Puccinia Striiformis Research Articles

Population studies of causative agent of wheat yellow rust in the Northwest Russia

Wheat yellow rust (causative agent Puccinia striiformis West.) is a widespread disease of common wheat in the North-West Russia. A comparative analysis of P. striiformis populations, collected in the Leningrad region in 2019 on wheat and triticale, by virulence and racial composition was carried out for the first time. Virulence and racial composition of P. striiformis were studied using international and European sets of differentiating varieties and isogenic lines of “Avocet” with 20 Yr genes. A total of 55 single-pustule isolates of P. striiformis were studied, including 31 from common wheat, 10 from durum wheat and 14 from triticale. Using differentiating varieties, 2 races (111E247, 110E247) were identified among isolates from common wheat, 3 races from durum wheat (111E213, 111E231, 111E247), and 5 races from triticale (111E247, 78E209, 79E209, 111E231, 78E215). Using “Avocet” isogenic Yr lines, all isolates were avirulent to lines with the Yr5, Yr10, Yr11, Yr12, Yr15, Yr24, Yr26 genes and virulent to Yr6, Yr7, Yr8, Yr9, Yr18, YrSp, YrJr(18), YrAS genes. Isolates virulent to Yr17 were revealed on triticale; avirulent to YrSk (27) – on durum wheat. and avirulent to YrAR – on triticale. Highly effective Yr genes can be recommended for wheat resistance breeding to to yellow rust.

Genome‐wide mapping of resistance to stripe rust caused by Puccinia striiformis f. sp. tritici in hexaploid winter wheat

AbstractStripe rust, caused by the fungus Puccinia striiformis f. sp. tritici (Pst), is a widespread disease and major limit to wheat (Triticum aestivum L.) production worldwide. The USDA‐ARS National Small Grains Germplasm Collection has proven to be a rich source of genetic diversity to improve stripe rust resistance in wheat breeding programs. The objective of the present study was to investigate the genetic diversity and sources of resistance to stripe rust in 441 accessions from the collection representing globally sourced winter wheat germplasm, as a complement to a previous study. A genome‐wide association study was conducted to identify loci conferring resistance to Pst based on phenotypic data from four field experiments and greenhouse seedling resistance screening against three races of the pathogen. A total of 5831 single nucleotide polymorphism markers were used to investigate population structure, linkage disequilibrium, and marker–trait associations. Our results showed 12 and 7 genomic regions significantly associated with Pst resistance based on field adult‐plant and greenhouse seedling responses, respectively, at a false discovery rate adjusted P value of <0.1. Four of the significantly associated genomic regions were mapped far from previously identified Pst resistance genes and quantitative trait loci, indicating that they represent potentially new Pst resistance loci. The present study provides additional insight into the usefulness of wheat germplasm collections as an important resistance source that can be used to incorporate diverse resistance genes into adapted wheat cultivars. Further work should aim to validate the identified genomic regions and the associated molecular markers to enhance their utility in marker‐assisted breeding.

Effects of Stripe Rust Infection on the Levels of Redox Balance and Photosynthetic Capacities in Wheat.

Wheat stripe rust (Puccinia striiformis f. sp. tritici, Pst) is the most destructive wheat disease and a major problem for the productivity of wheat in the world. To obtain a better understanding about different effects of redox homeostasis and photosystem (PS) to Pst infection in wheat, we investigated the differences in photosynthesis and the antioxidant defense system in wheat cultivar Chuanmai42 (CM42) in response to two Chinese Pst races known as CYR32 and V26. The results showed that V26-infected wheat accumulated a higher reactive oxygen species (ROS), cell death, and energy dissipation than CYR32-infected wheat when compared with the control. Furthermore, we found that the activities of three antioxidant enzymes (APX, GR, and GPX) and four resistance-related enzymes in CYR32-infected wheat were significantly higher than that in V26-infected wheat. In addition, quantitative RT-PCR indicated that the expression levels of two genes associated with resistant stripe rust in CYR32-infected wheat were clearly higher than that in V26-infected wheat. Compared with CYR32-infected wheat, lower photochemical efficiencies were observed in V26-infected wheat at the adult stage. Meanwhile, only a marked decline in D1 protein was observed in V26-infected wheat. We therefore deduced that wheat with stripe rust resistance could maintain high resistance and photosynthetic capacity by regulating the antioxidant system, disease-resistant related enzymes and genes, and the levels of PSII reaction center proteins.

