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First reports of Asian leaf rust on soybean in Malawi and Costa Rica


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

In this posting:

[1] Malawi

[2] Costa Rica

******

[1] Malawi

Date: March 2015
Source: Plant Disease [edited]
<http://apsjournals.apsnet.org/doi/abs/10.1094/PDIS-09-14-0924-PDN>

[Ref: HM Murithi et al (2015): First Report of _Phakopsora pachyrhizi_ Causing Rust on Soybean in Malawi. Plant Disease 99, 420; DOI:

10.1094/PDIS-09-14-0924-PDN]

---------------------------------------

Soybean rust (SBR) caused by _Phakopsora pachyrhizi_ has become established in Africa since the 1st report in Uganda in 1996. [It has] infested new regions of Africa, including Ghana and Democratic Republic of the Congo in 2007, and Tanzania in 2014. No refereed reports have been published about rust in Malawi, but some people have indicated that soybean rust may have been observed [there] as early as 2008.

Typical symptoms of SBR were observed in May 2014 in the major soybean-growing areas of Malawi, including the central and southern regions, in 9 out of 12 sites surveyed. To confirm the pathogen, DNA [from symptomatic leaves] was analyzed by quantitative PCR for _P.

pachyrhizi_ and _P. meibomiae_. _P. pachyrhizi_ DNA was detected in all samples, indicating a substantial infection. _P. meibomiae_ DNA was absent from all samples. Urediniospores from 3 leaves produced tan lesions after 2 weeks of incubation in a detached-leaf assay.

This is the 1st confirmed report of _P. pachyrhizi_ causing rust on soybean in Malawi, putting at risk 14 000 ha currently under soybean production. The reports of soybean rust in Malawi and adjoining countries will alter soybean production practices and research interests. In some cases, foliar application of fungicides has increased, and planting dates have been changed to avoid conditions that are most conducive for rust development. Efforts to understand the virulence and genetic diversity of the pathogen in the region are needed in order to develop and deploy resistant cultivars.

--

Communicated by: ProMED-mail <promed@promedmail.org>

******

[2] Costa Rica

Date: March 2015
Source: Plant Disease [edited]
<http://apsjournals.apsnet.org/doi/abs/10.1094/PDIS-06-14-0646-PDN>

[Ref: A Murillo-Williams et al (2015): First Report of _Phakopsora pachyrhizi_ on Soybean in Costa Rica. Plant Disease 99, 418; DOI:

10.1094/PDIS-06-14-0646-PDN]

------------------------------

_Phakopsora meibomiae_ has been reported to occur in several legume species in the tropical regions of Central and South America. In Costa Rica, this pathogen was initially reported as _P. pachyrhizi_; however, to our knowledge, _P. pachyrhizi_ has not [previously] been detected in Costa Rica.

In routine evaluations of a 0.2-ha field planted with soybean (_Glycine max_), symptoms similar to Asian soybean rust were observed in December 2012 and January 2013. Microscopic examination of symptomatic leaves showed the presence of uredinia and urediniospores on the lower surface of the leaf. While initial symptoms were on the southern side of the field, a substantial area of the field was infected at the 2nd evaluation.

DNA was extracted from urediniospores and excised leaf pieces and 8 samples amplified in real-time PCR with _P. pachyrhizi_-specific, but not with _P. meibomiae_-specific, primers. Nucleotide sequence alignment of the internal transcribed spacer (ITS) regions 1 and 2 that were amplified by PCR further confirmed the identification as _P. pachyrhizi_.

To the best of our knowledge, this is the 1st confirmation of soybean rust caused by _P. pachyrhizi_ in Costa Rica. CIGRAS-06 is the only soybean variety bred in the country as well as one of the very few varieties available for growers. Given that breeding for disease resistance is not a short-term option, alternative disease management strategies will have to be developed.

--

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. The related species _P. meibomiae_ (American rust) is also found in South America and the Caribbean causing similar symptoms but is less virulent and considered of lesser economic significance.

Spores are spread locally by splashing rain or by wind over long distances. Several epidemic cycles may occur in one cropping season.

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 (for example, see ProMED-mail posts 20090501.1644 & 20150102.3066955).

 

Maps

Malawi:

<http://nthambazale.com/wp-content/uploads/2009/05/malawi_map.gif>

Costa Rica:

<http://worldatlas.com/webimage/countrys/namerica/camerica/lgcolor/crcolor.htm>

 

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> and <http://www.wbhm.org/pics/features/soybean-icon.jpg>

Electron microscopy of leaf rust spores:

<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>

and

<http://nt.ars-grin.gov/taxadescriptions/factsheets/index.cfm?thisapp=Phakopsorapachyrhizi>

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>

 - Mod.DHA

 

ProMED HealthMaps can be accessed at:

Malawi: <http://healthmap.org/promed/p/176>,  

Costa Rica: <http://healthmap.org/promed/p/17>.]

 

[See Also:

Leaf rust, soybean - Brazil: (MS) 20150102.3066955

2012

----

Leaf rust, soybean - Brazil: (MT), pathogen reservoirs

20120911.1287887

2010

----

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

2009

----

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

2008

----

Soybean diseases - USA: (LA) 20081012.3231 Asian soybean rust - USA, Mexico: update 20080928.3070

2007

----

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] 



More news from: ISID (International Society for Infectious Diseases)


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Published: February 25, 2015

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