Stem Rust Epidemics Research Articles

Mapping rust resistance in European winter wheat: many QTLs for yellow rust resistance, but only a few well characterized genes for stem rust resistance

Key messageStem rust resistance was mainly based on a few, already known resistance genes; for yellow rust resistance there was a combination of designated genes and minor QTLs.Yellow rust (YR) caused by Puccinia striiformis f. sp. tritici (Pst) and stem rust (SR) caused by Puccinia graminis f. sp. tritici (Pgt) are among the most damaging wheat diseases. Although, yellow rust has occurred regularly in Europe since the advent of the Warrior race in 2011, damaging stem rust epidemics are still unusual. We analyzed the resistance of seven segregating populations at the adult growth stage with the parents being selected for YR and SR resistances across three to six environments (location–year combinations) following inoculation with defined Pst and Pgt races. In total, 600 progenies were phenotyped and 563 were genotyped with a 25k SNP array. For SR resistance, three major resistance genes (Sr24, Sr31, Sr38/Yr17) were detected in different combinations. Additional QTLs provided much smaller effects except for a gene on chromosome 4B that explained much of the genetic variance. For YR resistance, ten loci with highly varying percentages of explained genetic variance (pG, 6–99%) were mapped. Our results imply that introgression of new SR resistances will be necessary for breeding future rust resistant cultivars, whereas YR resistance can be achieved by genomic selection of many of the detected QTLs.

Wheat Stem Rust Detection and Race Characterization in Tunisia.

Climate changes over the past 25 years have led to conducive conditions for invasive and transboundary fungal disease occurrence, including the re-emergence of wheat stem rust disease, caused by Puccinia graminis f.sp. tritici (Pgt) in East Africa, Europe, and the Mediterranean basin. Since 2018, sporadic infections have been observed in Tunisia. In this study, we investigated Pgt occurrence at major Tunisian wheat growing areas. Pgt monitoring, assessment, and sampling from planted trap nurseries at five different locations over two years (2021 and 2022) revealed the predominance of three races, namely TTRTF (Clade III-B), TKKTF (Clade IV-F), and TKTTF (Clade IV-B). Clade III-B was the most prevalent in 2021 as it was detected at all locations, while in 2022 Pgt was only reported at Beja and Jendouba, with the prevalence of Clade IV-B. The low levels of disease incidence during these two years and Pgt population diversity suggest that this fungus most likely originated from exotic incursions and that climate factors could have caused disease establishment in Tunisia. Further evaluation under the artificial disease pressure of Tunisian wheat varieties and weather-based modeling for early disease detection in the Mediterranean area could be helpful in monitoring and predicting wheat stem rust emergence and epidemics.

Puccinia graminis f. sp. tritici Population Causing Recent Wheat Stem Rust Epidemics in Kazakhstan Is Highly Diverse and Includes Novel Virulence Pathotypes

Wheat stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is a reemerging disease that caused severe epidemics in northern Kazakhstan and western Siberia in the period of 2015 to 2019. We analyzed 51 stem rust samples collected between 2015 and 2017 in five provinces in Kazakhstan. A total of 112 Pgt races were identified from 208 single-pustule isolates. These races are phenotypically and genotypically diverse, and most of them are likely of sexual origin. No differentiation of phenotypes and single-nucleotide polymorphism genotypes was observed between isolates from Akmola and North Kazakhstan provinces, supporting the idea of a wide dispersal of inoculum in the northern regions of the country. Similarities in virulence profiles with Pgt races previously reported in Siberia, Russia, suggest that northern Kazakhstan and western Siberia constitute a single stem rust epidemiological region. In addition to the races of sexual origin, six races reported in Europe, the Caucasus, and East Africa were detected in Kazakhstan, indicating that this epidemiological region is not isolated, and spore inflow from the west occurs. Virulence alone or in combination to several genes effective against the Ug99 race group was detected, including novel virulence on Sr32 + Sr40 and Sr47. The occurrence of a highly diverse Pgt population with virulence to an important group of Sr genes demonstrated the importance of the pathogen's sexual cycle in generating new and potentially damaging virulence combinations.

