A ProMED-mail post
http://www.promedmail.org
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International Society for Infectious Diseases
http://www.isid.org

Date: Fri 23 Dec 2022
Source: Indiana Prairie Farmer [summ. Mod.DHA, edited]
https://www.farmprogress.com/commentary/tracking-mystery-soybeans


For over 3 years, a small number of soybean plants in some fields were observed with strange symptoms near harvest. They remain greener than other plants, are spindly with few nodes, and have small pods, independent of crop variety. There is no apparent yield loss and no annual increase in numbers, but the condition is present every year.

Initially, the symptoms were thought to resemble tobacco ringspot virus (TRSV) [ProMED post 20191024.6745616]. When confirmation was sought in 2021, diagnostic testing at Purdue Plant and Pest Diagnostic Laboratory strongly indicated TRSV but was not conclusive. At a 2nd sampling, no TRSV was detected; some well-known fungal diseases were present or suspected but could not account for the unusual symptoms. Scientists also noted that the plants had similar symptoms to another sample that tested positive for TRSV.

So far, the cause of the symptoms remains unclear, the report concluded. It is being speculated that, considering the percentage of affected plants and their presence in multiple fields during different years, the symptoms could be due to a virus, environmental conditions, or genetic abnormalities.

[Byline: Tom J. Bechman]

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Communicated by:
ProMED

[_Tobacco ringspot virus_ (TRSV) is the type member of genus _Nepovirus_. The name indicates that generally, members of the genus are transmitted by nematodes. TRSV has a wide host range, including herbaceous as well as woody species. Symptoms vary with hosts and virus strain. Several TRSV strains can infect soybean. Symptoms on this host may include terminal distortion of shoots ("shepherd's crook"); spots, distortion, proliferations, and necrosis on leaves and buds; stunting and wilting of plants; delayed maturity; and reduced number of pods. The highest yield losses of up to 100% have been reported if infection occurs before flowering as bud blight.

TRSV can be easily transmitted by plant sap, such as on agricultural equipment. The virus is also seed transmitted at a rate depending on the host and timing of infection. Systemically infected soybean plants often produce infected seeds, which are the main source of virus carry-over to the next cropping season. Natural vectors of the virus are adults and larvae of dagger nematodes (_Xiphinema_ species), but the efficiency of these vectors is low in soybean. Local spread of TRSV in soybean cultures has been reported to be mainly by insects, including some aphid species and thrips. Disease management relies mainly on phytosanitation and use of certified clean seeds. A number of soybean cultivars appear to have some resistance to some of the virus strains.

TRSV was first reported in the US in 1941 and is now known to be present in many states. Generally, symptoms are unsuitable for disease diagnosis since the same symptoms can be due to several different causes, both infectious and environmental. Especially in field crops, a combination of different pathogens and/or other factors is common and may lead to symptoms atypical for each of the causes. Furthermore, the amount of viruses in different tissues and at different stages of the infection can vary to the extent that they may not be detectable at some samplings, thus a negative TRSV diagnosis in some samples, as stated above, does not preclude the presence of the virus in the host. It will be interesting to see what factors may be involved in the condition above.

Maps
USA (with states):
https://www.orangesmile.com/common/img_country_maps/usa-map-0.jpg

Pictures
TRSV symptoms on soybean:
https://crop-protection-network.s3.amazonaws.com/articles/SoyFG_Fig149a-Bud-Blight-Delayed-Maturity_Kevin-Black.jpg,
https://crop-protection-network.s3.amazonaws.com/articles/photos/SoyFG_Fig148-Bud-Blight-Pith-Discoloration_Kevin-Black.jpg,
https://crop-protection-network.s3.amazonaws.com/articles/SoyFG_Fig145-TRSV_Shepherds-crook_Kevin-Black.jpg and
https://crop-protection-network.s3.amazonaws.com/articles/photos/TRSV_Shepherds-crook_Loren-Giesler.jpg

Links
Information on TRSV:
https://www.dpvweb.net/dpv/showdpv/?dpvno=309,
https://doi.org/10.1079/cabicompendium.54202,
https://gd.eppo.int/taxon/TRSV00 (with distribution) and via
https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/tobacco-ringspot-virus
TRSV causing disease on soybean:
http://soydiseases.illinois.edu/index808b.html?category=diseases&disease=80,
https://www.sciencedirect.com/science/article/pii/0042682274903006?via%3Dihub and via
https://soybeanresearchinfo.com/soybean-disease/viruses/
Information on nematodes:
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7489.html#LIFE
Virus taxonomy via:
https://ictv.global/taxonomy
- Mod.DHA]

See Also

2019
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Tobacco ringspot virus, soybean - USA: (IN) 20191024.6745616
2018
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Grapevine viruses: USA, Pakistan 20180117.5566550
Cherry viruses - USA: spread 20180108.5545889
2015
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Tomato ringspot virus, pomegranate - Italy ex USA 20151118.3799622

Website: http://www.isid.org

Published: January 13, 2023