Past, Current and Future of Wheat Diseases in Kenya

Wheat (Triticum aestivum L.) is an important cereal and is among the crops that contribute significantly to food security in Kenya. However, wheat diseases are among the biotic factors that affect wheat production. Considerable progress has been made to control wheat diseases through host plant resistance breeding and chemical applications. Frequent changes in the pathogens population still present a major challenge to achieving durable resistance. Disease surveillance and monitoring of the pathogens have revealed the changes in virulence across the region, justifying the need to develop and deploy more efficient and sustainable strategies to manage the diseases. Understanding the genetic variability and composition of the diseases is important for variety release with appropriate resistance gene combinations for sustainable disease management. This review highlights the prevalence, distribution of wheat diseases, host plant resistance in the key wheat-growing regions of Kenya, and future prospects in Kenya

Association mapping of stem rust race TTKSK resistance in US barley breeding germplasm

KEY MESSAGE: Loci conferring resistance to the highly virulent African stem rust race TTKSK were identified in advanced barley breeding germplasm and positioned to chromosomes 5H and 7H using an association mapping approach. ABSTRACT: African races of the stem rust pathogen (Puccinia graminis f. sp. tritici) are a serious threat to barley production worldwide because of their wide virulence. To discover and characterize resistance to African stem rust race TTKSK in US barley breeding germplasm, over 3,000 lines/cultivars were assessed for resistance at the seedling stage in the greenhouse and also the adult plant stage in the field in Kenya. Only 12 (0.3 %) and 64 (2.1 %) lines exhibited a resistance level comparable to the resistant control at the seedling and adult plant stage, respectively. To map quantitative trait loci (QTL) for resistance to race TTKSK, an association mapping approach was conducted, utilizing 3,072 single nucleotide polymorphism (SNP) markers. At the seedling stage, two neighboring SNP markers (0.8 cM apart) on chromosome 7H (11_21491 and 12_30528) were found significantly associated with resistance. The most significant one found was 12_30528; thus, the resistance QTL was named Rpg-qtl-7H-12_30528. At the adult plant stage, two SNP markers on chromosome 5H (11_11355 and 12_31427) were found significantly associated with resistance. This resistance QTL was named Rpg-qtl-5H-11_11355 for the most significant marker identified. Adult plant resistance is of paramount importance for stem rust. The marker associated with Rpg-qtl-5H-11_11355 for adult plant resistance explained only a small portion of the phenotypic variation (0.02); however, this QTL reduced disease severity up to 55.0 % under low disease pressure and up to 21.1 % under heavy disease pressure. SNP marker 11_11355 will be valuable for marker-assisted selection of adult plant stem rust resistance in barley breeding. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00122-014-2297-8) contains supplementary material, which is available to authorized users

Stem Rust Resistance in 1BL.1RS and 2RL.2BS Double Wheat-Rye Translocation Lines

The wheat stem rust pathogen, Puccinia graminis f.sp. tritici, is a significant and devastating disease of wheat crops worldwide. Wheat has many wild relatives in which to source new resistance genes, including the cereal crop of rye in the tertiary genepool. The aim of this study was to assess the reaction of 1BL.1RS and 2RL.2BS double wheat-rye translocation lines to virulent stem rust races from Africa and North America. BC1F3 and BC1F4 populations from a cross between the line KR99-139 (a double wheat-rye translocation line with 1BL.1RS and 2RL.2BS) and the bread wheat cultivar Topper were used in the study. Several of the populations homozygous for 1BL.1RS and heterozygous for 2RL.2BS showed resistance and low severity adult plant resistance (20RMR-50MSS) to the African stem rust race TTKSK in the field. None of the tested populations with varying chromosome combinations showed seedling resistance to any of the tested stem rust races. Thus, these resistant populations likely carry gene/s effective at the adult plant stage since all stage resistance genes with major effect appear to be absent based on the seedling assays. Resistant lines combined three chromosomes (1RS, 2RS and 2BS) which make their direct use in breeding more complicated. Mapping studies followed by potential transfer of genes between 2R and 2B will make the identified minor genes more useful in wheat breeding

Genome-Wide Association Study for Multiple Biotic Stress Resistance in Synthetic Hexaploid Wheat

Genetic resistance against biotic stress is a major goal in many wheat breeding programs. However, modern wheat cultivars have a limited genetic variation for disease and pest resistance and there is always a possibility of the evolution of new diseases and pests to overcome previously identified resistance genes. A total of 125 synthetic hexaploid wheats (SHWs; 2n = 6x = 42, AABBDD, Triticum aestivum L.) were characterized for resistance to fungal pathogens that cause wheat rusts (leaf; Puccinia triticina, stem; P. graminis f.sp. tritici, and stripe; P. striiformis f.sp. tritici) and crown rot (Fusarium spp.); cereal cyst nematode (Heterodera spp.); and Hessian fly (Mayetiola destructor). A wide range of genetic variation was observed among SHWs for multiple (two to five) biotic stresses and 17 SHWs that were resistant to more than two stresses. The genomic regions and potential candidate genes conferring resistance to these biotic stresses were identified from a genome-wide association study (GWAS). This GWAS study identified 124 significant marker-trait associations (MTAs) for multiple biotic stresses and 33 of these were found within genes. Furthermore, 16 of the 33 MTAs present within genes had annotations suggesting their potential role in disease resistance. These results will be valuable for pyramiding novel genes/genomic regions conferring resistance to multiple biotic stresses from SHWs into elite bread wheat cultivars and providing further insights on a wide range of stress resistance in wheat