Irrigation can create new green bridges that promote rapid intercontinental spread of the wheat stem rust pathogen

Wheat stem rust epidemics caused by the obligate pathogenic fungus Puccinia graminis f.sp. tritici have historically driven severe yield losses on all wheat growing continents and, after many decades of control, stem rust is re-emerging as a disease of concern. In 1998, a highly virulent race able to overcome 90% of world wheat cultivars, Ug99, was identified in Uganda. Since initial detection, the pathogen has evolved many new variants and spread to many countries. The original variant spread from East Africa to the Middle East with three years between detection in Ethiopia and subsequent detection in Yemen. In 2014, another Ug99 variant (TTKTT), with one of the most complex virulence profiles, was detected in Kenya. This variant also spread from East Africa to the Middle East, but with only one year between detection in Ethiopia and subsequent detection in Iraq. Here we investigate potential airborne migration routes to account for the rapid spread of TTKTT in East Africa and beyond to the Middle East by using an integrated model combining the outputs from a meteorology-driven fungal spore dispersion model with epidemiological models to account for seasonal availability of susceptible crops and conditions for spore release and infectivity. We find viable pathways in the 2018/19 season that incorporate critical stepping-stone locations in Yemen or Saudi Arabia, but only in the presence of newly irrigated regions in Ethiopia. Our results indicate the potential and increasing importance of irrigated wheat areas in Ethiopia, Yemen and Saudi Arabia for inter-regional stem rust movements. Future movement of stem rust races out of East Africa is considered likely as irrigated areas expand. Targeted surveillance and the use of mitigation strategies including the use of durable resistant varieties in regions of irrigation are required to reduce the risks of enhanced dispersal of stem rust to other regions.

Development and Validation of a Mechanistic, Weather-Based Model for Predicting Puccinia graminis f. sp. tritici Infections and Stem Rust Progress in Wheat.

Stem rust (or black rust) of wheat, caused by Puccinia graminis f. sp. tritici (Pgt), is a re-emerging, major threat to wheat production worldwide. Here, we retrieved, analyzed, and synthetized the available information about Pgt to develop a mechanistic, weather-driven model for predicting stem rust epidemics caused by uredospores. The ability of the model to predict the first infections in a season was evaluated using field data collected in three wheat-growing areas of Italy (Emilia-Romagna, Apulia, and Sardinia) from 2016 to 2021. The model showed good accuracy, with a posterior probability to correctly predict infections of 0.78 and a probability that there was no infection when not predicted of 0.96. The model’s ability to predict disease progress during the growing season was also evaluated by using published data obtained from trials in Minnesota, United States, in 1968, 1978, and 1979, and in Pennsylvania, United States, in 1986. Comparison of observed versus predicted data generated a concordance correlation coefficient of 0.96 and an average distance between real data and the fitted line of 0.09. The model could therefore be considered accurate and reliable for predicting epidemics of wheat stem rust and could be tested for its ability to support risk-based control of the disease.

Genetic Diversity and Population Structure Reveal Cryptic Genetic Variation and Long Distance Migration of Puccinia graminis f. sp. tritici in the Indian Subcontinent.

