This document discusses strategies for controlling yellow rust in wheat globally. It notes that aggressive yellow rust races have spread worldwide, causing earlier and more severe epidemics. It recommends promoting resistant wheat varieties with diverse combinations of resistance genes. Specifically, combining race-specific genes with adult-plant resistance genes conferred by multiple minor genes. Ongoing research efforts aim to identify new resistance genes and effectively pyramid them into varieties through molecular breeding. Gene cassettes containing multiple resistance genes may provide more durable resistance in the future. Fungicides can also be part of an integrated control strategy. Sustainable control requires surveillance of pathogen populations and developing varieties with complex resistance.

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2nd International Wheat Stipe Rust Symposium
Izmir, Turkey, May 2014
H-J Braun, RP Singh, J Huerta-Espino, SA Herrera-Foessel, C Lan & BR Basnet
Global status of yellow rust on wheat and
strategies for its short and long term control

2.  Trace to 100% yield losses depending on the
time of the epidemic initiation, susceptibility
of variety & climatic conditions
 Selection of virulence for resistance genes Yr9
and Yr27 associated with epidemics in the
last 25 years in Africa, Middle East and Asia
 Worldwide spread of aggressive, temperature
tolerant races and their further evolution:
 new areas, initiation of epidemics at earlier
growth stages and higher severity
 Fungicide application becoming a routine in
some countries
Global status of yellow rust

3. The good news:
 More productive spring, facultative and winter
wheat germplasm, resistant to current yellow
rust (and other rusts) populations has been
distributed through various international yield
trials and nurseries, and also developed by
various National Programs during past years
 Various yellow rust resistant varieties released
in different countries

4. Immediate action points:
 Reduce the area sown to susceptible varieties
by promoting new varieties with diverse
resistance
 Release and seed multiplication of resistant
varieties in countries lagging behind
 Release and promotion of additional varieties
with complex resistance base
If not done then in 3rd YR conference we
will assemble & discuss the same again

5. Long-term R4D for a sustainable
yellow rust control (1)
 Surveillance of pathogen population for a timely detection
of new virulences of relevance
 Trap nurseries (NILs and varieties)
 Race characterization (GRC-Denmark, Turkey/ICARDA in Izmir,
India, Pakistan, Iran, China, Mexico, USA, Australia, etc.)
 Information on the resistance genes present in commercial
varieties and important breeding materials
 Gene postulation, molecular mapping- robust diagnostic
markers
 High-throughput markers assays

6. Long-term R4D for a sustainable
yellow rust control (2)
 Identification of new resistance genes and their utilization
 Race-specific genes- large & small effect; seedling & adult-plant
 Slow rusting, pleiotropic adult-plant resistance genes
 Breeding more productive wheat varieties
 Utilizing race-specific effective resistance genes in combinations
(preferably 3 genes per combination)
 Strong molecular breeding approaches for a forward breeding
 Utilizing multiple minor, slow rusting genes, or their combination
with small/intermediate effect race-specific genes
 Field based selection in conjunction with other traits
 Fungicides are part of the solution

7. Reality with the utilization of
large effect race-specific genes
● Most often end up being used as a single gene –
stewardship / stacking
● Molecular breeding being practiced only in some
programs but often results in non-competitive varieties
● Few effective genes with reliable molecular markers
available at present
● Stronger emphasis required for marker assisted
pyramiding of genes together with forward breeding
approaches including genomic selection

8. Breeding minor, slow-rusting genes based adult plant
resistance in combination with small/intermediate effect race-
specific resistance genes for achieving near-immune resistance
from seedling growth stages
● Resistance based on 4-5 slow rusting APR genes is usually adequate in
areas where yellow rust infection initiates from stem elongation stages
onwards (APR genes are already functioning by then)
● However, with the aggressive races of yellow rust- infection in some areas
initiates as early as tillering stage when APR genes still not fully functional
● Utilize combinations of slow rusting APR genes with small/intermediate
effect race-specific resistance genes that have enhanced expression due
to additive effects

