2019
A current assessment of foliar soybean diseases in Iowa
Contributor/Checkoff:
Category:
Sustainable Production
Keywords:
Crop protectionDiseaseField management
Lead Principal Investigator:
Steve Whitham, Iowa State University
Co-Principal Investigators:
Daren Mueller, Iowa State University
Project Code:
450-49-04
Contributing Organization (Checkoff):
Leveraged Funding (Non-Checkoff):
This proposed project focuses on fields in Iowa. We have submitted a project with a similar scope and funding level to the North Central Soybean Research Program with a focus on seed-transmitted foliar diseases found in other North Central states.
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Institution Funded:
Brief Project Summary:

This project focuses on identifying pathogens affecting the Iowa soybean crop during recent growing seasons to answer questions about what pathogens are present, if known pathogens have changed and new problems that could emerge. It includes a survey for pathogens causing known and unknown diseases, and work to associate causal organisms with diseases of unknown cause. Sharing results with diagnostic clinics will help form strategy to develop diagnostic assays for unknown diseases. For example, such work has confirmed the identification of new virus, Clover yellow vein virus, in a soybean plot in the recent growing seasons, and evidence suggests that infection by multiple viruses may be common.

Key Benefactors:
farmers, agronomists, Extension agents, diagnostic clinics

Information And Results
Project Deliverables

This project has a direct benefit to soybean farmers in Iowa because:
1. Farmers, agronomists and ag industry will have a better idea of pathogens infecting soybean, including new ones. We have made preliminary identification of new virus, Clover yellow vein virus, in a soybean plot in the 2016 growing season, and we have preliminary evidence that infection by multiple viruses may be common.
2. Researchers and diagnosticians will have information about possible new detection methods for soybean pathogens.
3. Preliminary data will be collected to revisit recommendations for screening soybean seed for presence of pathogens.
4. Background data will be collected to start assessing the role of infection by new pathogens on yield loss.

Final Project Results

Update:
During the 2019 growing season, we have collected leaves found growing in several Iowa counties from late summer through the fall. Combined with the 2018 growing season, we now have a total of 46 plant samples that were collected from different Iowa counties and stored at minus 80 degrees Celsius. The samples represent a wide spectrum of different symptoms that included diseases that could be tentatively identified and those that could not be identified. Samples that were collected displayed an array of symptoms including: Virus-like symptoms with discoloration or mosaic patterns and vein necrosis, Phyllosticta-like lesions with water-soaking (3 “zone” lines), brown veins and necrosis, stunted plants with heavily chlorotic leaves, big brown spots on pods, complete chlorosis and necrosis on leaves, unknown spots, several with “top die back” symptoms, and several with unknown symptoms.

We are trying to determine if there are biotic causes of leaf tissues with virus-like symptoms. A sample from Bruner farm tested positive for TRSV using TRSV ImmunoStrip test (Agdia). Leaf samples collected from North of LaPorte city on S. Canfield Rd with Top Dieback symptoms with numerous yellow patch on leaves tested positive for TRSV. Leaf samples collected from North of Dysart on county Rd, V37 with Top Dieback symptoms and leaf samples collected from Nevada-Pioneer (2 varieties) with Top Dieback symptoms both were positive for TSV using TSV ImmunoStrip test (Agdia). We had previously found TRSV and/or TSV in some plants that had top die back symptoms, but the results of these tests from 2019 samples suggest that the cause may not be either of these two viruses since we cannot find a 100% correlation between symptoms and a particular virus. We completed RNA sequencing of the 46 samples for 2018 and 2019 in late December. We are currently analyzing the RNA sequencing data using our established sequencing analysis pipeline to identify viruses, bacteria or fungal pathogens present in the samples. To test our ideas about the need for more soybean pathogen genomes, we also generated the DNA sequencing data need to construct draft genome sequence of the fungal soybean pathogens Septoria glycines, Cercospora sojina, and Cercospora flagellaris. Once the draft genomes are assembled, we will use it to reanalyze our RNA sequencing data from the 2016, 2017, 2018, and 2019 field samples to determine if we can now better identify the pathogens.

The RNA sequencing platform has been very useful in identifying and characterizing viruses present in plant samples displaying virus-like symptoms. Major results from this work are:
1. We have discovered and validated a virus that is new to soybean fields in Iowa, Clover yellow vein virus.
2. Tobacco ring spot virus and tobacco streak virus are commonly occurring in Iowa soybean fields.
3. We used RNA sequencing to assist in identifying the cause of a disease tentatively called top die back. The results are inconclusive as far as the cause being biotic, abiotic, or some combination.

The RNA sequencing platform was used to identify some bacterial and fungal pathogens present in field-grown soybean samples. However, there currently are limitations and assumptions that need to be overcome by adding more reference genome sequences for soybean pathogens to public databases that were used to make identifications. We will use the data generated in this project to test our ideas for improving identification of bacterial and fungal pathogens.

The United Soybean Research Retention policy will display final reports with the project once completed but working files will be purged after three years. And financial information after seven years. All pertinent information is in the final report or if you want more information, please contact the project lead at your state soybean organization or principal investigator listed on the project.