Development and characterization of novel microsatellite markers in Puccinia striiformis f.sp. tritici and their transferability in Puccinia species

AbstractPuccinia striiformis f.sp. tritici (Pst) and P. striiformis f.sp. hordei (Psh) causing stripe rust disease in wheat and barley, respectively, are two devastating phytopathogens. Microsatellite/simple sequence repeat (SSR) markers are increasingly being utilized for analysis of genetic diversity, diagnosis, population structure and possible migratory routes of plant pathogens. In the current study, novel polymorphic SSR markers were designed for Pst using the genomic sequences of PST‐78 isolate. A total of 1,191 SSR motifs, comprising 30% each of di‐ and tri‐nucleotide type of repeats, 17% of penta‐nucleotide, 15% of tetra‐nucleotide and 8% of hexa‐nucleotide repeats, were detected through in silico scanning of PST‐78 genomic sequences. Polymorphism was detected by nine of the 50 designed SSRs (PsSSRs) in seven stripe rust pathotypes of wheat and barley. The mean number of alleles per SSR locus, mean polymorphism information content (PIC), mean heterozygosity, mean major allele frequency (MAF) and mean gene diversity were 2.33, 0.34, 0.33, 0.71 and 0.40, respectively. The dendrogram analysis suggested that newly developed PsSSR markers could distinguish stripe rust pathotypes based on their virulence phenotype. Further, the cross‐genera and cross‐species amplification test of these markers in 14 different rust pathotypes revealed that 9 PsSSRs are capable of amplification in Pst species infecting wild grass, followed by 6 PsSSRs in Pt, 3 PsSSRs in Pgt, 1 PsSSRs in Puccinia species on barberry and Melampsora lini. Thus, the transferability of PsSSRs to other species reduced with increasing genetic distance of target species. These newly designed SSR markers expand the available Pst SSR marker resources and allow better genetic studies.

AAC LeRoy Canada Western Red Spring wheat

AAC LeRoy (BW1049) is a hollow stemmed, awned, high-yielding Canada Western Red Spring (CWRS) wheat suited to the growing conditions in western Canada. AAC LeRoy was 10% higher yielding than Unity, the highest yielding check in the Central Bread Wheat Cooperative registration trials (2015–2017). Within the same test, AAC LeRoy was 13% higher yielding than Carberry, a popular CWRS wheat variety across the Canadian Prairies. AAC LeRoy matured 2 d earlier than Carberry and 1 d later than Unity, the earliest maturing check suited for eastern prairie growing conditions. AAC LeRoy was 6 cm shorter with better stem strength than Unity. The lodging score for AAC LeRoy was lower than the mean of the checks. The test weight of AAC LeRoy was similar to the mean of the checks. Over the 3 yr of testing (2015–2017), the 1000-kernel weight of AAC LeRoy was higher than all of the checks, with a grain protein content 0.6% units lower than Carberry. AAC LeRoy was rated as moderately resistant to Fusarium head blight (Fusarium graminearum Schwabe), leaf rust (Puccinia triticina Erikss.), stripe rust (Puccinia striiformis Westend.), and stem rust (Puccinia graminis Pers. f. sp. tritici Erikss. & E. Henn), including the Ug99 family of stem rusts. It also had a resistant reaction to loose smut [Ustilago tritici (Pers.) Rostr.] and an intermediately resistant reaction to common bunt [Tilletia caries (DC.) Tul. & C. Tul.]. AAC LeRoy was resistant to orange wheat blossom midge (Sitodiplosis mosellana Géhin). AAC LeRoy was registered under the CWRS market class.