Stem rust caused by Puccinia graminis f. sp. tritici (Pgt) is a devastating disease of wheat worldwide since time immemorial. Several wheat stem rust outbreaks have been reported worldwide including India. Approximately 7 mha wheat area in central and peninsular India is highly vulnerable to stem rust epidemics. In this study, a repository of 29 single genotype uredospore pathotypes, representing five geographical regions, was characterized by investigating their virulence phenotype and simple sequence repeat (SSR) genotypes using 37 reproducible polymorphic SSR markers, 32 of which had ≥ 0.50 polymorphic information content (PIC) value. Virulence phenotypes were used to evaluate the virulence frequency (VF) and construct a hypothetical evolutionary hierarchy of these pathotypes. We projected seven lineages to explain the evolutionary pattern of the Pgt population. The VF of these pathotypes ranged between 0% and 100%. The virulence-based neighbor-joining (NJ) cluster analysis grouped Pgt pathotypes into five virulence groups. Likewise, five molecular groups were categorized using molecular genotypes. The molecular grouping was supported by principal coordinate analysis (PCoA), which revealed 25% of the cumulative variance contributed by the first two axes. Analysis of molecular variance (AMOVA) revealed 8 and 92% of the variation among and within the populations, respectively. The Mantel test confirmed a positive but weak correlation (R2 = 0.15) between virulence phenotypes and SSR genotypes. The highest and lowest values of different genetic diversity parameters (Na, Ne, I, He, uHe, and %P) revealed maximum and minimum variability in the Pgt population from Maharashtra and Uttar Pradesh, respectively. The population structure analysis clustered 29 Pgt pathotypes into two subpopulations and an admixture. Our results demonstrated that there was significant genetic diversity among Pgt pathotypes resulting from their long-distance dispersal ability complemented by gene flow. These findings provide insights into the virulence patterns, genetic variations, and possible evolution of Pgt pathotypes, which would support strategic stem rust resistance breeding.

Aeciospore ejection in the rust pathogen Puccinia graminis is driven by moisture ingress

Fungi have evolved an array of spore discharge and dispersal processes. Here, we developed a theoretical model that explains the ejection mechanics of aeciospore liberation in the stem rust pathogen Puccinia graminis. Aeciospores are released from cluster cups formed on its Berberis host, spreading early-season inoculum into neighboring small-grain crops. Our model illustrates that during dew or rainfall, changes in aeciospore turgidity exerts substantial force on neighboring aeciospores in cluster cups whilst gaps between spores become perfused with water. This perfusion coats aeciospores with a lubrication film that facilitates expulsion, with single aeciospores reaching speeds of 0.053 to 0.754 m·s−1. We also used aeciospore source strength estimates to simulate the aeciospore dispersal gradient and incorporated this into a publicly available web interface. This aids farmers and legislators to assess current local risk of dispersal and facilitates development of sophisticated epidemiological models to potentially curtail stem rust epidemics originating on Berberis.

Analysis of the effects of climate, host resistance, maturity and sowing date on wheat stem rust epidemics

Under appropriate environmental conditions, stem rust (caused by Puccinia graminis Pers.:Pers. f. sp. tritici) is a destructive disease of wheat crops worldwide. Although fungicide application and host genotype resistance are the most commonly used methods to control wheat stem rust, efficient agronomic methods are needed to lower disease management expenses and improve sustainability of wheat production. However, an understanding of highly effective agronomic practices to control stem rust is still lacking. From 2013 to 2017, 282 wheat-stem-rust progress curves were studied using three agronomic, three climatic, and two disease variables at plot scale in Kermanshah province, Iran. Cultivar, sowing date, and disease-assessment time significantly affected stem rust severity. From principal component analysis (PCA), three principal components explained 79% of data variance. According to PCA and multiple regression model, AUDPC corresponded with disease-onset and maturity date, mean minimum temperature and number of rainy days over designated spring months, number of days with minimum temperature within 5–20 °C and maximum relative humidity above 60%, resistance index, and sowing date. These new findings suggested improvement of current predicting models by involving dates of maturation and sowing, and wheat resistance besides rainfall-temperature-wetness variables for more effective, economic and sustainable management of stem rust epidemics.

