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Gone with the wind: Revisiting stem rust dispersal between southern Africa and Australia

Despite being 10,000 km apart, the current study emphasizes the potential vulnerability of Australia to wind-borne Puccinia graminis f. sp. tritici (Pgt) spore introductions from southern Africa. Of four Pgt introductions into Australia since 1925, at least two (races 326-1,2,3,5,6 and 194-1,2,3,5,6) are thought to have originated from southern Africa. Microsatellite analysis of 29 Australian and South African Pgt races confirmed close genetic relationships between the majority of races in these two geographically separated populations, thus supporting previously reported phenotypic similarities. Using Lagrangian Particle Dispersion Model simulations with finely-resolved global meteorological data over a 14-year period and a three-day urediniospore survival time, the study showed that long distance dispersal of Pgt from southern Africa to Australia is possible, albeit rare. Transmission events occurred most frequently from central South Africa, but were also possible from southern South Africa and Zimbabwe; while none occurred from a representative source-location in Tanzania. Direct dispersal incursions into both the western and eastern Australian wheat belts were feasible. Together, the genetic and simulation data strongly support the hypothesis that earlier introductions of Pgt into Australia occurred through long-distance wind-dispersal across the Indian Ocean. The study thus acts as a warning of possible future Pgt dispersal events to Australia which could include members of the Ug99 race group. This emphasizes the continued need for Pgt surveillance on both continents.

Department of Plant Sciences, University of the Free State, South Africa
Marcel Meyer, Robert Park, Christopher Gilligan, Laura Burgin, Matthew Hort, David Hodson, Zacharias Pretorius
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Association mapping for validation of genomic regions influencing adult plant resistance to leaf rust in Australian barley breeding germplasm

Elite barley breeding lines from the Australian Northern Region Barley Breeding Program were evaluated at the seedling and adult growth stages for resistance to leaf rust (LR) caused by Puccinia hordei. F3:5 lines derived from parental germplasm of different geographic origins were screened in the glasshouse and field spanning four years of trials. The 2009 and 2011 breeding populations (BP1 and BP2) comprised 360 lines and were genotyped with 3,244 polymorphic diversity arrays technology (DArT) markers. The 2012 and 2013 breeding populations (BP3 and BP4) comprised 320 lines genotyped with the DArT GBS array (DArTseq), providing 15,400 high quality polymorphic markers. Association mapping (AM) using the DArT/DArT-seq datasets and phenotypic data from 15 independent LR response assays identified a number of genomic regions associated with resistance. The BP1 and BP2 study detected a total of 15 QTL; 5 QTL co-located with catalogued LR resistance genes (Rph1, Rph3/19, Rph8/14/15, Rph20, and Rph21), 6 QTL aligned with previously reported genomic regions and 4 QTL (3 on chromosome 1H and 1 on 7H) were novel. Markers in common between the DArT and DArTseq datasets enabled integration of mapping results for LR response across the four breeding populations and all QTL detected were visualised on a single map for validation. The adult plant resistance (APR) locus Rph20 was the only region detected in all field environments. Markers and their associated sequences identified in this study will be useful for building QTL combinations involving Rph20, thereby providing stable LR resistance in improved barley cultivars. We will also highlight the advantages of AM using breeding germplasm over traditional bi-parental mapping approaches that underutilise genetic diversity and divert valuable resources into populations of low breeding value.

The University of Queensland, Queensland Alliance for Agriculture and Food Innovation, Australia
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Expression of adult plant resistance to stripe rust in selected Australian wheat genotypes

Wheat stripe rust can be managed using adult plant resistance (APR). Knowledge of the growth stage at onset of APR is crucial for integrated management of this disease. Wheat varieties Annuello, Baxter, EGA Kidman, GBA Sapphire, Janz, Kennedy, Livingston, Spitfire, Sunstate, Sunvale, Wyalkatchem, Yitpi, and Avocet NILs carrying Yr18 and Yr18+29, all carrying adult plant stripe rust resistance, were compared for expression of APR under greenhouse conditions. Weekly plantings of all genotypes and susceptible controls Mace and Morocco permitted simultaneous comparisons of infection at the seedling, tillering, jointing, flag leaf, and head emergence stages. Ten replicates of each genotype at the five different growth stages were inoculated with Pst pathotype 134 E16,A+,17+,27+. Percentages of leaf area affected by stripe rust and host response were recorded at 14, 17, 20, 24, 28 and 31 days post inoculation to establish the latent period. With one exception all genotypes including the controls showed latent periods of less than 14 days when inoculated at the seedling, tillering and stem elongation, or jointing growth stages; Spitfire had a latent period of more than 17 days when inoculated at jointing. At the flag leaf and head emergence stages, all genotypes except the controls Mace and Morocco had latent periods of greater than 20 days. The results indicate that the onset of APR in Spitfire was earlier (at jointing) than the other genotypes, and that the resistance remained effective until head emergence and beyond. The information generated shows the importance of information regarding stripe rust development within specific wheat genotypes, and in supporting decisions on chemical intervention to control this disease.