AAC Magnet Canada Western Red Spring wheat

AAC Magnet (BW1045) is an awned, hollow-stemmed, high-yielding Canada Western Red Spring (CWRS) wheat adapted to growing conditions in the Canadian Prairies. AAC Magnet was 5% higher yielding than Glenn and yielded 2% more than Carberry, a popular CWRS wheat variety across the Canadian Prairies. AAC Magnet matured 2 d earlier than Carberry and a day later than Unity, the earliest maturing check. AAC Magnet had the same height as Glenn and was shorter with better stem strength compared with Unity. AAC Magnet had better lodging scores compared with Unity. Over the 3 yr of testing (2015–2017), the test weight of AAC Magnet was slightly lower than the lowest checks, whereas the 1000-kernel weight of AAC Magnet was higher than all of the checks. The grain protein content of AAC Magnet was 0.3% lower than Carberry. AAC Magnet was rated moderately resistant to Fusarium head blight (Fusarium graminearum Schwabe), resistant to leaf rust (Puccinia triticina Erikss.) and stem rust (Puccinia graminis Pers. f. sp. tritici Erikss. & E. Henn). AAC Magnet was moderately susceptible/susceptible to resistant to the Ug99 family of stem rusts, resistant to loose smut [Ustilago tritici (Pers.) Rostr.], intermediately resistant to stripe rust (Puccinia striiformis Westend.), susceptible to common bunt [Tilletia caries (DC.) Tul. & C. Tul.], and moderately susceptible to leaf spot complex. AAC Magnet was susceptible to orange wheat blossom midge (Sitodiplosis mosellana Géhin). Based on the milling and baking performance over 3 yr (2015–2017) evaluated by the Grain Research Laboratory, Canadian Grain Commission, AAC Magnet was classified as CWRS wheat.

Haustoria - arsenals during the interaction between wheat and Puccinia striiformis f. sp. tritici.

SummaryAs an obligate parasite, Puccinia striiformis f. sp. tritici (Pst) forms haustoria to obtain nutrients from plant cells for development, and these structures are essential for pathogen survival. To better understand the contribution of haustoria to the interactions with the host plants, we isolated haustoria from susceptible wheat leaves infected with Pst race CYR31 and sequenced their transcriptome as well as those of urediospores and germ tubes, and compared the three transcriptomes. A total of 3524 up‐regulated genes were obtained from haustoria, of which 73 genes were related to thiamine biosynthesis, glycolysis and lipid metabolic processes. Silencing seven of the genes reduced the growth and development of Pst in wheat. More interestingly, 1197 haustorial secreted proteins (HASPs) were detected in haustoria, accounting for 34% of the total proteins, indicating that these HASPs play important roles in haustorium‐mediated pathogenic progression. Furthermore, 69 HASPs were able to suppress Bax‐triggered programmed cell death in tobacco. Additionally, 46 HASPs significantly reduced callose deposition in wheat using the type III secretion system. This study identified a large number of effectors through transcriptome sequencing, and the results revealed components of metabolic pathways that impact the growth and colonization of the pathogen and indicate essential functions of haustoria in the growth and pathogenicity of Pst.

Identification of three Berberis species as potential alternate hosts for Puccinia striiformis f. sp. tritici in wheat-growing regions of Xinjiang, China

Since the recent discovery of barberry (Berberis spp.) as an alternate host for the stripe rust pathogen Puccinia striiformis, many Chinese Berberis species have been identified as alternate hosts for P. striiformis f. sp. tritici. However, little is known about Berberis species and their distribution in wheat-growing regions in Xinjiang, China, where stripe rust is endemic. As the largest province or autonomous region, Xinjiang represents a relatively independent epidemic region for wheat stripe rust in China. In this study, we conducted a survey of barberry plants in the main wheat-growing areas of Xinjiang. We identified three Berberis species, B. heteropoda, B. nummularia and B. kaschgarica, and confirmed their roles as potential alternate hosts for P. striiformis f. sp. tritici in the laboratory.