Virulence and race structure of Puccinia graminis f. sp. tritici in Kazakhstan

Severe epidemics of wheat stem rust caused by Puccinia graminis f. sp. tritici (Pgt) have been observed in recent years in major spring wheat producing regions in Kazakhstan. However, information on the virulence structure and race composition of Pgt is currently not available. Stem rust samples were collected in 2015–2018 in three regions of Kazakhstan to determine the virulence diversity and race distribution in the Pgt populations. A total of 203 single-pustule isolates were derived and evaluated on the stem rust differential and supplemental lines and 38 races were identified. Among them, the races QHHSF and THMTF were found in all the regions and in all the years. The races RFRTF, RHMRF, TKRPF and MHCTC were the most common races in the Akmola and Kostanay regions, and the races LHCSF, QKCSF and LKCSF were only widely distributed in East Kazakhstan. The virulence complexity (i.e., number of Sr genes on which the races were virulent) ranged from 5 to 16, with about 40% of the races having 14 or more virulence. The stem rust resistance genes Sr11, Sr13, Sr22, Sr26, Sr31, Sr33 and Sr35 were found to confer resistance to all the races identified during the study period. Hence, these genes can be used as sources of resistance in wheat breeding programmes in Kazakhstan.

First Report of Puccinia graminis f. sp. tritici Race TTKTT in Ethiopia

Stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is one of the most important diseases affecting wheat production in Ethiopia, with crop losses of up to 100% during epidemic years (Olivera et al. 2015). In recent years, different virulent races of Pgt have been identified in the major wheat production areas of the country. Ethiopia is considered as a hot spot for the development and spread of new Pgt races (Singh et al. 2006). During the 2018 main cropping season (June to December), wheat rust surveys were undertaken in major wheat growing regions of Ethiopia, and stem rust was recorded in 284 (39%) of the 727 wheat fields surveyed. A total of 333 stem rust samples were analyzed for race identity using 20 North American wheat lines to differentiate Pgt races following the standard protocol described by Jin et al. (2008). Race analysis was undertaken independently at Ambo Agricultural Research Center (AmARC), Ethiopia, and Regional Cereal Rust Research Center (RCRRC), Turkey. The analysis identified seven races, namely, TTTTF, TKTTF, TTKTT, TKKTF, TKPTF, TTRTF, and TTKTF. Of these, TTKTT was detected for the first time in Ethiopia. Race TTKTT was virulent to wheat differential lines containing Sr5, Sr6, Sr7b, Sr8a, Sr9a, Sr9b, Sr9d, Sr9e, Sr9g, Sr10, Sr11, Sr17, Sr21, Sr24, Sr30, Sr31, Sr38, SrTmp, and SrMcN and avirulent on lines with Sr36. This race was identified independently by AmARC and RCRRC in replicated tests from 18 isolates collected in Guji (southern Ethiopia), West Arsi (southeastern Ethiopia), Hararge (eastern Ethiopia), and East Wellega (western Ethiopia) zones of Oromia region and two isolates from East Gojam and South Wollo zones of Amhara region. Race TTKTT was identified from samples collected from commercial wheat varieties Hulluka, Senate, Shorima, Ogolcho, Hidase, and Danda’a. This race was reported in Kenya for the first time in 2014 and belongs to the TTKSK (Ug99) race group (Patpour et al. 2016). TTKTT has the most complex virulence combination of all known Ug99 races, and range expansion out of Kenya is highly significant. This first confirmation of TTKTT in Ethiopia is important because of its striking virulence combinations for SrTmp, Sr24, and Sr31, indicating the increase in virulence and variability in the Ethiopian Pgt population. Among these three genes, Sr24 is a valuable source of resistance that has remained effective in Ethiopia. Detection of TTKTT in Ethiopia is of great concern because a high frequency of widely grown commercial cultivars and advanced breeding lines in Ethiopia are known to possess the Sr24 resistance gene (Hundie et al. 2019). According to Jin et al. (2008), commercial cultivars with resistance based on Sr24, particularly in the East African region, must be viewed with caution in terms of potential stem rust epidemics. The current result also emphasizes the importance of regular monitoring to timely identify new races and utilize this information in screening and identification of effective sources of resistance. Moreover, the result highlighted the need for developing cultivars with combinations of effective resistance genes to enhance their longevity. Alternatively, cultivars with multiple minor genes to achieve durable resistance could be developed. Further spread of race TTKTT is considered likely and requires close monitoring.

