Simon R. Ellwood

The obligate biotrophic fungus Blumeria graminis f. sp. tritici ( Bgt ) is the causal agent of wheat powdery mildew. To investigate virulence diversity, 30 Australian Bgt isolates collected between 2020 and 2024 were screened against a panel of 24 wheat lines carrying defined resistance ( R ) genes. Pathotyping analysis indicated that the Australian Bgt population is diverse in its virulence profile. Non-metric multidimensional scaling analysis of disease phenotyping revealed evidence of six distinct Bgt pathotype groups. In addition, we identified wheat varieties carrying defined R genes and their combinations that remain effective against the contemporary Australian Bgt . Notably, wheat cvs Khapli and Tabasco, possessing Pm4a and Pm2 , respectively, in addition to unknown R gene(s), were highly resistant to all the isolates tested. Furthermore, elite European varieties that possess durable resistance, such as Alchemy and Cordiale, display complete to moderate resistance against all selected isolates representing the Australian pathotype groups.

Centuries of barley (Hordeum vulgare) domestication and selection has resulted in reduced genetic diversity in modern cultivars, limiting breeder's options to select desirable traits. Barley landraces, heirloom varieties, and wild relatives are substantially more variable and can be exploited to reintroduce favorable genes and alleles. Five doubled haploid populations were phenotyped for net form net blotch (NFNB) disease, caused by the pathogen Pyrenophora teres f. teres, at three growth stages. Major, moderate, and minor effect quantitative trait loci (QTLs) associated with NFNB resistance were detected on six of the seven barley chromosomes, with percentage of explained variance (PEV) ranging from less than 10% to over 70%. Previously established major (PEV > 50%) and moderate (PEV 10%–40%) effect QTLs on 3H and 6H were detected against the Australian isolate used, as well as moderate and minor QTLs (PEV < 10%) distributed on 2H, 3H, 4H, and 5H. Differences in effect sizes of individual QTL were apparent between growth stages, tapering up toward heading or down from seedlings, together with growth stage‐specific and synergistic QTL. Several of these QTL represent novel sources of resistance that may be combined for durable NFNB resistance.

Centuries of barley (Hordeum vulgare ) domestication and selection has resulted in reduced genetic diversity in modern cultivars, limiting breeder’s options to select desirable traits. Barley landraces, heirloom varieties and wild relatives are substantially more variable and can be exploited to reintroduce favourable genes and alleles. Five doubled haploid populations were phenotyped for net form net blotch (NFNB) disease, caused by the pathogen Pyrenophora teres f. teres, at three growth stages. Major, moderate and minor effect quantitative trait loci (QTLs) associated with NFNB resistance were detected on six of the seven barley linkage groups, with percentages of explained variance (PEV) ranging from less than ten to over 70%. Previously established major (PEV > 50%) and moderate (PEV 10 - 40%) effect QTLs on 3H and 6H were detected against the Australian isolate used, as well as moderate and minor QTLs (PEV < 10%) distributed on 2H, 3H, 4H and 5H. Differences in effect sizes of individual QTL were apparent between growth stages, tapering up towards heading or down from seedlings, together with growth stage-specific and synergistic QTL. Several of these QTL represent novel sources of resistance which may be combined for durable NFNB resistance.

Spot form of net blotch, caused by Pyrenophora teres f. maculata, is a significant necrotrophic disease of barley that spread world-wide in the 20th century. Genetic relationships were analysed to ...

Australia is one of the largest barley exporters in the world, with Western Australia accounting for some 40% of national production. The crop is predominantly grown in the south and south‐west of the state in winter and spring, where temperate conditions and higher rainfall levels are more suited to barley than northern and eastern regions. Between 2007 and 2013, prolonged outbreaks of barley powdery mildew (BPM) occurred. This was brought about by a combination of the extensive use of susceptible cultivars and an over‐reliance on a small number of single mode‐of‐action demethylation inhibitor fungicides, which select for mutations in the C14α‐demethylase ( Cyp51A ) gene. This review highlights the steps taken to reduce losses to BPM, breeding efforts to introduce resistance into cultivars and the success of pre‐breeding research to find new and durable resistance genes. We also draw comparisons with powdery mildew in Australian wheat, where similar factors are leading to substantial outbreaks.