Multi-Pathotype Testing of Selected Kenyan Wheat Germplasm and Watkin Landraces for Resistance to Wheat Stripe Rust (Puccinia striiformis f. sp tritici) Races

Wheat stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the key diseases of economic importance in wheat worldwide. Host resistance, which follows the gene-for-gene hypothesis between the host and pathogen, has been used in wheat lines to resolve resistance specificities and postulate resistant genes. The objective of this study was to elucidate stripe rust resistance in a collection of Kenyan wheat lines and Watkin landraces to identify new sources of stripe rust (Yr) resistance. In this study, the resistance in twenty wheat lines was determined by comparing their infection type with those of twenty differential lines using isolates representing twelve Puccinia striiformis races from Kenya, Denmark, U.K., Sweden, and Eritrea at the seedling stage. Among the twenty wheat lines, none was resistant to all the twelve Pst races and isolate DK02d/12 (“Kranich” race) was virulent on all the genotypes except wheat genotype “Kenya Tai.” This genotype (“Kenya Tai”) had the highest resistance as it was resistant to all the twelve stripe rust races used in this study. From this study, the introduction and utilization of wheat genotypes with adult plant resistant (APR) stripe rust genes, such as Yr15, are important in breeding wheat genotypes with effective resistance to wheat stripe rust in Kenya and worldwide.

The utility of NBS-profiling for characterization of yellow rust resistance in an F6 durum wheat population

Seedling and adult plant (field) resistance to yellow rust in the durum wheat (Triticum turgidum ssp. durum) cross Kunduru-1149 x Cham-1 was characterized using a functionally-targeted DNA marker system, NBS-profiling. Chi-squared analysis indicated a four gene model conferring seedling yellow rust resistance against Puccinia striiformis f. sp. tritici isolate WYR85/22 (virulent on Yr2, Yr6, Yr7 and Yr9). Interval mapping located two QTL for yellow rust resistance on the long arm of chromosome 1B, while Kruskal-Wallis single marker regression identified a number of additional marker loci associated with seedling and/or adult plant, field resistance to yellow rust. These results suggested that much of the yellow rust resistance seen in the field may be due to seedling expressed resistance (R) genes. Characterization of the DNA sequence of three NBS marker loci indicated that all showed significant homology to functionally-characterized R-genes and resistance gene analogues (RGAs), with the greatest homology being NBS-LRR-type R-genes and RGAs from cereal species.

Identification of New Wheat Genes for Durable Resistance of Adult Plants to Yellow Rust (Puccinia striiformis Westend. f. sp. Ericks)

Yellow rust is a wheat disease caused by Puccinia striiformis, this pathogen causes economic losses in susceptible materials, which represent up to 70% of wheat varieties. Currently, the incorporation of genetic resistance through molecular tools, is a process used in the generation of new varieties resistant to this pathogen. A strategy employed to identify genes involved in the resistance to yellow rust is to screen differential EST obtained by suppressive subtractive hybridization. In this research, cDNA was extracted from healthy and inoculated plants from the resistant line V-26 from INIFAP. A set of 200 differentially expressed EST were cloned and sequenced, and 31 of them were selected for expression profile analysis by RT-PCR; additionally, with the aim of validate RT-PCR results, five genes were selected for RT-qPCR analysis in genotypes inoculated by P. striiformis. The results showed high levels of expression of selected genes in genotypes classified as resistant in the field conditions (21, 143, 230, 242, 261 and 277), while in the susceptible genotype 16, few genes were induced by the rust. Expression profiles confirmed significant differences between resistant and susceptible lines.