Principal approaches and achievements in studying race composition of wheat stem rust

Wheat stem rust caused by thebiotrophic fungus Puccinia graminisf. sp.triticiis a dangerous disease that seriously damages the economics in many countries of the world. The review contains information about epidemics of wheat stem rust and causes of their emergence worldwide. Recently wheat stem rust epidemics have been recorded in the northern regions of Kazakhstan and on the territories adjacent to Omsk Region of Russia. It has been shown that severe wheat stem rust epidemics occur mainly due to the emergence of new virulent races of the disease agent and to growing susceptible wheat cultivars. New methods of studying the race composition of the fungus are described as well as the use of the previous and current differential sets for race determination ofP. graminisf. sp.tritici. The results of developing molecular markers and assessing their effectiveness in studying stem rust races are presented. Wheat stem rust races dominant in major grain-growing countries of the globe and their typical peculiarities are described. The paper contains information on identifcation of race Ug99 and of its variations including data on areas of their dissemination and on their virulence toSr-resistance genes. The existence and emergence of other races of the agent potentially dangerous for commercially important genes for stem rust resistance is also described. Currently in nature strongly virulent races ofP. graminisf. sp.triticiare circulating with wide geographical coverage and their virulence is absolutely different from the virulence of race Ug99. Historical and modern data on studying the race composition of the pathogen in Kazakhstan are summarized. It is stated that the use of the old standard differential set and an incomplete North American system of race nomenclature in experiments prevents measuring similarity between Kazakhstani races and the worldwide known races of the pathogen. It has been shown that there is a need to continue studies on the intraspecies structure of the disease agent’s population in Kazakhstan with the use of the modern differential set, on determination of race composition and ways of emergence of new races potentially dangerous for commercial wheat varieties.

Identification of New Sources of Resistance to Wheat Stem Rust in Aegilops spp. in the Tertiary Genepool of Wheat.

Recent stem rust epidemics in eastern Africa and elsewhere demonstrated that wheat stem rust is a re-emerging disease posing a threat to wheat production worldwide. The cultivated wheat gene pool has a narrow genetic base for resistance to virulent races, such as races in the Ug99 race group. Wild relatives of wheat are a tractable source of stem rust resistance genes. Aegilops species in the tertiary genepool have not been exploited to any great extent as a source of stem rust resistance. We evaluated 1,422 accessions of Aegilops spp. for resistance to three highly virulent races (TTKSK, TRTTF, and TTTTF) of Puccinia graminis f. sp. tritici. Species studied include Ae. biuncialis, Ae. caudata, Ae. comosa, Ae. cylindrica, Ae. geniculata, Ae. neglecta, Ae. peregrina, Ae. triuncialis, and Ae. umbellulata that do not share common genomes with cultivated wheat. High frequencies of resistance were observed as 977 (68.8%), 927 (65.2%), and 850 (59.8%) accessions exhibited low infection types to races TTKSK, TTTTF, and TRTTF, respectively. Contingency table analyses showed strong association for resistance to different races in several Aegilops spp., indicating that for a given species, the resistance genes effective against multiple races. Inheritance studies in selected accessions showed that resistance to race TTKSK is simply inherited.

Analysis of Climate Variability Effects on Wheat Stem Rust (<i>Puccinia graminis</i> f.sp <i>tritici</i>) Epidemics in Bale and Arsi Zones of Oromia Regional State, Ethiopia

