leaf rust

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Phenotypic and genotypic analysis of stripe rust, leaf rust and stem rust resistance genes in Tajik wheat varieties

The objective of this study was to characterize seedling and adult plant resistance to all three rusts in a set of 40 bread wheat varieties currently cultivated in Tajikistan. Gene postulation based on multi-pathotype seedling test data and adult plant responses identified Yr2, Yr9, Yr17 and Yr27; Lr10 and Lr26; and Sr5, Sr6, Sr10, Sr11, Sr31 and Sr38. The effects of slow rusting, adult plant, pleiotropic resistance genes Lr34/Yr18/Sr57 and Yr30/Lr27/Sr2 were observed in the field and confirmed with molecular markers. Furthermore, molecular markers diagnostic for Yr9/Lr26/Sr31 and Yr17/Lr37/Sr38 were assessed on all varieties. Genes Lr34/Yr18/Sr57, Yr9/Lr26/Sr31 and Yr27 were identified in varieties Sarvar, Vahdat, Oriyon, Isfara, Ormon, Alex, Sadokat, Ziroat-70, Iqbol, Shokiri, and Safedaki Ishkoshimi based on phenotypic and genotypic results. Some lines were highly resistant to stripe rust (4 varieties), leaf rust (5) and stem rust (9), but the genes responsible could not be identified. They may possess new resistance genes. We thus identified combinations of major and minor rust resistance genes in Tajik wheat varieties. These varieties can now be used by breeders in Tajikistan as crossing parents to develop new varieties with durable resistance to the rusts.

Tajik Agrarian University, 146, Tajikistan
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The bread wheat Lr34 adult plant rust resistance gene confers seedling resistance in durum wheat

The Lr34 resistance gene from Triticum aestivum encodes a putative ABC transporter protein that confers broad spectrum, partial adult plant resistance to all three rusts species and powdery mildew. It has remained a durable source of resistance for over 100 years in which time no increased virulence towards Lr34 has been observed. This gene is located on chromosome 7D and consequently cannot be readily transferred to durum wheat by traditional breeding. A transgenic approach was used to transfer Lr34 to durum wheat cultivar Stewart by Agrobacterium transformation. Homozygous progeny from a number of independent Stewart lines expressing Lr34 under regulatory control of its endogenous promoter showed high levels of rust resistance at the seedling stage. A correlation between seedling resistance and transgene expression levels was observed in these plants. In contrast seedlings from a near isogenic line of hexaploid wheat cultivar Thatcher containing Lr34 showed only a minor difference in rust growth when compared with Thatcher seedlings, typical of this adult plant resistance gene in hexaploid wheat. Little is known about how the Lr34 gene product functions; however, the seedling resistance of these durum transgenics enables functional assays to be readily undertaken without the need for adult plant material. By infecting seedlings we have shown that day length has an effect on Lr34 resistance to leaf rust, with higher levels of resistance observed under long days (16 h light) compared with short days (8 h light). This study demonstrates that Lr34 provides strong and presumably durable seedling resistance to rust in durum plants that can be used to further understand how this gene confers resistance.

CSIRO Agriculture Flagship, Australia
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Leaf rust on high yielding winter wheats in Chile

The physical environment and farming system in Chile are conducive to high yields from winter/alternate wheat cultivars. The national average yields for 2012-2014 were 6.5 t/ha for pasta wheat and 5.3 t/ha for bread wheat grown on 19,000 and 239,000 ha, respectively. The most efficient farmers obtain averages of 8-9 t/ha, and experimental plots at southern INIA sites are as high as 14 t/ha. The most important diseases are Septoria leaf blotch, stripe rust, powdery mildew, and BYDV. Recent increases of leaf rust on winter cultivars from near non-existence to the level of a major threat are a concern. Wheat cultivars such as Bicentenario INIA showed yield increases of 31.7% to reach 12.4 t/ha yield when sprayed twice with a mixture of strobilurin and triazol compared to 9.4 t/ha for the unsprayed control. Susceptible winter cultivars being introduced by private companies require complete chemical protection. In order to understand the virulences present in the pathogen population the Thatcher NILs were grown in 2014/15 under non-inoculated conditions in central [Chillan] and southern [Osorno] Chile. The Morocco check showed 100S, Thatcher 60S, TcLr1 40S, TcLr2b 30S, TcLr2c 40S, TcLr3a 30S, TcLr3ka 20S, TcLr3bg 30S, TcLr9 20S, TcLr10 60S, TcLr11 80S, TcLr12 60S, TcLr13 70S, TcLr14a 70S, TcLr15 50S, TcLr16 60S, TcLr17a 30S, TcLr18 20MR, TcLr19 0, TcLr20 30S, TcLr21 0, TcLr22a 0, TcLr23 70S, TcLr24 60S, TcLr25 0, TcLr26 60S, Lr27+31 80S, TcLr28 10MR, TcLr29 40MS, TcLr30 60S, TcLr32 70S, TcLr33 60S, TcLr34 70S, TcLr35 10MR–MS, Lr36 0, and TcLr37 0. The most significant differences (>40%) in response between the two locations were for TcLr2b, TcLr2c, TcLr11 and TcLr33. The Cereal Disease Laboratory (U.S.A.) tested 68 isolates from 55 samples from 2012/13 and identified 14 races, including one Triticum turgidum race (BBBQJ 26%). Significant breeding efforts are currently underway to address the leaf rust problem in Chile.

