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

Date: 28 Aug 2008
Source: Farm Weekly [edited]
<http://fw.farmonline.com.au/news/state/grains-and-cropping/general/stemming-the-spread-of-rust-in-wa/1256966.aspx>


WA [Western Australia] grain growers are being warned that stem rust has been observed in a 700-hectare (1730 acres) wheat crop at Warralakin in the eastern wheatbelt and are being asked to be vigilant checking their own crops. David Keamy of Merredin Rural Supplies found the rust in less than 10 percent of the crop, with the infection averaging 5-10 pustules on leaves and stems of scattered plants at early flag leaf emergence.

Dr Colin Wellings, plant pathologist at the Grains Research and Development Corporation [GRDC] supported Cereal Rust Laboratory [CRL], University of Sydney, described stem rust as "a disease to be nervous about." "It could represent the beginnings of an epidemic if conditions are favourable," he said. "Optimal average daily temperatures are 18-30 deg C [64.4-86 deg F] and the latent period, which is the time between infection and production of new spores, is
8 to 12 days. If moisture conditions become favourable, we could have a problem."

Samples are now being typed at CRL, but the process takes 3 weeks. Dr Wellings said Yitpi, a wheat variety grown in WA and widespread in the Mallee region of SA [South Australia], is vulnerable to stem rust.

Under favourable conditions, stem rust outbreaks could destroy up to 70 percent of a susceptible wheat crop, according to Dr Rohan Rainbow, GRDC Manager, Crop Protection. "The Warralakin outbreak is a classic example of green bridge proximity, where small patches of self-sown Yitpi, with advanced heading plants, spread the disease to the sown crop," Dr Rainbow said. "After this season's [2008] harvest, growers should be careful with variety selection for next year and make informed choices based on several factors, including degree of rust resistance."

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[Wheat stem rust (also called black rust) is caused by the fungus _Puccinia graminis_ f. sp. _tritici_. Yield losses of up to 70 percent are reported, but some fields are totally destroyed. If stem rust arrives early in the growing cycle, losses are higher. Spores are spread by wind and with infected straw, but the fungus needs living tissue to survive between seasons. Volunteer wheat plants forming a 'green bridge' are often a source of infection for a new crop.

New pathogen races are emerging with increased virulence. The most dangerous at present is strain Ug99, which is spreading eastwards on prevailing winds from its origin in Uganda. At this time Ug99 has not spread beyond the Middle East, nevertheless the typing of the strain in WA which is currently in progress is important. For more information on stem rust and Ug99 please see previous ProMED-mail posts and links listed below.

Maps
Australia:
<http://www.ga.gov.au/image_cache/GA4073.jpg>  and <http://healthmap.org/promed?v=-25.7,134.5,4>
Western Australia:
<http://www.ozhorizons.com.au/wa/map.htm>
 
Pictures of stem rust symptoms:
<http://www.cimmyt.org/english/wps/news/2005/sept/images/stem2.jpg>  and <http://www.gov.mb.ca/agriculture/crops/diseases/images/fac15s01.jpg>

Links
Information on wheat stem rust:
<http://www.ars.usda.gov/Main/docs.htm?docid=9910>
Information on Ug99:
<http://www.agbioworld.org/newsletter_wm/index.php?caseid=archive&newsid=2686>
and
<http://www.pestalert.org/viewNewsAlert.cfm?naid=36>
Ug99 distribution and potential global impact:
<http://www.cimmyt.cgiar.org/gis/pdf/UG99postH.pdf>
_P. graminis_ f.sp. _tritici_ taxonomy:
<http://www.indexfungorum.org/Names/NamesRecord.asp?RecordID=455607>
Global Rust Initiative:
<http://www.globalrust.org/>
GRDC:
<http://www.grdc.com.au/>.  - Mod.DHA]