Identification of New Wheat Genes for Durable Resistance of Adult Plants to Yellow Rust (Puccinia striiformis Westend. f. sp. Ericks)

Identification of New Wheat Genes for Durable Resistance of Adult Plants to Yellow Rust (<i>Puccinia striiformis</i> Westend. f. sp. Ericks)

Stripe Rust Effector PstGSRE1 Disrupts Nuclear Localization of ROS-Promoting Transcription Factor TaLOL2 to Defeat ROS-Induced Defense in Wheat

Puccinia striiformis f. sp. tritici (Pst), a biotrophic plant pathogen, secretes numerous effectors to modulate host defense systems. Understanding the molecular mechanisms by which Pst effectors regulate wheat immunity is of great importance for the development of novel strategies for durable control of stripe rust. In this study, we identified a glycine-serine-rich effector gene, PstGSRE1, which is highly induced during early infection. Transgenic expression of PstGSRE1 RNAi constructs in wheat significantly reduced virulence of Pst and increased H2O2 accumulation in wheat. PstGSRE1 was shown to target the reactive oxygen species (ROS)-associated transcription factor TaLOL2, a positive regulator of wheat immunity. PstGSRE1 disrupted nuclear localization of TaLOL2 and suppressed ROS-mediated cell death induced by TaLOL2, thus compromising host immunity. This work reveals a previously unrecognized strategy whereby rust fungi exploit the PstGSRE1 effector to defeat ROS-associated plant defense by modulating the subcellular compartment of a host immune regulator and facilitate pathogen infection.

Genetic diversity and population structure of Puccinia striiformis f. sp. tritici reveal its migration from central to eastern China

This study focused on Puccinia striiformis f. sp. tritici (Pst), the causal pathogen of wheat stripe rust in China. The central and eastern China are the main wheat-producing areas in the country, and the occurrence of wheat stripe rust can cause severe yield losses. To determine the population genetic structure and potential pathways of migration among regional populations of the pathogen, 264 isolates were sampled during spring 2017 and genotyped using 12 simple sequence repeat (SSR) loci. The highest genetic diversity was detected in Hubei Province, indicating its potential role as one of the source populations for the rust epidemics. A high gene flow was detected among regional populations, supporting migration of Pst among regions. The significant decrease in genotypic and genetic diversity from Hubei to Shandong suggested that the migration direction was from central to eastern China. Analysis of molecular variance (AMOVA), Bayesian assignment, and principle coordinate analysis all indicated two major groups, one was mainly distributed in Hubei and the another was mainly in the eastern region. Shared genotypes, gene flow among regions, and Bayesian assignment tests revealed that the migration of Pst from central to eastern of China during spring occurred mainly through stepwise movement.

QPCR and loop mediated isothermal amplification for rapid detection of Ustilago tritici.

Loose smut of wheat caused by the basidiomycete fungus Ustilago tritici, a seed-borne disease, is difficult to control because of the expanse of wheat planting area and difficulty in pathogen detection. In this study, real-time fluorescence quantitative PCR (qPCR) and loop-mediated isothermal amplification (LAMP) assays are used to rapidly amplify the DNA of U. tritici. Five pairs of primers for qPCR and two series primers for LAMP were designed. Primarily, the specificity of the primer was assessed by using genomic DNA of U. tritici, Fusarium graminearum, Blumeria graminis, Rhizoctonia cerealis, Puccinia striiformis, Bipolaris sorokiniana, and Alternaria solani as templates. Further, the amplification systems were optimized. Finally, the sensitivity of qPCR and LAMP assays were evaluated. The results showed that the primer Y-430 F/R, Y-307 F/R, Y-755 F/R, and Y-139 F/R for qPCR and primers L-139 and L-988 for LAMP could be used for U. tritici detection. In the sensitivity test, the detection limit of qPCR assay was identified as 10 pg μL−1 of genomic DNA, the detection limit for LAMP assay was 100 fg μL−1. We successfully performed qPCR and LAMP assays on wheat loose smut wheat samples. This paper establishes two methods for U. tritici detection, which can be used for diagnosis of wheat loose smut in the laboratory and in the field.

