lines and varieties for resistance to BSR has provided additional results. Initial results suggest that it may be feasible to evaluate resistance within a month after planting in a greenhouse vs 2 months. Additional experiments have been conducted using two different lab testing methods, and data are still being obtained and analyzed. Experiments will continue into the fall and winter. The initial results suggest that this method based on quantity of the pathogen in soybean stems, could potentially reduce cost, time, and challenges with evaluating resistance to BSR caused by both types of the BSR pathogen.

Objective two, focused on pod and stem blight.
•This project period focused on additional field and greenhouse studies to study development and management of pod and stem blight of soybean. One large experiment was completed in the greenhouse, but for reasons unknown very little developed following the use of the same methods and check varieties as used previously. Fluctuating greenhouse temperatures in the summer may have suppressed disease development. Another experiment was started in the greenhouse in mid-August. Field studies are also underway. Plants were inoculated using two methods in field studies in St Paul. Like BSR in the field studies, it is too early for full disease development and evaluation of the plots and plants, thus we will be evaluating the plants in September based on plant staging and development.

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This proposed project will address the need for improved understanding and management of key soybean stem diseases that reduce soybean yields across Minnesota. Enhanced understanding of where stem diseases occur and managing them more effectively should lead to increased soybean yields in many fields. Results will be transferred through newsletters, production meetings and field days, scientific meetings, and agricultural news outlets. The ultimate benefit to soybean growers will be increased yields and reduced risk of lost yields due to disease.