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Wheat stem (Sr), leaf (Lr) and stripe (Yr) rust pathogens are among the most destructive fungal diseases threatening global wheat production. We utilized 2300 wheat accession including worldwide landraces, cultivars, breeding materials and 341 synthetic accessions backcrossed with three widely grown Australian cultivars (Annuello, Yitpi and Correll) to investigate rust resistance under wide environmental conditions. The germplasm was genotyped with 90K SNP chip, and was phenotyped for two seasons in three different environments against Sr and Lr and in four different environments against Yr. Different environments for each trait showed significant correlation with mean r values of 0.53, 0.23 and 0.66 for Lr, Sr and Yr; respectively. Single-trait genome wide association (GWAS) revealed several environment-specific QTL and multi-environmental QTL distributed on all chromosomes except 6D. Multi-trait GWAS confirmed a cluster of Yr QTL on chromosome 3B (within 8.3 cM) as well as a QTL for Sr and Lr on chromosome 3D. Linkage disequilibrium and comparative mapping showed that at least three Yr QTL exists within the 3B cluster including the durable rust resistance gene Sr2/Yr30. The same region was effective against Sr resistance but did not pass the stringent significant threshold in two environments. The 3D QTL was found mainly in the synthetic germplasm with Annuello background which is known to carry the Ag. elongatum 3D translocation carrying Sr24/Lr24 resistance gene. Interestingly, estimating the SNP effect using BayesR method showed that the correlation among the highest 5% QTL effects across environments were lower than that for the small effect QTL with differences in r values of 0.25 and 0.2 for Lr and Yr respectively. These results indicate the importance of small effect QTL that cannot be captured using GWAS in achieving durable rust resistance. The detected QTL in this study are useful resources for improving bread wheat resistance to rust diseases.
To inform breeders and growers of important changes in virulence and to facilitate development and deployment of resistant cultivars, isolates of wheat rust fungi are routinely evaluated on seedlings of a set of differential wheat lines containing different resistant genes. However, the methods used to evaluate and report virulence changes in most regions of the world seem inadequate for accomplishing these goals and could be improved by adherence to three principles. Firstly, for each region, the resistance genes in the set of differentials should match the effective genes in contemporary cultivars and breeding lines. Most differential sets contain several resistance genes that have been ineffective for decades and do not contain genes found in cultivars and breeding lines. Given the importance of genes for race-specific adult-plant resistance, these should be included in differential sets. Secondly, intermediate reactions on differential lines that had been highly resistant are important warnings of gradual increases in virulence. Naming races requires isolates to be either virulent or avirulent on each line in a fixed set of differentials and is a hindrance to identifying gradual changes in virulence on currently effective genes. Utilizing virulence formulae with a designation for intermediate virulence (e.g. parentheses around the gene or differential) seems to be a simple solution for both documenting partial virulence and for easily changing differentials to match genes in cultivars and breeding lines. Thirdly, the method for evaluating virulence against a particular differential should predict the result of that host-pathogen interaction in the field. Growth stage and environmental conditions are important for expression of some resistance genes, and all currently effective genes are not likely to be expressed under the same conditions. Following these principles will make virulence surveys more predictive of important changes in the field and thereby contribute to more effective management of rust diseases.
Rusts (Puccinia spp.) are the most significant disease affecting wheat yield and quality in Turkey. Knowing the resistance status of wheat genotypes in crossing program is an important issue for breeding programs. The aim of the study was to determine of the resistance of the 106 wheat genotypes consisting of Crossing Block Spring Wheat (CBSW) nursery developed by the International Winter Wheat Improved Project (IWWIP). For this purpose, adult plant and seedling test were conducted for yellow rust while only seedling test were conducted for leaf and stem rust. Evaluations were carried out at the research facilities of CRIFC at İkizce and Yenimahalle in Ankara in the 2014 season. For adult plant reactions; the genotypes were inoculated with local Pst populations (virulent on Yr2,6,7,8,9,25,27,Sd,Su,Avs). Stripe rust development on each entry were scored using the modified Cobb scale when the susceptible check Little Club had reached 80S infection severity in June, 2014. Coefficients of infections were calculated and values below 20 were considered to be resistant. For seedling test; the seedling was inoculated with local Pgt (avirulent on Sr24, Sr26, Sr27, and Sr31), Pt (avirulent on Lr9, Lr19, Lr24, and Lr28) and Pst populations. Stripe, leaf and stem rust development on each entry were scored after 14 days with 0-4 and 0-9 scale for leaf-stem rust and yellow rust, respectively. In seedling stage, thirty nine (37%), 47 (44%), and 20 (19%) genotypes were resistant to local Pgt, Pt, and Pst populations, respectively. In adult plant test, 46 (43%) genotypes were resistant to Pst. The resistance genotypes to stem, leaf, and stripe rust were determined with this research.