National Institute of Agricultural Research [INIA], Chile
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Characterization of seedling and adult plant resistance to leaf rust in African wheat germplasm

Many of the catalogued leaf rust resistance genes in wheat deployed in agriculture have been overcome by variants of Puccinia triticina (Pt), the causal pathogen of leaf rust. Discovery and characterization of new sources of resistance in various germplasms using multipathotype tests and molecular markers could permit future diversification of the genetic base of leaf rust resistance in wheat. In searching for new sources of leaf rust resistance, 140 wheat lines from 14 African countries were tested with 8 Australian Pt pathotypes. Seedling tests revealed that 41% of the lines were susceptible to all pathotypes, 31% were postulated to carry either one of 10 resistance genes (Lr1, Lr2a, Lr3a, Lr13, Lr18, Lr23, Lr24, Lr26, Lr37 or Lr73) or one of five gene combinations (Lr2a+Lr3a, Lr1+Lr13, Lr1+Lr23, Lr1+Lr13+Lr73 and Lr23+Lr73). Twenty-eight percent of the lines were postulated to carry uncharacterized seedling resistance genes. Based on average coefficients of infection (ACI), 101, 25 and 11 lines showed high (ACI 0-19), moderate (ACI 21-38) and low (ACI 41-56) levels adult plant resistance, respectively, whereas three lines were moderately susceptible to susceptible (ACI 63-76). Genotyping of 74-78 lines that were anticipated to carry APR genes, using the molecular markers: csLV34 (linked to Lr34) and KASP SNP markers SNP1G22 and SNPT10 (linked to Lr46 and Lr67), respectively, revealed the presence of Lr34, Lr46 and Lr67 in 11, 22 and 14 wheat lines, respectively. The identities of the APR in the remaining 22 lines are unknown, and potentially represent new resistance sources. Genetic analyses of these uncharacterized APR sources are underway to select single gene lines and allow fine mapping.

The University of Sydney, Plant Breeding Institute, Australia
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Genetic characterization of slow rusting resistance to leaf rust in durum wheat cv. Bairds

The CIMMYT durum Bairds is susceptible to leaf rust (LR) at the seedling stage but shows an adequate level of slow rusting adult plant resistance (APR) in Mexican field environments. A recombinant inbred line (RIL) population developed from a cross of Bairds and the susceptible parent Atred#2 was phenotyped for LR response at Ciudad Obregon, Mexico, during 2013, 2014 and 2015 under artificial epidemics created with Pt race BBG/BP. Genetic analysis indicated that 3-4 additive genes conferred LR resistance. The RILs and parents were also genotyped with the 50K diversity arrays technology (DArT) sequence system and 93 SSR markers. A genetic map comprising 1,150 markers was used to map the resistance loci. Inclusive composite interval mapping analysis detected four quantitative trait loci (QTL) on chromosomes 1BL, 2BC (centromere region), 5BL and 6BL. These QTL, designated as QLr.cim-1BL, QLr.cim-2BC, QLr.cim-5BL and QLr.cim-6BL, explained 20.1-60.7%, 6.4-13.1%, 4.3-11.2%, and 7.1-28.0%, respectively, of the variation in leaf rust severity. QLr.cim-1BL was close to the previously reported APR gene Lr46, whereas QLr.cim-6BL, detected in all three seasons, is a new resistance locus in durum wheat. The four QTL combined showed a significant additive effect on resistance with a disease severity of 18-20%, whereas RILs carrying the individual QTL showed mean leaf rust severities ranging from 56 to 98%. Three QTL, except for QLr.cim-2BC, were derived from Bairds. The final LR severity of Bairds ranged from 15-25% across three years. This cultivar can be used as a source for complex APR in durum wheat breeding.

