leaf rust

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.

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.

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.

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 5S^s, 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, 5S^s and 6T chromosomes were crossed with Sears’ ph1b mutant to induce homoeologous recombination with related wheat chromosomes.