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http://www.promedmail.org

Source: Delta FarmPress, Louisiana State University (LSU) report [abridged, edited]
https://www.farmprogress.com/soybeans/lsu-student-identifies-fungus-causing-soybean-taproot-decline

LSU graduate student Teddy Garcia-Aroca has identified and named the new species of fungus _Xylaria necrophora_, the pathogen that causes soybean taproot decline. The fungus infects roots, causing them to become blackened while causing leaves to turn yellow or orange with chlorosis. The disease has the potential to kill the plant. Trey Price, LSU, said, "I've seen fields that suffered a 25% yield loss." [He] said the disease has been a problem for many years as pathologists struggled to identify it. Some incorrectly attributed it to related soybean diseases.

Garcia-Aroca compared samples from infected soybeans with samples from the LSU Herbarium and the US National Fungus Collections that were collected as far back as the 1920s. Some of the historical samples were collected [from] sugarcane but were not documented as pathogenic. Non-pathogenic samples from Martinique and Hawaii were also used in the comparison, along with the genetic sequence of a sample from China. Many of the historical specimens had [been] classified as _X. arbuscula_ that causes diseases on macadamia and apple, along with sugarcane in Indonesia. DNA sequencing showed a match for _X. necrophora_ for 5 of the historical, non-pathogenic samples, as well as sequences from the non-pathogenic specimen from China. All were consistently placed within the same group as the specimens causing taproot decline on soybeans.

"Events underlying the emergence of _X. necrophora_ as a soybean pathogen remain a mystery," the study concludes. A hypothesis is that the fungus could have been in the soil before soybeans were grown, feeding on decaying plant material, and eventually made the jump to live soybeans. Changes in the environment, new soybean genetics, and changes in the fungal population may have resulted in the shift.

The lifespan of the fungus is not known. [It] survives during the winter on buried plant debris. It is likely that soybean seeds become infected with the fungus after coming in contact with infected debris from previous crops. These hypotheses remain to be tested. Now that the pathogen has been identified, Price said, management strategies need to be refined. Crop rotation and tillage can be used to reduce incidence as well as tolerant varieties. The final chapter will be further research into the origins of this fungus.

[Byline: Bruce Shultz]

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ProMED
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[A new "mystery" disease of soybeans had been reported in the early 2000s as being widespread in the southern US (for example, ProMED post 20150609.3421625). It was named taproot decline and reported to occur in individual plants or clusters along the same row of the same area of a field if soybean was planted in consecutive years. This suggested soil transmission and possible mechanical transmission by farming activities.

An undescribed fungus was isolated from infected taproots (ProMED-mail post 20160211.4011151) and was found to have 97 percent sequence similarity, based on internal transcribed spacer regions, to species within the family Xylariaceae. Subsequently, it was found to be a member of the _Xylaria arbuscula_ species complex. In 2020, it was identified as a new species and named _X. necrophora_ (see links below).

Maps
USA:
https://www.mapsofworld.com/usa/large-us-map.html and
http://healthmap.org/promed/p/221
Individual states via:
http://www.mapsofworld.com/usa/usa-state-and-capital-map.html

Pictures
Taproot decline on soybean:
https://soybeansouth.com/wp-content/uploads/2016/02/taproot-decline-roots-web.jpg,
http://www.soybeansouth.com/wp-content/uploads/2015/12/nr20151218_taproot300.jpg,
https://agfax.com/wp-content/uploads/taproot-decline-soybeans-msu.jpg,
https://i0.wp.com/agfax.com/wp-content/uploads/soybean_taproot_decline_alabama_cooperative_extension.jpg,
http://www.mississippi-crops.com/wp-content/uploads/2017/07/SDS-vs-TRD.jpg,
http://www.mississippi-crops.com/wp-content/uploads/2015/05/IMG_1051.jpg, and
http://www.mississippi-crops.com/wp-content/uploads/2015/05/DSC_0590.jpg

Links
Information on soybean taproot decline:
https://cropprotectionnetwork.org/resources/articles/diseases/taproot-decline-of-soybean (with pictures),
https://www.lsuagcenter.com/profiles/aiverson/articles/page1536778578565,
https://news.utcrops.com/2018/09/taproot-decline-a-new-soybean-disease-for-tennessee/,
https://www.mssoy.org/uploads/files/2020-taproot-decline-ratings-lsu.pdf,
https://www.mssoy.org/article/progress-on-research-of-taproot-decline-trd-in-soybean,
http://agfax.com/2015/12/22/soybeans-new-taproot-decline-disease-hurting-south/, and
https://soybeansouth.com/departments/industry-news/mississippi-state-researchers-id-new-soybean-disease/
Characterisation of taproot decline pathogen:
https://www.sciencedirect.com/science/article/pii/S2352340917307552
https://doi.org/10.1094/PDIS-05-20-0947-PDN, and
https://doi.org/10.1094/PHP-01-17-0004-RS
_X. necrophora_ identification and taxonomy:
00275514.2020.1846965" target="_blank" rel="noopener noreferrer">https://doi.org/10.1080/ 00275514.2020.1846965
Genus _Xylaria_ taxonomy and current species list via:
http://www.indexfungorum.org/names/Names.asp

ProMED thanks the contributor who wishes to remain anonymous. - Mod.DHA]