Xianming Chen

Stripe rust (caused by Puccinia striiformis f. sp. tritici), a destructive disease of wheat, can be controlled by planting resistant cultivars and/or applying fungicides. This study was conducted in a field near Pullman, WA, in 2025 to determine yield losses by stripe rust and yield protection by fungicide applications on 23 commercially grown winter wheat cultivars, plus a susceptible check (PS279). The fungicide applications significantly reduced stripe rust, increased grain test weight, and increased yield in PS279 and some commercial cultivars. The results are useful for growers to determine if fungicide application is needed based on their planted cultivars.

Stripe rust (caused by Puccinia striiformis f. sp. tritici) is a damaging disease of wheat and can be controlled by planting resistant cultivars and/or applying fungicides. This study was conducted in a field near Pullman, WA, in 2025 with artificial inoculation of the pathogen to determine yield losses by stripe rust and yield protection by the applications of fungicides Trivapro and Tilt on 23 commercially grown spring wheat cultivars, plus a susceptible check (AvS). The fungicide applications significantly reduced stripe rust in AvS and two commercial cultivars but did not increase test weight or yield in any tested cultivars including AvS. This was likely due to slow and low rust development under dry weather conditions. These results indicate that fungicide application is not necessary when stripe rust pressure is low.

Stripe rust is a destructive disease of wheat, and its damage can be reduced by applying fungicides. This study was conducted in a field near Pullman, WA, under both natural infection and artificial inoculation of Puccinia striiformis f. sp. tritici in 2025 to determine the efficacy of 23 fungicide treatments on the control of stripe rust on a susceptible winter wheat variety. All fungicide treatments significantly reduced relative area under the disease progress curve (rAUDPC); three of them had significantly higher test weight; and 19 of them produced significantly higher grain yield than the nontreated check. The significant yield differences compared with the nontreated check ranged from 15.2 bushels/acre (14.5%) to 46.6 bushels/acre (44.7%). The results are useful for growers to select fungicides and the timing of application for controlling stripe rust on winter wheat.

Puccinia striiformis f. sp. tritici (Pst) and P. striiformis f. sp. hordei (Psh) cause stripe rust epidemics on wheat and barley, respectively. Demethylation inhibitor (DMI) fungicides have been used for decades in managing stripe rust in the United States, but the pathogen tolerance was not clear. To determine the dynamics of DMI fungicide targeting gene Cyp51 mutants, Pst isolates collected from 1968 to 2021 and Psh isolates from 1993 to 2021 were tested using a Kompetitive allele-specific PCR (KASP) marker for the Y134F point mutation in the Cyp51 gene. The mutant allele was found in the Pst and Psh populations as early as in 1968 and 1993, respectively, and the mutant frequencies fluctuated from year to year. The KASP test of the Pst-infected leaf samples from fungicide-testing plots in 2024 revealed that the application of the DMI fungicide Tilt increases the mutant frequency. The urediniospore germination tests with 22 selected isolates at different concentrations of Tilt showed that the mean half-maximal effective concentration (EC ₅₀ ) value of the homozygous mutant isolates was 2.7 times higher than that of the wild-type isolates, but those of the heterozygous and wild-type isolates were not significantly different from each other. The results indicate that the KASP marker is useful in monitoring the DMI fungicide targeting gene mutants, and DMI fungicides are likely less effective in controlling the homozygous mutant population than the wild-type and heterozygous populations. The information is useful for managing stripe rust using diverse fungicides and growing resistant cultivars.

Stripe rust, caused by Puccinia striiformis f. sp. tritici ( Pst), is a devastating wheat disease worldwide, and novel stripe rust resistance genes that can be readily used in wheat breeding are needed for sustainable wheat production. Winter wheat cultivar Samara exhibits high resistance to predominant Pst races in the United States. An F _2:3 population and an F ₇ recombinant inbred line (RIL) population derived from the cross Samara × Jagalene were evaluated for responses to Pst race PSTv-37 at the seedling stage, leading to the conclusion that a single dominant gene conditions stripe rust resistance in Samara. A subset of F ₂ plants was genotyped using genotyping-by-sequencing (GBS), and over 44,000 single-nucleotide polymorphisms (SNPs) were identified. Allele distribution analyses at each SNP locus revealed a genomic region harboring a stripe rust resistance gene, temporarily designated YrSA, in the terminal region of chromosome arm 2AL in Samara. Some GBS SNPs in the target region were converted to KASP markers, and linkage analysis delimited YrSA to an interval of 1.65 Mb between 784.71 and 786.36 Mb in the Chinese Spring IWGSC RefSeq v.2.1 reference sequence. The mapping results were further confirmed in the F ₇ RIL population. YrSA is a new stripe rust resistance gene different from other genes previously reported on chromosome arm 2AL based on their locations, origins, and responses to Pst races. The physical order of YrSA with other permanently named Yr genes on 2AL is Yr1-Yr86-Yr32-YrSA. YrSA can be widely used to enhance wheat stripe rust resistance in the United States, and the newly developed KASP markers can be used in marker-assisted selection.

