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International Society for Infectious Diseases

Date: Thu 21 Dec 2017
Source: Farm Futures [edited]
< 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]
Communicated by:

[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 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 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 20170505.5014584 and 20160516.4225677). Rotating or mixing
chemical classes of fungicides is vital to extend the useful life of
any of the compounds.

Brazil (with states):
< 96/Map-Brazil.jpg>
South America, overview:
< south-america/political-map- of-south-america.jpg>

Leaf rust on soybean:
< images/plantpath/soybean/ asianrust/asian_soyrust_ lesions.jpg>,
< agriculture/plant/soybeanrust. jpg>,
< 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:
< newsletters/rust2.jpg> and
< is/image/eidupont/cp_A-327_ soybean_rust_callout_2>
Electron microscopy of leaf rust pustules and spores:
<https://pesticideguy.files. soybean-rust.png>
< features/soybean-spores.jpg>

Information on soybean leaf rust:
< edcenter/intropp/lessons/ fungi/Basidiomycetes/Pages/ SoybeanRust.aspx>,
< datasheet/40019>,
< taxadescriptions/factsheets/ index.cfm?thisapp= Phakopsorapachyrhizi>,
< plantdisease/soybean/soybean- rust>,
< pubmed/20447267>,
< site/us/agronomy/crop- management/soybean-insect- disease/soybean-rust/>
<https://www.extension.umn. edu/agriculture/crop-diseases/ soybean/soybeanrust.html>
Disease history and pathogen evolution:
< php?pid=S1415- 47572008000500020&script=sci_ arttext>
_P. pachyrhizi_ taxonomy:
< Names/NamesRecord.asp? RecordID=121037>
_P. meibomiae_ taxonomy:
< Names/NamesRecord.asp? RecordID=102469>
Information on carboximide fungicides via:
< group/sdhi-fungicides/ introduction-and-general- information>
< NWREC/sites/default/files/ upload/pubs/McReynolds_Robert/ project26/Ocamb.pdf>
< international>. - Mod.DHA]

[See Also:
Leaf rust, soybean - Brazil: (MS) 20150102.3066955
Leaf rust, soybean - Brazil: (MT), pathogen reservoirs 20120911.1287887
Nematodes, white mould, rust, soybean - Brazil: (BA, MT) 20100408.1128
Frogeye & leaf rust, soybean - Argentina, Brazil 20100207.0420
Asian leaf rust, soybean - Cuba: 1st rep. 20100204.0374
Fungal diseases, soybean - USA 20090817.2912
Leaf rust, soybean - Brazil: new strains 20090501.1644
Leaf rust, soybean - USA: update 20090428.1594
Leaf rust, soybean - Australia: (NSW) alert 20090407.1351
Soybean diseases - USA: (LA) 20081012.3231
Asian soybean rust - USA, Mexico: update 20080928.3070
Asian soybean rust - USA: 2007 final report 20071230.4184
Asian soybean rust - Canada: 1st report, (ON) 20071120.3751
Asian soybean rust - Congo DR: 1st report 20070917.3086
Asian soybean rust - Ghana: 1st report 20070820.2710
Soybean rust, Asian strain - Brazil 20060611..1628
Soybean rust - Mexico: 1st report 20060219.0546
and older items in the archives]
............................... lm
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