An important component of the management of wheat stem rust is an understanding of the population diversity of the pathogen, Puccinia graminis f. sp. tritici (Pgt). The discovery of “Ug99” resulted in renewed efforts on pathogen surveys, sample collections and pathotyping of Pgt, with a primary focus on Africa. In the last few years these efforts have been expanded to include other targeted regions, however a global effort is needed. The aims of the “Global Pgt Initiative” is: to capture and maintain living cultures that collectively reflect the entire global diversity of Pgt in the years 2014 - 2016; pathotype and genotype this collection; develop DNAbased diagnostic tools that will be able to rapidly detect shifts in Pgt populations, and provide an early warning system of the vulnerability of wheat to new virulent strains; and provide a genetic baseline for comparison of Pgt populations over time, both forward and backwards. This initiative will provide the wheat rust community with a geographically distributed, well characterized, living culture collection that represents the global diversity of Pgt; a global open access knowledge bank on Pgt pathotypes and genotypes; and advanced molecular diagnostic tools for rapid detection and tracking of Pgt populations. The Global Pgt Initiative represents the most comprehensive effort to capture and characterize the global diversity of Pgt and provide a unique resource to the global wheat rust community.
Wheat contributes directly to food security and the national economy in Nepal. Of the rusts of wheat, stripe rust causes the most frequent and severe yield losses. Race changes can lead to damaging epidemics. To better understand factors that influence regional diversity of the stripe rust and stem rust pathogens, we surveyed rusts on barberry in 2012 and 2013. Nepal has a high diversity of barberry (30 species) and elevational habitats that extend the seasonal distributions of wheat and barberry. The greatest diversity occurs from 2,700 m and above, and distributions range from 1,200 to 4,500 m. We surveyed locations in all regions (central, eastern, western, and far-western) of the hill zone. Barberry was common between 1,300 and 1,800 m where wheat is grown. In the far-western region, barberry was found near all the wheat fields we surveyed. Between 1,300 and 1,800 m, Berberis asiatica is the most common species. B. aristata is present at the upper end of this range. Aecial infections on barberry occurred in patchy distributions in both 2012 and 2013. Collections of aecia on barberry were made at 5 locations and are being identified by inoculation studies using a range of grass hosts. Additionally, the rust samples are being evaluated by real-time PCR assays using species-specific ITS primer/probes for detection of Puccinia graminis or P. striiformis. Preliminary results for 32 single-aecia samples from 2012 were negative for P. graminis; 7 were positive for the P. striiformis complex.
Targeted breeding to develop high yielding wheat germplasm resistant to Ug99 and other rusts initiated at CIMMYT in 2006. Ug99 resistant materials, especially those with adult plant resistance (APR), were used in crossing. F3 and F4 populations from simple, BC1 and top crosses were grown for two generations under high rust pressures at Njoro, Kenya in a Mexico-Kenya shuttle breeding scheme. Parallel populations were also grown in Mexico for comparison. Approximately 5,000 advanced lines were tested for grain yield performance at Ciudad Obregon, Mexico in 2009/10 season, and phenotyped for resistance to Ug99 and other rusts. The 728 retained lines were evaluated for grain yield performance in five environments during the 2010/11 season in Mexico. About 68% of the 728 lines had nearimmune (16.5% entries) to adequate APR to Ug99. An additional 13.6% lines carried one of the six (Sr25, Sr26, SrTmp, SrHuw234, SrSha7, and an unidentified gene) race-specific resistance genes often in combination with APR gene Sr2. About 80% entries were highly resistant to yellow rust in Kenya and Mexico, and 90% entries to leaf rust in Mexico. Yield distribution of lines derived from Mexico-Kenya shuttle breeding was similar to lines selected only in Mexico. Sufficient lines with >5% superior yields than the Mexican checks varieties in 2 years testing were identified. Our results indicate that targeted crossing and shuttle breeding are powerful tools for a simultaneous improvement of grain yield potential and resistance to rusts.