Wheat stripe rust, caused by the biotrophic fungal pathogen Puccinia striiformis f. sp. tritici ( Pst), is among the top crop diseases incurring huge economic losses worldwide. Identification of new stripe rust-resistant sources that can be easily used in wheat cultivar development is essential for food security. PI 622129, an Iranian wheat landrace, exhibits high resistance to the predominant U.S. Pst races. A recombinant inbred line (RIL) population from the cross PI 622129 × Stardust was genotyped using single-nucleotide polymorphisms generated by genotyping-by-sequencing. The RIL population was evaluated for responses to the Pst race PSTv-37 at the seedling stage in three environments, and quantitative trait loci (QTLs) analysis revealed four QTLs for stripe rust resistance on chromosome arms 2DS, 5BS, 2AL, and 7BL. Of these, QYr.stars-2DS and QYr.stars-5BS are major QTLs that explained 21 to 38% and 11.6 to 27.2% of the total phenotypic variance, respectively, in three experiments. QYr.stars-2DS is a new stripe rust resistance locus that was identified in the interval of 2.58 to 5.54 Mb on chromosome arm 2DS based on the Chinese Spring IWGSC RefSeq v.2.1 reference genome. Another QTL, QYr.stars-5BS, is close to Yr47 and was delimited to the interval 8.1 to 9.0 Mb in the reference genome. QYr.stars-2AL and QYr.stars-7BL were mapped to the terminal and QTL-rich regions on chromosome arms 2AL (750.8 to 752.5 Mb) and 7BL (718.1 to 721.2 Mb), respectively. KASP markers were developed to facilitate rapid introgression of these QTLs into locally adapted lines via marker-assisted selection.

Puccinia striiformis f. sp. hordei (Psh), causing stripe rust epidemics on barley, has been continually changing in virulence since its introduction in the United States in the early 1990s. To monitor virulent changes, 344 Psh isolates recovered from stripe rust samples collected from barley, grasses, and wheat in 2010-2020 were tested on the set of 12 barley differentials. Among the 12 virulence factors, the frequency of virulence to Topper was the highest (99.4%), followed by those to Abed Binder 12 (77.9%), Trumpf (35.2%), and Bancroft (27.6%). The virulence frequencies of all remaining differentials were below 20.0% with 16.6% on Hiproly; 15.7% on Bigo; 14.5% on Varanda; 10.8% on Emir; 9.9% on Astrix and Heils Franken, 8.1% on Muzurka, and 2.6% on I 5. Except the consistently high frequencies of the virulence to Topper from different years, epidemiological regions, and hosts, virulence frequencies of other differentials varied from year to year, region to region, and host to host. A total of 41 Psh races were identified from the isolates, with the numbers of virulence factors ranging from 0 to 12. Among these races, 12 were first identified in the United States in the present study while the other 29 races were first detected before 2010. Race PSH-33 (virulent on differentials Topper and Abed Binder 12) had the highest frequency (36.0%), followed by PSH-48 (virulent only on differential Topper) with a frequency of 20.6%. Cluster analysis and discriminant analysis of principal components revealed differentiations among years with major changes occurring between 2016 and 2017, and to a less extent to regions and hosts. The results should be useful for developing resistant barley cultivars for control of stripe rust.