TaXa21, a Leucine-Rich Repeat Receptor-Like Kinase Gene Associated with TaWRKY76 and TaWRKY62, Plays Positive Roles in Wheat High-Temperature Seedling Plant Resistance to Puccinia striiformis f. sp. tritici.

Puccinia striiformis f. sp. tritici causes wheat stripe rust, one of most important diseases of wheat worldwide. High-temperature seedling plant (HTSP) resistance of wheat to P. striiformis f. sp. tritici is one specific type of host resistance, induced by high temperature (HT). Receptor-like kinases (RLKs) play key roles in regulating plant development and signaling networks, but there have been no reports on possible roles played by RLKs in wheat HTSP to P. striiformis f. sp. tritici. In the present study, a leucine rich repeat (LRR)-RLK gene, TaXa21, with a high homology with rice bacterial blight resistance gene Xa21, was cloned from wheat cultivar Xiaoyan 6 (XY 6). TaXa21 expression was up-regulated by the exposure to HT (20°C) for 24 h at 8 days postinoculation with P. striiformis f. sp. tritici and was induced by ethylene (ET) and hydrogen peroxide (H2O2). Knocking down TaXa21 using virus-induced gene silencing reduced HTSP resistance to P. striiformis f. sp. tritici compared with the control plants. In addition, the expression level of TaCAT in the H2O2 pathway was induced and TaACO in the ET signal pathway was reduced in the HT-treated TaXa21-silenced plants. Transient expression of TaXa21 in tobacco leaves confirmed its subcellular localization in plasma membrane, consistent with the prediction from bioinformatics analysis. The transmembrane and kinase domain of TaXa21 can interact with TaWRKY76 in the nucleus and cell membrane, which is different from the localization of Xa21 in rice. The interaction between TaWRKY76 and TaWRKY62 (positively involved in the HTSP resistance of XY 6) were observed. Together, these results indicated that TaXa21 is a RLK associated with TaWRKY76 and TaWRKY62 and functions as a positive regulator of wheat HTSP resistance to P. striiformis f. sp. tritici. Furthermore, the host defense is mediated by the H2O2 and ET signal pathways.

Histopathological analysis of incompatible and compatible interaction of Puccinia striiformis f.sp. tritici on wheat at early infection stage

Stripe rust of wheat (Puccinia striiformis f.sp. tritici; Pst) incurs substantial yield losses in wheat worldwide. Two Indian Pst pathotypes 38S102 (I) and 46S119 (Yr9 virulence) showed incompatible and compatible interaction respectively in variety PBW 343. The histopathological study using confocal microscopy revealed that during incompatible interaction comparatively longer abortive germ tubes are produced with a rapid response at early stages of infection but with no fungal colonization. Whereas, in compatible interaction, germ tubes were shorter with infection peg formed above the stomata subsequently leading to colonization. The detailed microscopic observations showed that necrotic regions were more prominent in incompatible interaction than compatible interaction. This information would aid in understanding the growth and virulence mechanism of stripe rust pathogen in wheat.

Identification of a new source of stripe rust resistance Yr82 in wheat.

Stripe rust resistance gene, Yr82, was mapped in chromosome 3BL using SNP markers. Yr82 interacted with Yr29 to produce lower stripe rust responses at the adult plant stage. Landrace Aus27969 produced low infection types against Australian Puccinia striiformis f. sp. tritici (Pst) pathotypes. A recombinant inbred line (RIL) F7 population from the Aus27969/Avocet S cross was developed. Monogenic segregation for seedling stripe rust response was observed among the RIL population, and the resistance locus was named Yr82. Bulk segregant analysis performed using the iSelect wheat 90K Infinium SNP array located Yr82 in the long arm of chromosome 3B. The RIL population was screened against stripe rust under field conditions and was genotyped with targeted genotyping-by-sequencing assay. QTL analysis detected the involvement of chromosomes 1B and 3B in controlling stripe rust resistance carried by Aus27969. Incorporation of Yr82 and marker SNPLr46G22 into the linkage map showed that the QTL in 1B and 3B represented Yr29 and Yr82, respectively. Kompetitive allele-specific PCR (KASP) markers sun KASP_300 and KASP_8775 flanked Yr82 distally and proximally, respectively, each at 2cM distance. These Yr82-linked markers were polymorphic among 84% of Australian cultivars and can be used for marker-assisted selection of Yr82.

