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: Thu 21 Dec 2017
Source: Farm Futures [edited]
<http://www.farmfutures.com/fungicide/fungus-threatens-brazilian-farms>
Over almost 2 decades, Asian rust has infiltrated soybean fields across Brazil. The pathogen has become so resistant to chemicals meant to kill it that many farmers spray several times during their growing seasons, and still endure losses. The chemicals were ineffective almost half the time.
"Resistance [development] has been faster than research" into preventive measures, said Claudia Godoy, Brazilian Agricultural Research Corporation (Embrapa). There is mounting concern that fungicide used on soybeans may have reached a limit, at least until new chemicals are developed.
Fungicides have switched active ingredients at least twice as rust adapted. Most of the products in Brazil use carboxamide, but increased resistance to that [has emerged], said Godoy. "If fungus resistance to carboxamide spreads, soybeans may lose as much as 30 percent of output." There is no substitute for carboxamide, and the next chemicals probably won't arrive for [some] years. [There are also] delays on approvals of chemicals.
There are also signs that weather and wind make it difficult to control the spread. "Soybeans are in South American fields 12 months per year, making crops more susceptible to fungus," said Ricardo Balardin, Federal University of Santa Maria. "If we had 6 months with no soybeans, the situation would be quite different."
[Byline: Tatiana Freitas]
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Communicated by: ProMED-mail <promed@promedmail.org>
[Leaf rust (also called soybean rust, Asian rust) caused by the fungus _Phakopsora pachyrhizi_ can result in yield losses of soybean crops of up to 90 percent. It is widespread in Asia and Africa and spreading in the Americas, but data are lacking on its establishment in Europe.
Besides soybean, over 30 other species in the family of _Fabaceae_ can also be affected and may serve as pathogen reservoirs.
Spores are spread locally by splashing rain, or by wind over long distances. Several epidemic cycles may occur in one cropping season.
In Brazil, volunteer soybeans as pathogen reservoirs have been identified as a threat to new crops (ProMED-mail post http://promedmail.org/post/20120911.1287887). Fungicides may be used in disease management, but timing of applications is considered important. Partially resistant soybean lines are available. However, new fungal strains with increased virulence and fungicide resistances are emerging in soybean producing countries, including Brazil (ProMED-mail post http://promedmail.org/post/20090501.1644).
While the related species _P. meibomiae_ is also found in South America and the Caribbean causing similar symptoms, is not referred to as 'Asian' rust. This species is less virulent and considered of lesser economic significance. The item above refers to _P. pachyrhizi_ which has been established in Brazil since 2002.
Carboxamides are part of the SDHI (succinate dehydrogenase inhibitors) group of fungicides which generally induce cross resistance within the group. SDHI resistance has also been reported to emerge for other fungi (for example see ProMED-mail posts
http://promedmail.org/post/20170505.5014584 and http://promedmail.org/post/20160516.4225677). Rotating or mixing chemical classes of fungicides is vital to extend the useful life of any of the compounds.
Maps
Brazil (with states):
<http://en.18dao.net/images/9/96/Map-Brazil.jpg>
South America, overview:
<http://ontheworldmap.com/south-america/political-map-of-south-america.jpg>
Pictures
Leaf rust on soybean:
<http://www.ent.iastate.edu/images/plantpath/soybean/asianrust/asian_soyrust_lesions.jpg>,
<http://www.state.nj.us/agriculture/plant/soybeanrust.jpg>,
<http://www.wbhm.org/pics/features/soybean-icon.jpg> and <https://brunswick.ces.ncsu.edu/wp-content/uploads/2017/10/soybeanrustuuky.jpg>
Close-up of rust pustules:
<http://www.mississippi-crops.com/wp-content/uploads/2012/07/DSC00127.jpg>
Fungicide treated vs. untreated crops:
<http://www.cropprotech.com/newsletters/rust2.jpg> and <http://eidupont.scene7.com/is/image/eidupont/cp_A-327_soybean_rust_callout_2>
Electron microscopy of leaf rust pustules and spores:
<https://pesticideguy.files.wordpress.com/2013/02/30-soybean-rust.png>
and
<http://www.wbhm.org/pics/features/soybean-spores.jpg>
Links
Information on soybean leaf rust:
<http://www.apsnet.org/edcenter/intropp/lessons/fungi/Basidiomycetes/Pages/SoybeanRust.aspx>,
<https://www.cabi.org/isc/datasheet/40019>,
<https://nt.ars-grin.gov/taxadescriptions/factsheets/index.cfm?thisapp=Phakopsorapachyrhizi>,
<https://cropwatch.unl.edu/plantdisease/soybean/soybean-rust>,
<https://www.ncbi.nlm.nih.gov/pubmed/20447267>,
<https://www.pioneer.com/home/site/us/agronomy/crop-management/soybean-insect-disease/soybean-rust/>
and
<https://www.extension.umn.edu/agriculture/crop-diseases/soybean/soybeanrust.html>
Disease history and pathogen evolution:
<http://www.scielo.br/scielo.php?pid=S1415-47572008000500020&script=sci_arttext>
_P. pachyrhizi_ taxonomy:
<http://www.indexfungorum.org/Names/NamesRecord.asp?RecordID=121037>
_P. meibomiae_ taxonomy:
<http://www.indexfungorum.org/Names/NamesRecord.asp?RecordID=102469>
Information on carboximide fungicides via:
<http://www.frac.info/working-group/sdhi-fungicides/introduction-and-general-information>
and
<http://oregonstate.edu/dept/NWREC/sites/default/files/upload/pubs/McReynolds_Robert/project26/Ocamb.pdf>
Embrapa:
<https://www.embrapa.br/en/international>. - Mod.DHA]
[See Also:
2015
----
Leaf rust, soybean - Brazil: (MS)
http://promedmail.org/post/20150102.3066955
2012
----
Leaf rust, soybean - Brazil: (MT), pathogen reservoirs
http://promedmail.org/post/20120911.1287887
2010
----
Nematodes, white mould, rust, soybean - Brazil: (BA, MT)
http://promedmail.org/post/20100408.1128
Frogeye & leaf rust, soybean - Argentina, Brazil
http://promedmail.org/post/20100207.0420
Asian leaf rust, soybean - Cuba: 1st rep.
http://promedmail.org/post/20100204.0374
2009
----
Fungal diseases, soybean - USA
http://promedmail.org/post/20090817.2912
Leaf rust, soybean - Brazil: new strains
http://promedmail.org/post/20090501.1644
Leaf rust, soybean - USA: update
http://promedmail.org/post/20090428.1594
Leaf rust, soybean - Australia: (NSW) alert
http://promedmail.org/post/20090407.1351
2008
----
Soybean diseases - USA: (LA) http://promedmail.org/post/20081012.3231
Asian soybean rust - USA, Mexico: update
http://promedmail.org/post/20080928.3070
2007
----
Asian soybean rust - USA: 2007 final report
http://promedmail.org/post/20071230.4184
Asian soybean rust - Canada: 1st report, (ON)
http://promedmail.org/post/20071120.3751
Asian soybean rust - Congo DR: 1st report
http://promedmail.org/post/20070917.3086
Asian soybean rust - Ghana: 1st report
http://promedmail.org/post/20070820.2710
2006
----
Soybean rust, Asian strain - Brazil
http://promedmail.org/post/20060611.1628
Soybean rust - Mexico: 1st report
http://promedmail.org/post/20060219.0546
and older items in the archives]
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