Emerging virulent stem rust race (Ug99)

The emerging virulent stem rust race (Ug99) and vulnerability of wheat in the U.S. and worldwide.

Yue Jin (yuejin@umn.edu)

USDA-ARS, Cereal Disease Laboratory

New virulence in East Africa

In a nursery in Uganda, Africa in 1999, susceptible type stem rust pustules (designated Ug99) were found on wheat lines known to have the stem rust resistance gene Sr31, a gene for which no virulence had been reported previously anywhere in the world.  Similar virulence was observed in 2001 in Kenya and 2003 in Ethiopia.  (Race identification of earlier observations prior to 2001 could not be confirmed because of a lack of samples).  Race typing (race TTKS based on Pgt system of nomenclature, see Phytopathology 78:526-533) and DNA confirmed the presence of Ug99 in Kenya in 2005.

Sr31 is on the 1B/1R chromosomal translocation, a piece of rye chromosome that has been introduced into many wheat cultivars.  In addtion to Sr31, the leaf rust resistance gene Lr26 and the stripe rust resistance gene Yr9 are also on the 1B/1R translocation.

Stem rust vulnerability of wheat worldwide--the Sr31 factor

Stem rust resistance gene Sr31 is widely utilized in wheat worldwide, particularly in the India subcontinent, China, Europe, and South America. From the CIMMYT report: Impacts of International Wheat Breeding Research in Developing Countries, 1966-97, developing countries planted 69 million hectares (~170 million acres) of spring wheat in 1997, of which nearly 80% were planted to CIMMYT-related varieties.  Susceptibility of this material will provide little barrier to the spread of a virulent race (e.g. TTKS).  The use of the rust resistance gene Sr31 might have hindered the use of other Sr resistance genes (masking effect)--resulted in a genetic uniformity of narrowly based resistance in a large magnitude.

Environments in East Africa that may have contributed to new races

• Wheat production in the highlands (>2000m or higher).
• Wheat is present year-around due to staggered plantings and volunteer plants.
• Daily dews and mild temperatures permit continued survival of the fungus and re-infections on host plants.

Potential migration routes for Ug99

The current know status of Ug99 in east Africa (courtesy of R. Singh, CIMMYT).

The potential migration routes for Ug 99 based on prevailing air-flows and regional wheat production areas. Route A via Arabian peninsular, Middle-East, and South Asia is considered to have a higher probability (courtesy of R. Singh, CIMMYT).

Current research emphasis

? Screening U.S. wheat in Kenya against Ug99

? Characterizing sources of resistance

? Mapping effective resistance genes--ARS Genotyping Labs

Characterization of resistance to TTKS

     - postulation of genes

Characterization of resistance to TTKS

     -inheritance

Effectiveness of Sr genes to TTKS

Not effective

Effective (including moderate levels)

No data

Resistance to TTKS in US & CIMMYT wheat

-Based on seedling & 2005 Kenyan nursery results