9. Pleiotropic multi-pathogen resistance:
a subclass of adult plant resistance genes
● Lr34 [ Syn. = Yr18=Sr57=Pm38 =Ltn1=Sb1= Bdv1]
chromosome 7DS
(leaf rust, yellow rust, stem rust, powdery mildew, leaf tip necrosis, spot blotch, barley yellow dwarf virus )
● Lr46 [ Syn.= Yr29=Sr58=Pm39=Ltn2=Ts?]
chromosome 1BL
● Lr67 [Syn.= Yr46=Sr55=Pm46=Ltn3]
chromosome 4DL
Formation of cell wall appositions (instead of
hypersensitivity with race-specific genes)

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Chromosome 1A
Chromosome 7DChromosome 6DChromosome 5DChromosome 4DChromosome 3D
Chromosome 7BChromosome 6BChromosome 5BChromosome 4BChromosome 3BChromosome 2B
Chromosome 7AChromosome 6AChromosome 5AChromosome 4AChromosome 3A
Chromosome 2D
Chromosome 2A
Chromosome 1D
Chromosome 1B
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Sr2/Yr30
Diversity for Yellow Rust QTL (marked in yellow): > 35 regions through consensus maps
(Rosewarne et al. TAG 2013, 126:2427-2449 )

11. Examples of small/moderate effect race-specific
resistance genes characterized recently at CIMMYT and
their interaction with slow rusting genes
● Yr54 in Quaiu3 on 2DL
● YrF in Francolin on 2BS
● YrSuj in tall variety Sujata on 7BL
● Yr60 in Lalbahadur on 4AL
Contrasting example: A race-specific gene with minor effect
in seedlings but immunity in adult plants:
● YrKK in tall variety Kenya Kudu on 2BS

12. APR QTL interaction in enhancing yellow rust
resistance of Avocet x Quaiu3 RILs
Yr29 Yr30
Yr54
Minor
QTL
Yr29+
Yr30+
3D QTL
Yr29+
Yr30+
3D QTL+
Yr54
Source: Basnet et al. Plant Dis. 2013

13. Moderately effective race-specific gene YrF on 2BS and slow rusting genes
together confer a high level of YR resistance in Francolin#1
Other APR genes/QTL
• Lr46/Yr29 on 1BL
• Yr30/Sr2 on 3BS
• Two additional minor QTL
PVE: 10.3–21.1%
Source: Lan et al. (2014) Mol. Breed. DOI 10.1007/s11032-014-0075-6
Francolin#1 is an improved CIMMYT semidwarf & shows high level of
resistance to yellow rust- 50% severity when YrF present alone

14. Race-specific gene YrSuj on 7BL and slow rusting APR genes together
confer a high level of resistance in Avocet/Sujata mapping population
Other APR genes/QTL
Lr46/Yr29 (PVE:5.6-6.2%)
Lr67/Yr46 (PVE: 6.9-10.7%)
QYrLr.cim-7BL (PVE: 11.7-20.9%)
QYr.cim-1AS (PVE: 5.7-8.9%)
QYr.cim-3DS (PVE: 6.3-8.2%)
Sujata is an improved tall variety from India & shows high level of
resistance to yellow rust- 40% severity when YrSuj present alone
Source: Lan et al. (unpublished)

15. Future: GM intervention to achieve resistance durability
Gene Cassettes- multiple resistance genes inherited as a single unit:
simplifying breeding and enhancing resistance durability
Lr34/Yr18 Yr36 Lr21
SynR1 SynR2 SynR3
Natural gene cassette
(currently developed)
Synthetic gene cassette
(future possibility)
 Natural gene cassettes currently in the pipeline at CSIRO
Lr34/Yr18/Sr57 +Lr67/Yr46/Sr55 +Lr21 +Yr36
• Cloning many resistance genes for diversity
• Technology for insertion of large DNA “packages” – gene expression
• Synthetic R genes?
• Cisgenic vs Transgenic crops
Scientific/Commercialisation Challenges
Source: CSIRO-GRDC Triple Rust Initiative

16. Conclusions
 Sustainable yellow rust control will require growing resistant
varieties in disease prone areas around the world
 Wheat varieties should be enriched with multiple pleiotropic
APR genes, representing a novel class of resistance to rusts and
other wheat pathogens, to curtail/slow down pathogen
evolution
 Although near-immune level of durable resistance to all three
rusts can be achieved through field selection, molecular markers
can aid better utilization & diversity
 New approaches, such as “resistance gene cassettes” could
further expedite breeding for durable rust resistance
 Fungicides should be part of the rust control strategy