Puccinia striiformis f. sp. tritici, causing stripe rust, is one of the most prominent pathogens of wheat worldwide. The biotrophic and obligate fungus is capable of rapidly developing new virulent races that can overcome race-specific resistance in host plants. The traditional virulence characterization of the pathogen requires strict conditions for testing isolates on wheat differentials with specific resistance genes, which is time-consuming. Developing molecular markers for avirulence genes could provide an efficient method for monitoring virulence changes in the pathogen population. In this study, secreted protein gene-based single-nucleotide polymorphism markers previously identified to be associated with avirulence genes of the pathogen were converted to kompetitive allele-specific PCR (KASP) markers. The KASP markers were screened with a diverse panel of 192 isolates selected from various countries based on their virulent races and molecular genotypes. The markers significantly correlated with the avirulence/virulence phenotypic data of the 192 isolates were further validated with 845 isolates collected from the United States in 2019 to 2021. Based on the results of both the screening and validation data, 21 KASP markers significantly associated with different avirulence genes were developed. Seventeen of the 21 markers were significantly associated with two or more avirulence genes, and except AvrYr10, and the remaining 15 avirulence genes had two or more markers. Different combinations of up to three markers could be used for specific detection of 16 avirulence genes in monitoring the pathogen population.

Common bunt, caused by Tilletia caries and T. laevis, is a fungal disease of wheat that can cause significant yield and quality losses. Field screening for common bunt is often hampered by environmental factors and limited to one evaluation per year. In contrast, greenhouse screening potentially enables two to three assessments per year. We evaluated three greenhouse inoculation methods and two scoring techniques using two highly susceptible cultivars (‘Apogee’ and ‘Red Bobs’) and four moderately susceptible cultivars (‘Fielder’, ‘LCS Star’, ‘SY Capstone’, and ‘UI Platinum’). In the first inoculation method, seeds were inoculated with a teliospore suspension and planted on the same day. In the second method, inoculated seeds were allowed to dry and were planted three days later. In the third method, seedlings were inoculated at the three-leaf stage by spraying a teliospore suspension directly onto the leaves. Disease incidence (percentage of infected spikes per plant) and severity (percentage of infected spikelets) were measured at physiological maturity. Seedling inoculation by spraying produced the lowest infection levels across all cultivars, with overall incidence ranging from 7.91 to 19.11% and severity from 5.01 to 13.69%. The two seed inoculation methods showed greater disease symptoms, with incidence ranging from 49.35 to 98.02% and severity from 43.04 to 94.06 %, with no significant differences between the methods. Severity and incidence were correlated (0.95 to 0.97) across the two seed inoculation methods. Thus, the seed inoculation methods were superior to seedling spray inoculation method for the response of wheat germplasm to common bunt infection. This finding was validated with a set of wheat cultivars and differential lines. This study underscores the importance of considering inoculation techniques in screening germplasm to select for common bunt resistance in wheat.

Key message The genome-wide association study utilizing a diverse global collection of 318 barley accessions identified 44 loci for stripe rust resistance, including 14 potentially novel loci. Abstract Stripe rust is an important disease of barley in temperate regions worldwide. Identification and genetic characterization of stripe rust resistance are essential for development and deployment of durable resistance in barley cultivars. A total of 318 spring barley accessions from a global barley collection were evaluated for resistance to the stripe rust pathogen Puccinia striiformis f. sp. hordei (Psh) at the seedling stage in the greenhouse (4–20 °C) using five Psh races (PSH-33, PSH-48, PSH-72, PSH-117 and PSH-118) and at the adult-plant stage in the greenhouse under a high-temperature profile (15–25 °C) and in four field environments in Washington, USA. The frequencies of resistant accessions ranged from 15 to 54% at the seedling stage and 47–76% at the adult-plant stage. Genome-wide association analysis using 6332 markers identified 60 significant SNPs corresponding to 44 loci for stripe rust resistance. These loci include 22 all-stage resistance (ASR) loci and 22 high-temperature adult-plant resistance (HTAP) loci, distributed across all seven barley chromosomes. Three ASR loci and eleven HTAP loci are potentially novel. Five ASR loci (QRpsh_1H.3, QRpsh_3H.1, QRpsh_5H.5, QRpsh_5H.6 and QRpsh_7H.4) and five HTAP resistance loci (QRpsh_1H.4, QRpsh_3H.2, QRpsh_4H.3, QRpsh_5H.4 and QRpsh_7H.2) are major-effect loci, each explaining over 10% of the phenotypic variation. PCR Allele Competitive Extension markers were developed for eight broad-spectrum ASR loci, which may facilitate resistance screening and targeted locus introgression in barley breeding programs. The genetic resources developed in this study should be valuable for breeding efforts aimed at enhancing resistance to stripe rust in barley cultivars.