To reduce losses caused by rusts, regular and timely replacement of susceptible varieties with new high yielding, rust resistant varieties must occur. Data from a farmer survey carried out across Pakistan (Punjab, Sindh, KPK and Baluchistan) in 2014 enabled an analysis of the uptake of rust resistant variety NARC 2011. The empirical results indicated that the major sources of information that farmers obtained about NARC 2011 were research stations (83%), seed companies (7%) and fellow farmers (5%). Although production inputs were applied equally to both rust resistant NARC 2011 and rust susceptible wheat varieties the average yield of NARC 2011 (5,063 kg/ha) was superior to high yielding but rust susceptible varieties (4,446 kg/ha). Quality attributes of NARC 2011, including taste, color, dough kneading and chapatti making properties, were preferred by >70% of farmers). Seed availability and accessibility of NARC 2011 were major issues. Farmer awareness of rusts, especially the threat of exotic Pgt race Ug99, needs to be improved.
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Stem rust is a potentially destructive fungal disease of wheat worldwide. In 1998 Pgt pathotype TTKSK virulent to Sr31 was detected in Uganda. The same pathotype was confirmed in Lorestan and Hamedan provinces of Iran in 2007. We used a derivative of race TTKSK to phenotype 62 Iranian wheat landraces (resistant to stripe rust in a previous study) at the seedling stage to this new pathotype (TTSSK). Twenty eight accessions were evaluated for the presence of resistance genes Sr2, Sr22, Sr24, Sr25, Sr26, Sr35, Sr36 and Srweb using SSR markers. None carried Sr2, Sr24 or Sr26, but the presence of Sr22, Sr25, Sr35 and Sr36 was indicated. Some susceptible landraces predicted to carry Sr2 by marker analysis require further investigation. To evaluate defense gene expression in compatible and incompatible stem rust interactions we sampled resistant and susceptible cultivars at 0, 12, 18, 24, 72 hours post-inoculation (hpi). ?-1,3 glucanase expression was studied using qGLU-S and qGLUU-AS primers and a real-time PCR step-one ABI machine, with ?-tubulin and EF1-? genes used as internal controls. In incompatible interactions defense gene expression was increased at 24 hpi, but in compatible interactions the highest level of expression occurred at 12 hpi and was significantly decreased at 18 hpi. The results revealed that expression of defense genes such as ?-1,3 glucanase was earlier in compatible than in incompatible interactions but the expression level was less in incompatible interactions. On the other hand, in susceptible genotypes the expression of defense genes increased immediately after inoculation and declined sharply after infection. In contrast defense gene expression in resistant genotypes began to increase after establishment of the pathogen.
Wheat landrace PI 177906 has seedling and field resistance to Pgt races TTKSK and TTKST. From a cross between PI 177906 and LMPG-6, 138 doubled haploid (DH) lines and 144 recombinant inbred lines (RILs) were developed and tested for seedling resistance to Pgt race TTKSK. Goodness-of-fit tests from both populations indicated that two dominant genes in PI 177906 conditioned resistance to race TTKSK. Parents and the 138 DH lines were evaluated in the field in two experiments in Kenya; one in the main season and one in the off-season. The 90K wheat iSelect SNP genotyping platform was used to genotype the parents and DH lines and data were used to construct a genetic linkage map. Two loci for seedling resistance were mapped to chromosomes 2BL and 4BL. Two major QTL for field resistance mapped to the same regions, a 14.4 cM interval on 2BL and an 8.5 cM interval on 4BL. The QTL on 2BL and 4BL explained, respectively, 31.9-32.3% and 18.2-19.1% of the variation in the off-season and 28.3-30.4% and 5.4-6.5% of the variation in the main-season. Based on the mapping results, race specificity, and the seedling infection types, the resistance gene in 2BL could be Sr28, whereas the gene on chromosome 4BL could be novel. The mapping results will be verified in the RIL population using the flanking SNP markers in KASP assays.