Genetic Divergence and Diversity in Himalayan Puccinia striiformis Populations from Bhutan, Nepal, and Pakistan

The western Himalayan region in Pakistan has been shown to be the center of diversity of Puccinia striiformis; however, little is known about its genetic relations with the eastern part of the Himalayas. We studied the genetic structure of P. striiformis from Nepal (35 isolates) and Bhutan (31 isolates) in comparison with 81 Pakistani samples collected during 2015 and 2016, through microsatellite genotyping. Genetic analyses revealed a recombinant and highly diverse population structure in Pakistan, Bhutan, and Nepal. A high level of genotypic diversity (>0.90) was observed for the three countries of Pakistan (0.96), Bhutan (0.96), and Nepal (0.91) with the detection of 108 distinct multilocus genotypes (MLGs) in the overall population; 59 for Pakistan, 27 for Bhutan, and 26 for Nepal. Mean number of alleles per locus and gene diversity were higher in Nepal (3.19 and 0.458, respectively) than Bhutan (3.12 and 0.458, respectively). A nonsignificant difference between the observed and the expected heterozygosity in all populations further confirmed the recombinant structure. A clear population subdivision between the Himalayan region of Nepal, Bhutan, and Pakistan was evident, as revealed by FST values (ranging between 0.111 to 0.198), discriminant analysis of principal components, and resampling of MLGs. Limited gene flow could be present between Nepal and Bhutan, while the population from Pakistan was clearly distinct, and no divergence was present between two populations from Pakistan (Bajaur and Malakand). The overall high diversity and recombination signature suggested the potential role of recombination in the eastern Himalayan region (Nepal and Bhutan), which needs to be considered during host resistance deployment and in the context of aerial dispersal of the pathogen. Further surveillance should be made in the Himalayan region for disease management in the region and in the context of worldwide invasions.

QTL Mapping of Wheat Stripe Rust Resistance in Relation to Epidermal Flavonoids Measured Using a Leaf‐Clip, Optical Sensor

Epidermal flavonoids have long been suggested as targets for selection in stress‐adaptive crop breeding, but routine selection is neither practicable nor cost effective because of the time‐consuming process involved in phenotyping. In this research, we found that the content of epidermal flavonoids, measured using the chlorophyll fluorescence excitation ratio method, was inversely related to stripe rust (Puccinia striiformis f. sp. tritici [Pst]) response in wheat (Triticum aestivum L.). We investigated the genetic basis by conducting quantitative trait locus (QTL) mapping with a 9K single nucleotide polymorphism (SNP) array in a cross of ‘Drysdale’ × ‘Waagan’ segregating for adult‐plant stripe rust resistance. Data from 3 yr of field experimental trials were analyzed, and heritability estimates for epidermal flavonoids varied from 0.16 to 0.47 across the experiment‐years. In contrast, heritability for stripe rust response varied from 0.79 to 0.84. Eleven QTL were identified for epidermal flavonoids, with a major locus on chromosome 4D, explaining between 19.5 and 47.3% of the phenotypic variation across years. Thirteen QTL were identified for stripe rust response, with the major locus explaining 68% of the phenotypic variation. Quantitative trait loci associated with epidermal flavonoids and stripe rust response were colocated on chromosomes 2B and 3B, but other genetic loci detected for the respective traits were independent. The study has established a genetic basis for epidermal flavonoids measured using the nondestructive, leaf‐clip sensor in wheat, which will encourage further research on the possible exploitation of the compound by breeding and selection.