Stem rust caused by Puccinia graminis f.sp tritici, is the most destructive disease of wheat in the world. Bale and Arsi zones are conducive for stem rust epidemics. The study was conducted in 2013 to assess and model the effect of climate variability on wheat stem rust epidemics. The meteorological and disease data for the year 2004 to 2013 of Sinana and Kulumsa were obtained from the respective research centers and were analyzed. Four bread wheat cultivars, namely Kubsa, Madawalabu, Sofumer and Tusie were included in the study. The present study showed that stem rust epidemics was higher at kulumsa than at Sinana over the last 10 years. The increased stem rust severity at Sinana and Kulumsa were due to the total seasonal rainfall increase during wheat growing seasons. A reduction in stem rust severity was manifested at Sinana and kulumsa as seasonal mean minimum temperature increased, whereas a reduction in stem rust severity occurred at Kulumsa as the seasonal average relative humidity increased. The total seasonal rainfall had positive effect on the development of wheat stem rust, while the seasonal mean minimum temperature and seasonal average relative humidity significantly affected the development of wheat stem rust in the field.

Large-Scale Atmospheric Dispersal Simulations Identify Likely Airborne Incursion Routes of Wheat Stem Rust Into Ethiopia.

In recent years, severe wheat stem rust epidemics hit Ethiopia, sub-Saharan Africa's largest wheat-producing country. These were caused by race TKTTF (Digalu race) of the pathogen Puccinia graminis f. sp. tritici, which, in Ethiopia, was first detected at the beginning of August 2012. We use the incursion of this new pathogen race as a case study to determine likely airborne origins of fungal spores on regional and continental scales by means of a Lagrangian particle dispersion model (LPDM). Two different techniques, LPDM simulations forward and backward in time, are compared. The effects of release altitudes in time-backward simulations and P. graminis f. sp. tritici urediniospore viability functions in time-forward simulations are analyzed. Results suggest Yemen as the most likely origin but, also, point to other possible sources in the Middle East and the East African Rift Valley. This is plausible in light of available field surveys and phylogenetic data on TKTTF isolates from Ethiopia and other countries. Independent of the case involving TKTTF, we assess long-term dispersal trends (>10 years) to obtain quantitative estimates of the risk of exotic P. graminis f. sp. tritici spore transport (of any race) into Ethiopia for different 'what-if' scenarios of disease outbreaks in potential source countries in different months of the wheat season.

Barberry as Alternate Host Is Important for Puccinia graminis f. sp. tritici But Not for Puccinia striiformis f. sp. tritici in the U.S. Pacific Northwest

Common barberry (Berberis vulgaris) is the alternate host of the wheat stem rust pathogen, Puccinia graminis f. sp. tritici, under natural conditions in the U.S. Pacific Northwest. Barberry was recently shown to be infected by basidiospores of the wheat stripe rust pathogen, Puccinia striiformis f. sp. tritici, under controlled conditions, but it is unclear if barberry plays any role in stripe rust epidemics under natural conditions. Aecial samples of Puccinia spp. collected from barberry plants in the Pacific Northwest from 2010 to 2013 were characterized to species by inoculation on wheat plants under controlled conditions and by molecular markers and sequences of the internal transcribed spacer (ITS) region of nuclear ribosomal DNA. Inoculation of wheat plants with bulked aecia-bearing barberry samples resulted in most P. graminis f. sp. tritici uredia and some P. striiformis f. sp. tritici uredinia. Virulence tests demonstrated that the P. graminis f. sp. tritici isolates were sexually produced, whereas the P. striiformis f. sp. tritici isolates were clonal based on both virulence and simple sequence repeat marker tests, indicating urediniospores from wheat fields landing on barberry leaves as the possible source of P. striiformis f. sp. tritici inoculum. A method for simultaneously testing individual aecia for identifying of P. graminis f. sp. tritici and P. striiformis f. sp. tritici by pathogenicity and ITS markers. Using the method together with ITS sequencing, tested individual aecia were mostly P. graminis f. sp. tritici and occasionally some other formae speciales of P. graminis, but not P. striiformis. The results imply that barberry is essential for stem rust epidemics, but not for stripe rust under the natural conditions in the U.S. Pacific Northwest.