Key message: Gene expression at the RBgh2 locus indicates involvement in cAMP/G-protein-coupled signalling and innate immunity in barley powdery mildew adult plant resistance. Barley powdery mildew is a globally significant disease, responsible for reduced grain yield and quality. A major effect adult plant resistance gene, RBgh2, was previously found in a landrace from Azerbaijan. The atypical phenotype suggested different underlying genetic factors compared to conventional resistance genes and to investigate this, genome-wide gene expression was compared between sets of heterogeneous doubled haploids. RBgh2 resistance is recessive and induces both temporary genome-wide gene expression changes during powdery mildew infection together with constitutive changes, principally at the RBgh2 locus. Defence-related genes significantly induced included homologues of genes associated with innate immunity and pathogen recognition. Intriguingly, RBgh2 resistance does not appear to be dependent on salicylic acid signalling, a key pathway in plant resistance to biotrophs. Constitutive co-expression of resistance gene homologues was evident at the 7HS RBgh2 locus, while no expression was evident for a 6-transmembrane gene, predicted in silico to contain both G-protein- and calmodulin-binding domains. The gene was disrupted at the 5′ end, and G-protein-binding activity was suppressed. RBgh2 appears to operate through a unique mechanism that co-opts elements of innate immunity.

Spot form net blotch, caused by Pyrenophora teres f. maculata, is a major foliar disease of barley worldwide. Knowledge of the pathogen's genetic diversity and population structure is critical for a better understanding of inherent evolutionary capacity and for the development of sustainable disease management strategies. Genome-wide, single nucleotide polymorphism data of 254 Australian isolates revealed genotypic diversity and an absence of population structure, either between states, or between fields and cultivars in different agro-ecological zones. This indicates there is little geographical isolation or cultivar directional selection and that the pathogen is highly mobile across the continent. However, two cryptic genotypic groups were found only in Western Australia, predominantly associated with genes involved in fungicide resistance. The findings in this study are discussed in the context of current cultivar resistance and the pathogen's adaptive potential.

The fungusBlumeria graminisf. sp.triticicauses wheat powdery mildew disease. Here, we study its spread and evolution by analyzing a global sample of 172 mildew genomes. Our analyses show thatB.g. triticiemerged in the Fertile Crescent during wheat domestication. After it spread throughout Eurasia, colonization brought it to America, where it hybridized with unknown grass mildew species. Recent trade brought USA strains to Japan, and European strains to China. In both places, they hybridized with local ancestral strains. Thus, although mildew spreads by wind regionally, our results indicate that humans drove its global spread throughout history and that mildew rapidly evolved through hybridization.

Powdery mildew isa major disease of barley ( Hordeum vulgare L.) for which breeders have traditionally relied on dominant, pathogen race‐specific resistance genes for genetic control. Directional selection pressures in extensive monocultures invariably result in such genes being overcome as the pathogen mutates to evade recognition. This has led to a widespread reliance on fungicides and a single broad‐spectrum recessive resistance provided by the mlo gene. The range of resistance genes and alleles found in wild crop relatives and landraces has been reduced in agricultural cultivars through an erosion of genetic diversity during domestication and selective breeding. Three novel major‐effect adult plant resistance (APR) genes from landraces, designated R esistance to B lumeria g raminis f. sp. h ordei ( Rbgh1 to Rbgh3 ), were identified in the terminal regions of barley chromosomes 5HL, 7HS, and 1HS, respectively. The phenotype of the new APR genes showed neither pronounced penetration resistance, nor the spontaneous necrosis and mesophyll cell death typical of mlo resistance, nor a whole epidermal cell hypersensitive response, typical of race‐specific resistance. Instead, resistance was localized to the site of attempted penetration in an epidermal cell and was associated with cell wall appositions and cytosolic vesicle‐like bodies, and lacked strong induction of reactive oxygen species. The APR genes exhibited differences in vesicle‐like body sizes, their distribution, and the extent of localized 3,3‐diaminobenzidine staining in individual doubled haploid lines. The results revealed a set of unique basal penetration resistance genes that offer opportunities for combining different resistance mechanisms in breeding programs for robust mildew resistance.