A ProMED-mail post
http://www.promedmail.org
ProMED-mail is a program of the
International Society for Infectious Diseases
http://www.isid.org

Date: Wed 9 Nov 2022
Source: Western FarmPress [summ. Mod.DHA, edited]
https://www.farmprogress.com/crop-disease/viruses-devastate-salinas-valley-lettuce


Salinas Valley lettuce growers are incurring serious losses from a set of plant diseases that continue to increase in frequency and intensity across the region. A grower said 2022 was a disaster due to the widespread outbreak, with large fields without a single head of harvestable lettuce. Of the 20% of lettuce acreage in Salinas Valley thought to be impacted this year [2022], about 1/3 was lost from disease.

Laboratory tests point to _Impatiens necrotic spot virus_ (INSV) and pythium wilt. The symptoms may be similar, which can make it difficult to determine the primary cause of death for a plant. Both pathogens largely responsible for the lettuce deaths occur independently, but one may enhance the other. INSV seems to be the trigger for pythium wilt as plant stress increases.

[Byline: Todd Fitchette]

--
Communicated by:
ProMED

[INSV
----
_Impatiens necrotic spot virus_ (INSV) was considered a strain of _Tomato spotted wilt virus_ (TSWV; type species of genus _Orthotospovirus_, previously _Tospovirus_) but has since been classified as a separate species in the genus. It has a wide host range and is of significant economic importance for many crop and ornamental hosts. The virus is distributed in Europe and North America and has also been identified in a few locations elsewhere. INSV is transmitted by the western flower thrips _Frankliniella occidentalis_; other thrips vectors have so far not been recorded.

Tospoviruses are an extremely important group of plant viruses able to infect a large range of crops. They are transmitted exclusively by thrips species, which are found worldwide. Weed species can serve as pathogen or vector reservoirs; tospovirus epidemiology is complex. Disease management is difficult but may include vector control (for glasshouse crops), use of clean planting material, control of weed hosts of virus and vectors, phytosanitary measures, as well as use of resistant crop cultivars. Due to their agricultural importance, research on tospoviruses has increased dramatically in recent years. As a result, many new species are being found and many more can be expected to exist.

Pythium wilt
---------------
_Pythium_ species cause root and stem rots on a wide range of plant species, including lettuce and other crops. Stress on plants from other infections (such as viruses, as mentioned above for INSV) and/or environmental factors increases their susceptibility to pythium diseases. The pathogens can survive in most soil types for long periods. They can be spread with infected materials (such as equipment, soil, plant debris), by mechanical means, and with water. Disease management may include the use of well-drained planting sites, crop rotation, and preventative fungicide treatments of planting materials.

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

Pictures
INSV symptoms on lettuce varieties:
https://agfstorage.blob.core.windows.net/misc/FP_com/2020/07/17/Lettuce_INSV_Greece.JPG,
https://www.growingproduce.com/wp-content/uploads/2020/06/necrosis_lettuce.jpg,
https://ucanr.edu/blogs/SalinasValleyAgriculture/blogfiles/11100.jpg,
https://ucanr.edu/blogs/SalinasValleyAgriculture/blogfiles/11105.jpg and
https://www.tricaldiagnostics.com/wp-content/uploads/2018/07/INSVGrnLf1.jpg
Thrips:
http://www.insectimages.org/images/768x512/4387048.jpg
Pythium wilt on lettuce:
https://ucanr.edu/blogs/SalinasValleyAgriculture/blogfiles/75228.jpg,
https://ucanr.edu/blogs/SalinasValleyAgriculture/blogfiles/75213_original.jpg and
https://vegetableswest.com/wp-content/uploads/sites/8/2020/10/Screen-Shot-2020-10-16-at-11.02.49-AM-850x590.png (co-infection with INSV)

Links
Information on INSV:
https://gd.eppo.int/taxon/INSV00
https://www.researchgate.net/publication/257652670_Impatiens_necrotic_spot_virus,
https://extension.psu.edu/impatiens-necrotic-spot-virus and
https://www.plantmanagementnetwork.org/pub/php/brief/2005/insv/
INSV in lettuce:
https://www.growingproduce.com/vegetables/how-to-manage-impatiens-necrotic-spot-virus-in-lettuce/ and
https://www2.ipm.ucanr.edu/agriculture/lettuce/Tospoviruses/
Review on tospoviruses, impact, and global crop risks:
https://doi.org/10.1146/annurev-virology-100114-055036
Virus taxonomy via:
https://ictv.global/taxonomy
Information on thrips vector:
https://www.planthealthaustralia.com.au/pests/western-flower-thrips/ and
https://www2.ipm.ucanr.edu/agriculture/onion-and-garlic/thrips/
Information on pythium wilt of lettuce:
https://www.tricaldiagnostics.com/2018/10/26/pythium-wilt-main-root-rot-disease-of-lettuce/
https://www.agrivi.com/blog/pythium-disease-management/ and
https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=43736 (co-infection with INSV; with pictures)
Genus _Pythium_ taxonomy and species list via:
http://www.indexfungorum.org/names/names.asp
- Mod.DHA]

See Also

2021
----
Impatiens necrotic spot virus, lettuce - USA: (CA) 20211008.8698914
2020
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Impatiens necrotic spot virus, lettuce - Greece: 1st rep, emerging 20200728.7618822
2019
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Impatiens necrotic spot virus, capsicum - USA: (MA) 20190611.6514103
2008
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Tospoviruses, lettuce - USA: (CA) 20080601.1758
2005
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Impatiens necrotic spot, potato - USA (NY) 20050312.0735
2004
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Pythium leaf blight, lettuce - Spain (Asturias) 20041221.3363
2002
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Pythium sp., lettuce - South Africa 20021005.5473
2000
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Impatiens necrotic spot tospovirus, peanuts - USA 20000530.0869
1999
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Impatiens necrotic spot tospovirus - Israel 19991107.1993
and additional items in the archives

Website: http://www.isid.org

Published: November 18, 2022