Assessing the vulnerability of wheat germplasm from Bangladesh and Nepal to Ug99 stem rust

Bread wheat (Triticum aestivum) is an important crop in South Asia and epidemics of stem rust, caused by Puccinia graminis f. sp. tritici, can impact negatively on food security. In a pro-active assessment of the risk posed by this disease, germplasm collections received from Bangladesh and Nepal in 2011 were screened for their response to Sr31-virulent stem rust races belonging to the Ug99 group. According to a molecular marker assay, 44% of Bangladesh lines and 48% of Nepal lines contained Sr31. In seedling tests, most entries were susceptible to P. graminis f. sp. tritici race PTKST, confirming the ineffectiveness of Sr31. No lines contained Sr24 or Sr36, two major genes that have been overcome by races within the Ug99 group. Despite seedling susceptibility, many lines exhibited high levels of adult plant resistance in the field. The presence of the Sr2 marker allele of csSr2 in some of these lines indicated that this gene is likely to play a role in the adult plant resistance observed. This study showed that sufficient protection to Ug99 exists in Bangladesh and Nepal wheat germplasm, and that the appropriate release of resistant cultivars will insure against a possible future incursion of this dangerous pathogen.

Emergence and Spread of New Races of Wheat Stem Rust Fungus: Continued Threat to Food Security and Prospects of Genetic Control.

Race Ug99 (TTKSK) of Puccinia graminis f. sp. tritici, detected in Uganda in 1998, has been recognized as a serious threat to food security because it possesses combined virulence to a large number of resistance genes found in current widely grown wheat (Triticum aestivum) varieties and germplasm, leading to its potential for rapid spread and evolution. Since its initial detection, variants of the Ug99 lineage of stem rust have been discovered in Eastern and Southern African countries, Yemen, Iran, and Egypt. To date, eight races belonging to the Ug99 lineage are known. Increased pathogen monitoring activities have led to the identification of other races in Africa and Asia with additional virulence to commercially important resistance genes. This has led to localized but severe stem rust epidemics becoming common once again in East Africa due to the breakdown of race-specific resistance gene SrTmp, which was deployed recently in the 'Digalu' and 'Robin' varieties in Ethiopia and Kenya, respectively. Enhanced research in the last decade under the umbrella of the Borlaug Global Rust Initiative has identified various race-specific resistance genes that can be utilized, preferably in combinations, to develop resistant varieties. Research and development of improved wheat germplasm with complex adult plant resistance (APR) based on multiple slow-rusting genes has also progressed. Once only the Sr2 gene was known to confer slow rusting APR; now, four more genes-Sr55, Sr56, Sr57, and Sr58-have been characterized and additional quantitative trait loci identified. Cloning of some rust resistance genes opens new perspectives on rust control in the future through the development of multiple resistance gene cassettes. However, at present, disease-surveillance-based chemical control, large-scale deployment of new varieties with multiple race-specific genes or adequate levels of APR, and reducing the cultivation of susceptible varieties in rust hot-spot areas remains the best stem rust management strategy.

A simulation model for epidemics of stem rust in ryegrass seed crops.

A simulation model (STEMRUST_G, named for stem rust of grasses) was created for stem rust (caused by Puccinia graminis subsp. graminicola) in perennial ryegrass grown to maturity as a seed crop. The model has a daily time step and is driven by weather data and an initial input of disease severity from field observation. Key aspects of plant growth are modeled. Disease severity is modeled as rust population growth, where individuals are pathogen colonies (pustules) grouped in cohorts defined by date of initiation and plant part infected. Infections due to either aerial spread or within-plant contact spread are modeled. Pathogen cohorts progress through life stages that are modeled as disease cycle components (colony establishment, latent period, infectious period, and sporulation) affected by daily weather variables, plant growth, and fungicide application. Fungicide effects on disease cycle components are modeled for two commonly used active ingredients, applied preinfection or postinfection. Previously validated submodels for certain disease cycle components formed the framework for integrating additional processes, and the complete model was calibrated with field data from 10 stem rust epidemics. Discrepancies between modeled outcomes and the calibration data (log10[modeled]-log10[observed]) had a mean near zero but considerable variance, with 1 standard deviation=0.5 log10 units (3.2-fold). It appears that a large proportion of the modeling error variance may be due to variability in field observations of disease severity. An action threshold for fungicide application was derived empirically, using a constructed weather input file favorable for disease development. The action threshold is a negative threshold, representing a level of disease (latent plus visible) below which damaging levels of disease are unable to develop before the yield-critical crop stage. The model is in the public domain and available on the Internet.