With ongoing threats of rust from both internal and international sources it has become a priority at CIMMYT and for Pakistan national programs to accelerate the rate of seed increase and to popularize new Pgt race Ug99 resistant varieties to avert future disasters. Seed of Ug99 resistant varieties NARC-11, Pak-2013, Dharabi-2011 and BARS-09 was produced under the Wheat Productivity Enhancement Program (WPEP). The country-wide participatory approach involves a partnership of farmers, seed companies and research institutes. In 2014 16,020 and 6,085 kg of seed of NARC-11 and Pak-13, respectively, were distributed all over the country, including Azad Jammu Kashmir and Gilgit-Baltistan. Comparative yields across Pakistan show that the rust resistant varieties are equal, or superior, to current stem rust susceptible varieties grown by farmers. Deployment and use of these varieties by farmers in Balochistan will have a significant impact not only on productivity, but may also avert the consequences of possible introduction of race Ug99.
Four Ug99 pathotypes occur in southern Africa. Although South African bread wheat cultivars and lines are regularly screened against representative isolates, the stem rust reactions of Zimbabwean germplasm to these variants were largely unknown. A collection of 49 wheat cultivars and lines, obtained from Seed-Co (Ltd.) and the Crop Breeding Institute in Zimbabwe, were tested as seedlings against pathotypes TTKSF, TTKSF+, TTKSP and PTKST. Twelve varieties and 21 experimental lines showed low infection types with all four pathotypes. Using molecular marker assays Sr31 was detected in 26 entries, Sr24 in five and Sr36 in one. The csSr2 marker suggested the presence of Sr2 in 20 entries. Screening of adult plants in the greenhouse using pathotype PTKST showed 34 entries with low infection types and 15 had high infection types. Stem rust field records in 2012 showed 5 susceptible entries with stem rust scores between 50S and 80S, whereas only 4 susceptible entries were identified in 2014 with scores ranging from 30S to 80S. Three lines were susceptible in both seasons. The study exposed the vulnerability of Zimbabwean wheat germplasm to Ug99 variants, but also identified suitable lines that can be used in breeding and possible commercialization.
In the past decade Pgt race Ug99 and its variants have been a challenge to wheat production in Kenya. Towards identifying suitable varieties, 37 lines selected from rust screening nurseries and 3 checks were tested for yield and adult plant reaction to natural stem rust epidemics across 11 diverse Kenyan environments in 2013 and 2014. Trial locations were chosen to mainly represent key wheat growing areas as well as three new sites. Evaluations based on the AMMI linear-bilinear model indicated significant (P≤0.01) genotype (G), environment (E), and GE interactions with the first three principal components (PC) explaining ~70% of the observed variation. With a contribution of over 90% to total sum of squares, environment was the predominant source of variation and the genotypic effect was approximately twofold higher than the GE effect. Based on biplot projections, clusters of lines were most closely associated with specific environments. Biplots also pointed to at least five environments, clearly those in traditional wheat growing areas that were highly correlated and associated with positive PC suggesting a similar ability to discriminate genotypes. Each non-traditional testing environment was associated with negative PC and was uncorrelated in its discriminatory ability. Combined yield and stability results achieved through classifying genotypes based on Shukla’s stability variance and Kang’s stability rating, revealed four genotypes (R1357, R1362, R1372, and R1374) as desirable candidates. The hitherto popular variety Robin, used as the ‘best check’ for yield, posted an at least 10% lower yield relative to the highest yielding genotype (R1357). Moreover, Robin which was released as a high yielding variety with adult plant resistance in 2009, was not stable in performance across environments, perhaps due its current susceptibility to a new Pgt race (TTKTT) within the race Ug99 group, that is virulent to the SrTmp-based resistance.