The University of Sydney, Faculty of Agriculture and Environment, Plant Breeding Institute, Australia
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Divergent evolution of the Jackie pathotype of Puccinia striiformis f. sp. tritici between Australia and New Zealand

Based on historical data, Australia and New Zealand (NZ) form a single epidemiological unit for cereal rusts. The dominant westerly wind pattern produces a one-way pathway of pathogen movement from Australia to NZ. Until 2002, pathotype analysis of cereal rust pathogens for NZ was conducted at the University of Sydney, Plant Breeding Institute. Over that time, windborne dispersal of members of the Pst 104 pathotype lineage to New Zealand was confirmed. Historically, pathotypes of Pst introduced to New Zealand have taken different evolutionary pathways to their Australian relatives, including a higher diversity of step-wise mutant isolates, often with different virulence profiles. A preliminary screen of Pst in NZ was conducted in January 2013 and a broader survey was conducted in 2014. Initial results confirmed that the Australian pathotype (pt.) 134 E16 A+ YrJ+ had crossed to NZ. The designation “YrJ+was allocated to indicate virulence for an unidentified, probably rye-derived, resistance gene in the Australian triticale cultivar ‘Jackie’. The divergent evolution of this pathotype in NZ relative to Australia is of interest. In NZ, this pathotype subsequently acquired virulence for Yr10 to produce pt. 150 E16 A+ YrJ+. In Australia, Yr10 virulence had previously evolved in pt. 134 E16 A+, the progenitor of pt. 134 E16 A+ YrJ+. Only two mutational derivative pathotypes have evolved from pt. 134 E16 A+ YrJ+ in Australia. The first acquired virulence for an adult plant resistance gene in another triticale variety, ‘Tobruk’, and the second acquired virulence for Yr27. Despite being present in both Australia and NZ, pt. 134 E16 A+ Yr17+ has dominated the Australian Pst population whereas in NZ the predominant pathotype appears to be 134 E16 A+ YrJ+. Since the rust resistance genotypes of NZ varieties are poorly characterised, no conclusions can yet be reached as to whether this difference in dominant pathotype is due to selection or chance.

New South Wales Department of Primary Industries, Elizabeth Macarthur Agricultural Institute, Australia
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Genetic characterization of stripe rust resistance in a common wheat landrace AWCC618

Stripe rust of wheat was estimated to cause losses of A$127 m annually in Australia. Although stripe rust can be controlled through the use of chemicals, breeding for resistance is considered to be the best means of control. Identification and characterization of diverse sources of resistance is essential to achieve durable stripe rust control. A common wheat landrace AWCC618 showed resistance (IT 1CN) to Australian Puccinia striiformis f. sp. tritici (Pst) pathotypes. AWCC618 was crossed with the susceptible genotype Avocet S (AvS) to determine the genetic basis of resistance. Seedling tests on 123 AWCC618/AvS F3 families using Australian Pst pathotype 134 E16 A+ 17+ 27+ indicated monogenic inheritance of resistance (22HR:68SEG:33HS; χ21:2:1=3.34, non-significant at P=0.05 and 2 d.f.). The resistance locus was temporarily named YrAW3. Selective genotyping of eight homozygous resistant (HR) and eight homozygous susceptible (HS) F3 families using the 90K SNP Infinium assay tentatively located YrAW3 on chromosome 6A. The AWCC618/AvS population was advanced to F6 for detailed mapping of the target region. YrAW3 appears to be a new locus. AWCC618 was crossed with three current Australian cultivars to transfer YrAW3 to modern wheat backgrounds. Backcross-derivatives will also be useful for validation of linked markers.


The University of Sydney Plant Breeding Institute, Australia
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A whole of industry approach to the assignment of stripe rust, stem rust and leaf rust disease response ratings to Australian wheat cultivars

A whole of industry, co-ordinated system for the assignment of reliable stripe rust, stem rust and leaf rust disease response ratings of current wheat cultivars and advanced breeding lines was developed to meet the needs of the Australian grains industry. Previously there was a lack of national consistency in ratings with each state independently publishing response ratings. However, privatisation of public wheat breeding during the early 2000s, recurrent stripe rust epidemics following a foreign pathotype incursion in 2002, and the dramatic increase in fungicide use provided the impetus to develop a consistent approach to assignment of ratings. Factors that became critical to the process were: 1) access to dedicated rust nurseries in diverse cereal production environments, 2) use of single seed sources for consistent variety identity, 3) control lines that provided consistent rust responses in field nurseries across regions, 4) a centralised repository of current and historical data (provided by the National Variety Trials program), and 5) pathotype identification (provided by the Australian Cereal Rust Control Program (ACRCP)). This system was subsequently documented by the Cereal Pathology Working Group (a sub-committee of the ACRCP consultative committee) and included a dispute resolution process. Currently, with GRDC funding, approximately 200 lines comprising current cultivars and advanced breeding lines, are assessed nationally in dedicated rust nurseries by members of the Working Group. Both current and historical data are reviewed annually by the Working Group with consensus rust responses developed and then scrutinised by wheat breeding companies prior to public release. This whole of industry system has improved timeliness, reliability and consistency of information with ratings used widely by Australian farmers to select cultivars to minimise losses due to rust and to plan appropriate chemical control strategies should resistance be inadequate.

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Department of Economic Development, Jobs, Transport and Resources, Australia
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