International Maize and Wheat Improvement Center (CIMMYT), México
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Leaf rust on triticale in the Czech Republic

farming because of lower demands for pesticide applications. Triticale as a hybrid of wheat and rye may possess disease resistances derived either or both from wheat and rye. In the Czech Republic the leaf rust pathogen isolated from triticale is Puccinia triticina. Triticale is usually resistant to the rye leaf rust pathogen (P. recondita). Limited information is available comparing P. triticina isolates collected from wheat and triticale. Manninger (2006, Acta Phytopathologica and Entomologica Hungarica 41: 93-100) pathotyped 82 isolates collected from wheat and triticale on 15 Thatcher NILs. More than 50% of isolates from wheat were virulent to Lr2b, Lr2c, Lr3, Lr11, Lr17, Lr21 and Lr26. Of 12 isolates from triticale 9 were virulent only to Lr2b and Lr2c and the other 3 isolates were virulent to Lr2b, Lr2c and Lr11. We inoculated 15 NILs and 7 triticale cultivars with 36 Pt isolates from wheat and 36 isolates from triticale. Characteristic differences between the reactions on NILs of isolates from triticale and wheat were not found except that virulence to Lr1 was much more frequent in isolates from wheat. Whereas isolates from triticale were virulent to all 7 tested triticale cultivars, isolates from wheat were virulent to only 3 triticales. In another experiment isolates from wheat and triticale were collected at four locations. Although paired isolates came from the same locations the responses of the NILs were different. Isolates from triticale for all four locations were virulent to a higher number of triticale cultivars than those from wheat. It seems that P. triticina races infecting triticale have changed over the last decade from having a narrow virulence range on bread wheat to the current situation of typical bread wheat races becoming specialized on triticale.

Research Institute of Crop Production, Czech Republic
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Phylogenetic relationships among Puccinia triticina pathotypes on wheat in India

Leaf rust of wheat causes considerable losses worldwide. New pathotypes may cause previously resistant varieties to become susceptible. Identification of pathotypes and their relationships provide information for breeding efforts and designing management strategies. Traditional identification of pathotypes is based on responses of differential hosts. At present 50 pathotypes of P. triticina are maintained in the National collection. To determine variability and relationships at the molecular level we conducted analyses with 26 SSR primers, eight of which were polymorphic. Binary (0 or 1) molecular data generated by NTSYS-pc was used to construct a phylogenetic tree. Cluster analysis was done using the unweighted pair group arithmetic means (UPGMA) method in the SAHN program of NTSYS-pc. Pathotype groups and subgroups were determined based on the Jaccard similarity coefficients (JC). Manual observations indicated seven major groups. Among them, two groups each have one pathotype (pathotypes 16 and 17). Jaccard similarity coefficients supported groupings based on pathogenicity data. For example, pathotypes in the race 12 group (12, 12-1, 12-3, 12-4, 12-6, 12-7, 12-8, but excluding 12-2 and 12-5) had similarity coefficients greater than 0.7. Similar observations were recorded for the race 77 group. Maximum similarity was observed between 12-3 and 12-7 (JC value: 0.89) followed by 12-3, 12-7 and 12-6 (JC value: 0.82). Based on the phylogenetic tree and similarity coefficients data, there was substantial diversity among pathotypes. Thus SSR marker data can be used for effective characterization of pathotypes and for making evolutionary inferences.

Indian Council of Agricultural Research
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Physiological specialization of Puccinia triticina on wheat in Argentina, 2013