The durability of stem rust resistance in wheat varieties is strengthened by the use of polygenic, and broad-spectrum sources of resistance. Adult plant resistance (APR) was observed in the mid-20th century Ecuadorian bread wheat cv. Morocho Blanco (PI 286545) in field tests at Njoro, Kenya, and at St. Paul. Morocho Blanco was susceptible to races TTKSK, RCRSC and TPMKC at the seedling stage. A doubled haploid (DH) mapping population was created from a cross between Morocho Blanco and the susceptible line LMPG-6 to identify loci associated with APR phenotypes. Eighty-eight DH lines were genotyped with approximately 90,000 SNPs using a custom Infinium assay from Illumina. Sixty-seven additional DH lines were used to verify SNPs associated with reduced stem rust levels. Severity and infection type were assessed on adult plants at the stem rust screening facility in Kenya in 2013 and 2014, and in two single race nurseries inoculated with races RCRSC and TPMKC at St. Paul in 2014. Two identified and verified QTL reducing stem rust severity were located on chromosome arms 2BS and 6AS. The QTL on 6AS also reduced infection type at Njoro, but a similar reduction was not observed at St. Paul suggesting a genotype x environment or genotype x race interaction. The QTL on 2BS was associated with reduced stem rust severity at both Njoro and St. Paul. It is a strong candidate for use in breeding for APR to stem rust.
Most of the current stem rust resistance genes (Sr) in Canadian wheat varieties are ineffective against the Pgt race Ug99 lineage, which pose a major threat to wheat production worldwide. Several stem rust resistance genes, including Sr33, Sr35, Sr36, SrCad/Sr42 and Sr43, are effective against race TTKSK. Although Sr36 is ineffective against Ug99 race TTTSK, it is still potentially useful for pyramiding genes to develop germplasm with durable stem rust resistance. For this purpose, we made crosses among RL5405 (Sr33), RL6099 (Sr35), Lang (Sr36), AC Cadillac (SrCad/Sr42), and RWG34 (Sr43) containing the respective Sr genes. A total of 54 doubled haploid (DH) lines were produced from the F1 from AC Cadillac/Lang//RWG34/RL5405, and 82 DH lines were obtained from RWG34/RL5405//RL6099. The DH progeny were tested at the seedling stage with race TTKSK and susceptible lines were discarded. We putatively developed 12 genotypes with multiple Sr gene combinations, including Sr33+Sr36+SrCad/Sr42+Sr43, Sr33+Sr36+SrCad/Sr42, Sr33+Sr36+Sr43, Sr33+SrCad/Sr42+Sr43, Sr36+SrCad/Sr42+Sr43, Sr35+Sr33+Sr43, Sr33+Sr36, Sr33+Sr43, Sr36+SrCad/Sr42, Sr36+Sr43, Sr35+Sr33, and Sr35+Sr43, based on positive association with linked PCR markers. Another population with 63 DH lines was derived from (Hoffman*2/RL6099)//(Hoffman*2/Lang) to combine the Fusarium head blight (FHB) resistance of Hoffman (Fhb1) with Sr35 and Sr36. We found 17 of 63 DH lines containing both Sr35 and Sr36 based also on linked PCR markers. This indicated that the combination Sr35+Sr36 was pyramided into the Canadian cultivar Hoffman; this derivative will be useful for development of cultivars resistant to Ug99 and FHB in Canada.
The stem rust resistance gene SrTmp carried by Triumph 64 confers resistance to Pgt race TTKSK and other members of the Ug99 race group. While some previous studies have mapped resistance postulated to be SrTmp, none used Triumph 64 as a parent. The purpose of this study was to genetically map SrTmp with DNA markers using a DH population from LMPG/Triumph 64 and compare the map position of SrTmp to previously mapped Sr genes. The DH population was tested with Pgt race TTKSK at the seedling stage. A single gene conditioned resistance to TTKSK (n = 144; χ21:1 = 0.44, p = 0.50). SrTmp was mapped to the distal region of chromosome 6DS with SSR markers. The map location of SrTmp was similar to SrCad and Sr42, which likely represent the same gene. In a concurrent study SNP markers were developed to fine-map SrCad. SNP markers were identified and/or developed using a public SNP database (http://www.cerealsdb.uk.net) and sequence information from an Aegilops tauschii genome sequencing project. Further SNP markers were developed by using resistance gene analogs from chromosome 6D to BLAST exome capture sequences from a set of Canadian wheat cultivars followed by searching for unique SNPs found in SrCad carriers. These SNP markers were added to the map of SrTmp. The map positions of SrTmp and SrCad/Sr42 are very similar. While preliminary data show functional differences between SrTmp and SrCad/Sr42, further analysis is needed to determine whether these genes are allelic or closely linked.