This paper offers projections of potential effects of climate change on rusts of wheat and how we should factor in a changing climate when planning for the future management of these diseases. Even though the rusts of wheat have been extensively studied internationally, there is a paucity of information on the likely effects of a changing climate on the rusts and hence on wheat production. Due to the lack of published empirical research we relied on the few published studies of other plant diseases, our own unpublished work and relevant information from the vast literature on rusts of wheat to prepare this overview. Potential risks from a changing climate were divided into three major groups: increased loss from wheat rusts, new rust races evolving faster and the reduced effectiveness of rust resistances. Increased biomass of wheat crops grown in the presence of elevated CO2 concentrations and higher temperatures will increase the leaf area available for attack by the pathogen. This combined with increased speed of the pathogen’s life cycle, may increase the rate of epidemic development in many environments. Likewise, should the effects of climate change result in more conducive conditions for rust development there will also be a corresponding increase in the rate of evolution of new and presumably virulent races. The effectiveness of some rust resistance genes are influenced by temperature, crop development stage and even nitrogen status of the host. It is likely that direct and indirect changes on the host from climate change may influence the effectiveness of some of these resistance genes. Currently the likely effects of climate change on the effectiveness of disease resistance is not known and since disease resistance breeding is a long term strategy it is important to determine if any of the important genes may become less effective due to climate change. Studies must be made to acquire new information on the rust disease triangle to increase the adaptive capacity of wheat under climate change. BGRI leadership is needed to broker research on rust evolution and the durability of resistance under climate change.
In China, wheat is grown on approximately 24 million hectares with an annual yield of 100 million tonnes. Stem rust, caused by Puccinia graminis f. sp. tritici, is a threat mainly to spring wheat in northeastern China. Leaf rust, caused by P. triticina, occurs on crops in the late growth stages in the Yellow-Huai-Hai River regions. Stripe rust, caused by P. striiformis f. sp. tritici (Pst), is destructive in all winter wheat regions and is considered the most important disease of wheat in China. During the last 20 years, widespread stripe rust epidemics occurred in 2002, 2003, and 2009, and localized epidemics occurred in many other years. In recent years, major yield losses were prevented by widespread and timely applications of fungicides based on accurate monitoring and prediction of disease epidemics. A total of 68 Pst races or pathotypes have been identified using a set of 19 differential wheat genotypes. At present, races CYR32 and CYR33 virulent to resistance genes Yr9, Yr3b, Yr4b, YrSu and some other resistance genes are predominant. Moreover, these races are virulent on many cultivars grown in recent years. Of 501 recent cultivars and breeding lines 71.9% were susceptible, 7.0% had effective all-stage resistance, mostly Yr26 (= Yr24), and 21.2% had adult-plant resistance. Several resistance genes, including Yr5, Yr10, Yr15, Yr24/Yr26, YrZH84 and some unnamed genes, are still effective against the current Pst population. All have been widely used in breeding programs. Lines with one or more of Yr1, Yr2, Yr3, Yr4, Yr6, Yr7, Yr8, Yr9 and other unnamed resistance genes are susceptible to currently predominant races. Durable adult plant resistance sources are being increasingly used as parents in breeding programs. Progress has been made in genomics and population genetics of Pst, molecular mapping of resistance genes, and cytological and molecular mechanisms of the host-pathogen interactions involved in stripe rust.
Rust diseases remain a significant threat to the production of most cereals including wheat. New sources of resistance are continually sought by breeders to combat the emergence of new pathogen races. Rice is atypical in that it is an intensively grown cereal with no known rust pathogen. The resistance of rice to cereal rust diseases is referred to as nonhost resistance (NHR), a resistance mechanism that has only recently become genetically tractable. In this report, the mechanisms of rice NHR to wheat stem rust and other cereal rust diseases are explored and the potential for transferring this durable disease resistance to wheat is considered. Approaches being undertaken for the molecular-genetic dissection of rice NHR to rust are described.
Two broad categories of resistance genes in wheat have been described. One group represents the so called seedling resistance or the ‘gene for gene’ class that often provides strong resistance to some but not all strains of a rust species. The other category referred to as adult plant resistance provide partial resistance that is expressed in adult plants during the critical grain filling stage of wheat development. A few seedling rust resistance genes have been cloned in wheat and other cereals and are predominantly from the nucleotide binding site/leucine rich repeat class which is associated with localized cell death at the pathogen entry site. Until recently, the molecular basis of race non-specific, partial and slow rusting adult plant resistance genes were unknown. Gene products that differ from known plant resistance genes were revealed from the recent cloning of the Yr18, Yr36 and Lr34 adult plant genes in wheat. The available range of diverse resistance gene sequences provide entry points for developing genebased markers and will facilitate selection of germplasm containing unique resistance gene combinations.