Meeting the Challenge of Yellow Rust in Cereal Crops - ICARDA

1980s. Occurrence of virulence for Yr9, widespread monoculture of wheat
cultivars with similar genetic composition, favourable environmental
conditions, overlapping crop calendars and high frequency of west-to-east
wind movements during cropping seasons are the major factors in regional
epidemics of yellow rust in CWANA. The recent wide spread of yellow rust in
CWANA is possibly a consequence of such similar conditions. Transboundary
rust pathogens can produce new virulence that could overcome resistance of
commonly grown wheat cultivars and cause epidemics in a short period.
Monitoring rust occurrences and movement would enable forecasting of the
development of new races and would allow timely use of appropriate control
measures by wheat growers to protect their production. Analysis of pathogenic
variation in rust pathogens is fundamental to understanding pathogen
population structure, host-pathogen co-evolution, and breeding for durable
resistance. Over the last 80 years, rust workers have conducted pathogenicity
surveys and race analysis using differential host genotypes for seedling and
adult-plant resistance genes. Currently, the threat of long-distance migration of
rust pathogens, monoculture of mega-genotypes and a lack of standard
facilities and expertise in many developing countries have increased the
importance of international collaboration in monitoring cereal rusts. Many
national, regional and international biological rust trap nurseries comprising
differential genotypes, known cultivars with widely used rust resistance genes
and local commercial cultivars have been established successfully in almost all
wheat growing areas worldwide. For example, rust trap nurseries were
successfully distributed by ICARDA to 244 testing sites for the 2008/09
cropping season, with the 3rd International Yellow Rust Trap Nursery (3rd
IYRTN-09) distributed to 76 sites in 31 countries. Although environmental
conditions were unfavourable for yellow rust because of drought conditions in
most CWANA countries, occasional severe yellow rust outbreaks were
recorded on commercial cultivars mostly known to carry Yr27 alone or in
combination with Yr9 and Yr18. Among the Yr genes present in the differential
genotypes and commercial cultivars in YRTN, only a few known genes have
been widely used in agriculture (such as Yr1, Yr6, Yr7, Yr9, Yr27 and Yr18).
Virulence for Yr1 is varied among these genes, and virulence for Yr6, Yr7 and
Yr9 is very common (nearly fixed) in CWANA regions, and it has been
indicated that yellow rust can not be effectively protected by genotypes with
Yr18 alone. Yr9 and Yr27 are the most common Yr genes that have been used
in many wheat cultivars grown in CWANA, such as PBW 343, Inqilab 91,
MH 97, Chamran, Shiroudi, Kubsa, Imam and many advanced lines. During
the 3rd Regional Yellow Rust Conference, detection of virulence for Yr27 was
highlighted, particularly for cultivars PBW 343 in India and Inqilab in
Pakistan, and Chamran and Shiroudi in Iran. Severe infections of commercial
cultivars were noted in India, Pakistan, Iran, Uzbekistan, Afghanistan,
Azerbaijan, Turkey and Morocco during 2008/09. Field-based pathogenicity
surveys have provided sufficient evidence of the usefulness of yellow rust
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