WHEAT STEM RUST, Ug99 GROUP: NEW RACES

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A ProMED-mail post

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International Society for Infectious Diseases


Date: Wed 26 May 2010
Source: Physorg [edited]



Mutant fungus threatens global wheat supply
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Four new mutations of Ug99, a strain of a deadly wheat pathogen known
as stem rust, have overcome existing sources of genetic resistance
developed to safeguard the world's wheat crop. Leading experts at a
global event organized by the Borlaug Global Rust Initiative, said the
evolving pathogen may pose an even greater threat to global wheat
production than the original Ug99.

The new "races" have acquired the ability to defeat 2 of the most
important stem rust-resistance genes. "With the new mutations,
countries cannot afford to wait," said Ravi Singh, International Maize
and Wheat Improvement Center (CIMMYT). "Already, most of the varieties
worldwide are vulnerable to the original form of Ug99. We will now
have to make sure that every new wheat variety has iron-clad
resistance to both Ug99 and the new races."

New mutations identified in South Africa will make wheat crops more
vulnerable as pathogens now will find new wind trajectories for
migration. "We do not have as much information as we would like on the
aggressiveness of the pathogen," said David Hodson, FAO. "The original
Ug99 does not seem to have increased as much as originally feared. But
the new variants pose a grave challenge."

Wealthy farmers have chemicals for dealing with wheat rust, but
chemical control is costly and unaffordable for most resource-poor
farmers. The best strategy is replacement of susceptible varieties
with new resistant varieties. CIMMYT and ICARDA [International Centre
for Agricultural Research in Dry Areas], in collaboration with
countries under threat, have developed high-yielding Ug99-resistant
varieties that are now being distributed in the most threatened areas.

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[Wheat stem rust is caused by the fungus _Puccinia graminis_ f. sp.
_tritici_. Overall yield losses of up to 80 percent are reported, but
some fields are totally destroyed. New races are emerging, and the
most dangerous at present is strain Ug99 (discovered in Uganda in
1999) which has overcome the major resistance gene Sr31 used in our
current wheat varieties. Since then Ug99 has appeared in Kenya,
Ethiopia, Sudan, Yemen, and Iran, and even more virulent variants of
Ug99 able to overcome additional resistance genes have emerged.

It is estimated that Ug99 could reduce world wheat production by 60
million tons and the pathogen is considered a global threat to food
security. Resistance breeding programmes have been set up with
international cooperation (Delhi Declaration, see link below) to
establish wheat varieties resistant to Ug99.

Rust spores are carried on prevailing winds, and regions at high risk
of a Ug99 incursion were identified accordingly to the east of Uganda
in the Middle East and South Asia. With a new source of Ug99 strains
now reported in South Africa, additional routes of aerial transmission
have opened up increasing the threat to regions in the southern
hemisphere. It is suggested that at least one new strain in South
Africa may be an introduction rather than a mutation from local races,
and this poses the question of where the source may be and how the
incursion may have occurred.

Maps
Africa and Middle East:

South Africa:


Pictures of stem rust symptoms on wheat:
and


Links
Additional news stories:
and

Information on wheat stem rust:

Information on Ug99:
,
,
, and

New Ug99 strain in South Africa:

_P. graminis_ f.sp. _tritici_ taxonomy:

Delhi Declaration on Ug99:

Global Rust Initiative:
. - Mod.DHA]

[see also:
2009
----
Stem rust, wheat - India: new strain 20090717.2548
Wheat stem rust, strain Ug99: resistance breeding 20090326.1168
Wheat rusts - Kenya, India, Australia 20090312.1019
2008
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Wheat stem rust Ug99 - Australia ex Ethiopia: interception 20081203.3807
Wheat stem rust, strain Ug99 - multicountry: update 20081120.3660
Wheat stem rust, strain Ug99 - Iran: 1st rep, alert 20080307.0925
2007
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Wheat stem rust, strain Ug99 - Kenya: new variant 20071217.4054
Wheat stem rust, strain Ug99 - multistate: resistance screening 20071208.3957
Barley yellow dwarf virus & stem rust, cereals - Kenya 20070705.2132
Wheat stem rust, strain Ug99 - Yemen (02): government response 20070430.1399
Wheat stem rust, spread: FAO, Global Rust Initiative 20070414.1241
Wheat stem rust, strain Ug99 - Yemen: 1st report 20070117.0229
2006
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Wheat stem rust, strain Ug99 - Pakistan: susp., 1st report 20060514.1366
Stem rust, wheat - multicountry: new strains 20060406.1039
2005
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Wheat stem rust, Ug99, new strain - East Africa 20050928.2849
Wheat stem rust, new strain - Uganda 20050912.2698
2002
----
Wheat stem rust fungus, new virulence genes - So Afr 20020814.5049
2000
----
Wheat stem rust in resistant wheat lines - Uganda 20000702.1092
1999
----
Stem-rust fungus, mutant, wheat - Uganda 19990519.0812]
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