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Date: 8 February, 2008
Source: Farmers Weekly [edited]
<http://www.fwi.co.uk/Articles/2008/02/08/109089/devastating-usa-strain-of-yellow-rust-could-infect-european-wheat.html>

European wheat could be potentially at risk from a new strain of yellow rust [stripe rust] that has caused large-scale yield losses across the major wheat belt in the USA. The strain survives at higher temperatures than current races, produces many more spores and is more aggressive in its attack, Mogens Hovmoller, from Denmark's University of Aarhus, together with Eugene Milus from the University of Arkansas have discovered. And with just one or 2 mutations it could easily infect European varieties, Dr Hovmoller says.

Prior to 2001 yellow rust was confined mainly to a few parts of the northwestern states of the USA. Then it spread and is now the most severe wheat disease in the region, with annual losses of 10 million tonnes/year. The disease extended into areas previously considered too hot. The new strain grows much faster on universally rust-susceptible wheat and is also able to produce enough spores at high temperatures to keep the rust epidemic going. DNA analysis has confirmed the new race has a completely different origin to European rust.

Worryingly for European farmers, isolates with a very similar make up have been found in rust screens set up in Denmark. These atypical races are able to grow on certain varieties used in the screen. At the moment, European varieties are protected by varietal resistance, but Dr Hovmoller suggests the new strain would only require maybe one, 2, or 3 mutations before it would be able to infect many new European varieties.

Research has found the rate of evolution for yellow rust to adapt to new cultivars is very fast. "It has a high mutation rate. That could be because the disease has no sexual stage, so it needs to mutate to continue to evolve," he explains.

And the new strains ability to produce many more spores increases its chance of succeeding, he warns. "We definitely need resistance to this strain otherwise it will be a big problem."

[Byline: Mike Abram]

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[Wheat stripe rust (also called yellow rust) is caused by the fungus _Puccinia striiformis_ var. _striiformis_ and occurs worldwide mostly in cooler climates. It causes yellow leaf stripes, stunting of plants, and reduced grain number and size on developing heads. Yield losses may vary from 40 up to 100 percent. It affects wheat, some barley varieties, triticale (wheat/rye hybrids), and a number of grass species. Spores are wind dispersed in several cycles during the cropping season. Since the fungus interferes with seed development, infection early in the crop cycle is more damaging than infection after seed fill has concluded. Early discovery of infection at any stage of the crop cycle is important so action can be taken to limit the spread of the pathogen as well as build-up of inoculum.

Spores are dispersed by wind and mechanical means. The fungus needs living plants to survive between seasons, it cannot survive on seed, stubble, or in soil. Volunteer plants may generate a "green bridge"
providing inoculum to infect new crops. Disease management includes the use of resistant varieties, fungicide applications, and control of volunteer cereals. New pathogen strains with increased virulence have been reported from several wheat growing areas. For example in Australia, strains apparently overcoming one of the major stripe rust resistance genes used in wheat breeding have emerged in recent years posing a serious problem to farmers.

The report above states that strains genetically 'very similar' to the US heat tolerant strain have been found apparently spontaneously infecting susceptible varieties in experimental screening plots. No information is given, however, whether the Danish strains are thought to be introduced variants of the US strain, or due to mutations having occurred locally but resulting in similar properties of enhanced virulence. In any case, this early alert may give European wheat breeders and farmers a chance to be prepared.

Maps
Denmark:
<http://home6.inet.tele.dk/vlh/danmark.JPG> and <http://healthmap.org/promed?v=56,10,5>
US states:
<http://www.census.gov/geo/www/us_regdiv.pdf> and <http://healthmap.org/promed?v=40,-97.6,4>

Pictures
Stripe rust leaf symptoms on wheat:
<http://www.ars.usda.gov/Main/docs.htm?docid=9918&pf=1&cg_id=0>  and <http://utextension.tennessee.edu/fieldCrops/wheat/Wheat_photos/Wheat_StripeRust.jpg> 
Symptom comparison on resistant and susceptible wheat cultivars:
<http://www.oznet.ksu.edu/path-ext/factSheets/wheat/Wheatimages/wheats1.jpg>

Links
Information on wheat stripe rust:
<http://pnw-ag.wsu.edu/smallgrains/Stripe%20Rust.html>,
<http://www.ars.usda.gov/Main/docs.htm?docid=9918&pf=1&cg_id=0> and <http://www.oznet.k-state.edu/path-ext/factSheets/Wheat/Wheat%20Stripe%20Rust.asp>
Stripe rust management:
<http://www.grdc.com.au/uploads/documents/striperustmgt.pdf>
_P. striiformis_ taxonomy:
<http://www.indexfungorum.org/Names/NamesRecord.asp?RecordID=427989>
Explanation of strains, pathotypes, and races of rusts:
<http://www.grdc.com.au/director/events/groundcover?item_id=publication-issue53&article_id=482B978094DEDBDD18CDA8C0DCD4F6C2>
Research group M. Hovmoller:
<http://www.agrsci.org/content/view/full/18054>
Research group E. Milus:
<http://www.uark.edu/depts/plntpath/PLPA/HTML/prof.eugenenmilus.html>. 
- Mod.DHA]