Leaf rust is the most important wheat disease in Argentina; 4.2 M ha of wheat were grown in 2014. The objective of the study was to identify avirulence/virulence phenotypes of the Pt population in leaf rusted samples collected in wheat-growing areas during 2013. Single uredinial isolates were taken from samples and tested on Thatcher near-isogenic lines and some local varieties. Resistance genes in sets of four included: Lr1, Lr2a, Lr2c, Lr3; Lr9, Lr16, Lr24, Lr3ka; Lr11, Lr17, Lr30, Lr10; Lr14a, Lr19, Lr20, Lr21; Lr23, Lr25, Lr26, Lr27+31; Lr29, Lr36, Lr39/41, Lr42, Lr43; Lr44 and Lr47. Race designations were based on the first three sets proposed by Long and Kolmer (1989, Phytopathology, 79: 525-529) and the gene designations Lr10 and/or Lr20 were appended to indicate virulence on lines with those genes. Among 141 single uredinial isolates, 18 races were identified. Race MFP, was the most frequent, accounting for 27% of isolates; race MDP was second at 22.7%. Both races were isolated for the first time in 2005. The first three races were present in similar frequencies to 2012. Two new races were found, MKJ 10 and MGJ 10. Virulence was not found for genes Lr19, Lr21, Lr25, Lr29, Lr36, “Lr43”, Lr44 and Lr47. Race DBB 10,20 was the most frequent race on durum wheat. Virulence to Lr16 appears to be increasing.

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National Institute of Agricultural Technology (INTA), Argentina
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Chromosomal locations of stem and leaf rust resistance genes from Ae. caudata, Ae. searsii and Ae. Mutica

The wild relatives of wheat represent a vast resource of potentially useful genes for agriculture. The genus Aegilops has provided several rust resistance genes used in commercial cultivars. Here we report progress on mapping of potentially new stem and leaf rust resistance from Ae. caudata, Ae. searsii and Ae. mutica (Amblyopyrum muticum). Addition lines derived from the amphiploids Alcedo/ Ae. caudata, TA3368, CS/ Ae. mutica, TA8024 (both from Wheat Genetics Resource Center, Kansas State University, USA) and CS/ Ae. searsii TE10 (kindly provided by Dr Moshe Feldman, Weizmann Institute, Rehovot, Israel) were produced after backcrossing the amphiploids with Australian cv. Angas or Westonia. Backcrossed generations were screened for stem rust and leaf rust responses and both resistant and susceptible plants were sampled for DNA marker analysis. Stem rust resistant plants derived from the Ae. caudata amphiploid and leaf rust resistant plants derived from the Ae. searsii amphiploid showed the presence of non-wheat marker bands after hybridizing restricted genomic DNA with the Triticeae group 5 RFLP probe PSR128, and after PCR using EST-based primers specific for Triticeae group 5. Susceptible plants did not show those non-wheat molecular markers. Hence, stem rust resistance from Ae. caudata was allocated to chromosome 5C, and the resistance gene is temporarily named SrAec1t. Leaf rust resistance from Ae. searsii was allocated in a similar manner to chromosome 5Ss, and is temporarily named LrAesr1t. Leaf rust resistance transferred from Ae. mutica was traced to a 6T chromosome after associating resistance with the presence of Triticeae group 6 RFLP probes (including BCD001, BCD269, BCD276, BCD1426, CDO772, CDO1380, WG933) and that gene is temporarily named LrAmm1t. The addition lines involving the 5C, 5Ss and 6T chromosomes were crossed with Sears’ ph1b mutant to induce homoeologous recombination with related wheat chromosomes.

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School of Agriculture, Food and Wine, The University of Adelaide, Australia.
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Recent changes in the UK wheat yellow rust and brown rust pathogen populations

Management of the cereal rusts currently relies on the use of fungicides and varietal resistance. Host resistance can be rapidly overcome by mutations in the pathogen population and for this reason virulence surveys are undertaken worldwide to give early warning on any changes. The UK Cereal Pathogen Virulence Survey (UKCPVS) was established in 1967 following an outbreak of yellow rust on the variety Rothwell Perdix. The UKCPVS currently monitors the wheat yellow rust and brown rust pathogen populations as well as the wheat and barley powdery mildew pathogen populations. A watching brief is maintained on barley yellow rust. In 2011 a new Pst race was detected; named the Warrior race after the variety on which it was first found. Initial tests on differential host lines suggested that the race was another stepwise mutation with an added virulence for Yr7 in addition to a combination of virulences to Yr6, Yr9, Yr17 and Yr32. Other characteristics of this race, however, suggested something different from previous race changes with an increase in telial production seen under field conditions. In addition, this new race was seen simultaneously in multiple locations throughout Europe (www.wheatrust.org) in contrast to the more gradual appearance of new variants seen previously. Subsequent genotypic analysis of isolates (Hubbard et al. 2015, Genome Biol. 16:23) demonstrated that the new race was an exotic incursion. Results presented at the meeting will summarize some of the key changes in the pathogen populations, such as the arrival of the Warrior Pst race in 2011 and the re-emergence of the Glasgow race of the brown rust pathogen in 2014. 


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