A FIASCO-Based Approach for Detection and Diagnosis of Puccinia graminis f. sp. tritici in China

Stem or black rust of wheat, caused by the fungus Puccinia graminis Pers. f. sp. tritici Eriks. & E. Henn. (Pgt), has historically caused severe losses to wheat (Triticum aestivum L.) production worldwide. In the Fujian and Guangdong provinces of China, six moderate-to-severe epidemics of wheat stem rust have occurred, which caused destructive losses of wheat between 1949 and 1966, although these were brought under control by integrated management. A rapid and reliable detection of the pathogen will contribute to the accurate forecast and seasonal control of this disease. The objective of this study was to develop a diagnostic molecular marker generated from simple sequence repeats (SSR) for the early rapid identification of P. graminis. The genomic DNA of P. graminis, Puccinia striiformis, Puccinia triticina and seven other species was amplified by a pair of SSR primers generated by the FIASCO (fast isolation by AFLP sequences containing repeats) enrichment protocol. The primer set Pgtw (f)/Pgtw (r) generated a polymorphic pattern displaying a 330-bp DNA fragment specific for P. graminis whereas no DNA fragment was obtained from other non-target wheat fungal pathogens. The detection limit of the primer was 1 ng DNA in a 25-μL PCR reaction. The SSR markers of P. graminis can also be used to detect the presence of latent hyphae in Pgt-infected wheat leaves as early as 30 h post-inoculation. A rapid approach to distinguish P. graminis from similar pathogenic fungi would be anticipated in further study.

Lr67/Yr46 confers adult plant resistance to stem rust and powdery mildew in wheat

We demonstrate that Lr67/Yr46 has pleiotropic effect on stem rust and powdery mildew resistance and is associated with leaf tip necrosis. Genes are designated as Sr55, Pm46 and Ltn3 , respectively. Wheat (Triticum aestivum) accession RL6077, known to carry the pleiotropic slow rusting leaf and yellow rust resistance genes Lr67/Yr46 in Thatcher background, displayed significantly lower stem rust (P. graminis tritici; Pgt) and powdery mildew (Blumeria graminis tritici; Bgt) severities in Kenya and in Norway, respectively, compared to its recurrent parent Thatcher. We investigated the resistance of RL6077 to stem rust and powdery mildew using Avocet × RL6077 F6 recombinant inbred lines (RILs) derived from two photoperiod-insensitive F3 families segregating for Lr67/Yr46. Greenhouse seedling tests were conducted with Mexican Pgt race RTR. Field evaluations were conducted under artificially initiated stem rust epidemics with Pgt races RTR and TTKST (Ug99 + Sr24) at Ciudad Obregon (Mexico) and Njoro (Kenya) during 2010-2011; and under natural powdery mildew epiphytotic in Norway at Ås and Hamar during 2011 and 2012. In Mexico, a mean reduction of 41 % on stem rust severity was obtained for RILs carrying Lr67/Yr46, compared to RILs that lacked the gene, whereas in Kenya the difference was smaller (16 %) but significant. In Norway, leaf tip necrosis was associated with Lr67/Yr46 and RILs carrying Lr67/Yr46 showed a 20 % reduction in mean powdery mildew severity at both sites across the 2 years of evaluation. Our study demonstrates that Lr67/Yr46 confers partial resistance to stem rust and powdery mildew and is associated with leaf tip necrosis. The corresponding pleiotropic, or tightly linked, genes, designated as Sr55, Pm46, and Ltn3, can be utilized to provide broad-spectrum durable disease resistance in wheat.