Rust expression browser: an open source database for simultaneous analysis of host and pathogen gene expression profiles with expVIP

BackgroundTranscriptomics is being increasingly applied to generate new insight into the interactions between plants and their pathogens. For the wheat yellow (stripe) rust pathogen (Puccinia striiformis f. sp. tritici, Pst) RNA-based sequencing (RNA-Seq) has proved particularly valuable, overcoming the barriers associated with its obligate biotrophic nature. This includes the application of RNA-Seq approaches to study Pst and wheat gene expression dynamics over time and the Pst population composition through the use of a novel RNA-Seq based surveillance approach called "field pathogenomics". As a dual RNA-Seq approach, the field pathogenomics technique also provides gene expression data from the host, giving new insight into host responses. However, this has created a wealth of data for interrogation.ResultsHere, we used the field pathogenomics approach to generate 538 new RNA-Seq datasets from Pst-infected field wheat samples, doubling the amount of transcriptomics data available for this important pathosystem. We then analysed these datasets alongside 66 RNA-Seq datasets from four Pst infection time-courses and 420 Pst-infected plant field and laboratory samples that were publicly available. A database of gene expression values for Pst and wheat was generated for each of these 1024 RNA-Seq datasets and incorporated into the development of the rust expression browser (http://www.rust-expression.com). This enables for the first time simultaneous 'point-and-click' access to gene expression profiles for Pst and its wheat host and represents the largest database of processed RNA-Seq datasets available for any of the three Puccinia wheat rust pathogens. We also demonstrated the utility of the browser through investigation of expression of putative Pst virulence genes over time and examined the host plants response to Pst infection.ConclusionsThe rust expression browser offers immense value to the wider community, facilitating data sharing and transparency and the underlying database can be continually expanded as more datasets become publicly available

Wheat Improvement, Management and Utilization

The Wheat Improvement, Management, and Utilization book covers some of the most recent research areas that touch on enhancement of wheat productivity. It is obvious that wheat is one of the major staple crops grown globally. This crop has widely been researched on considering that, for instance, it is afflicted by various abiotic and biotic stresses that limit its growth and productivity. Today?s goal of wheat improvement consistently is to develop varieties that are high yielding with good processing and technological qualities, well adapted and tolerant to prevailing biotic and abiotic stresses. Therefore, this is a valuable reference book on wheat improvement, agronomy, and end-use qualities, particularly for those who work in research organizations and higher academic institutions. Moreover, it provides an invaluable resource for readers interested in a quick review of trending topics in wheat

Multi-Pathotype Testing of Selected Kenyan Wheat Germplasm and Watkin Landraces for Resistance to Wheat Stripe Rust (Puccinia striiformis f. sp tritici) Races

Wheat stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the key diseases of economic importance in wheat worldwide. Host resistance, which follows the gene-for-gene hypothesis between the host and pathogen, has been used in wheat lines to resolve resistance specificities and postulate resistant genes. The objective of this study was to elucidate stripe rust resistance in a collection of Kenyan wheat lines and Watkin landraces to identify new sources of stripe rust (Yr) resistance. In this study, the resistance in twenty wheat lines was determined by comparing their infection type with those of twenty differential lines using isolates representing twelve Puccinia striiformis races from Kenya, Denmark, U.K., Sweden, and Eritrea at the seedling stage. Among the twenty wheat lines, none was resistant to all the twelve Pst races and isolate DK02d/12 (“Kranich” race) was virulent on all the genotypes except wheat genotype “Kenya Tai.” This genotype (“Kenya Tai”) had the highest resistance as it was resistant to all the twelve stripe rust races used in this study. From this study, the introduction and utilization of wheat genotypes with adult plant resistant (APR) stripe rust genes, such as Yr15, are important in breeding wheat genotypes with effective resistance to wheat stripe rust in Kenya and worldwide

Microsatellite mapping identifies TTKST-effective stem rust resistance gene in wheat cultivars VL404 and Janz

Wheat cultivar VL404 carries seedling resistance to Puccinia graminis f. sp. tritici pathotype TTKST. Monogenic segregation for seedling resistance was observed in a VL404/WL711 recombinant inbred line population and the resistance locus was temporarily designated SrVL. Bulked segregant analysis using Diversity Arrays Technology markers located SrVL on chromosome 2BL. Detailed simple sequence repeat mapping placed SrVL between gwm120 and wmc175, both at genetic distances of 3.3 cM. Based on adult plant responses of Janz and VL404 in India and Kenya, we expected these cultivars to carry the same gene against TTKST. A subset of Diamondbird/Janz doubled haploid (DH) population showed monogenic segregation, when tested against TTKST and the locus was temporarily named SrJNZ. SrVL-linked markers gwm120 and wmc175 flanked SrJNZ at a similar genetic distance, thereby confirming our hypothesis. Chromosome 2BL carries Sr9, Sr16 and Sr28. Sr9 is a multi-allelic locus and all known alleles of Sr9 and Sr16 are ineffective against TTKSK and its derivatives. A recombination value of 16.7 cM between Sr9g-linked stripe rust resistance gene Yr7 and SrJNZ in Diamondbird/Janz DH population suggested that SrJNZ is not an allele at the Sr9 locus. Based on comparison of published genetic distances between Lr13,Sr9, Sr28 and Sr16 with that observed in this study, we concluded SrVL and SrJNZ to be Sr28. This gene was contributed by a common parent Gabo, which also exhibited resistance against TTKST. Sr28-linked markers gwm120 and wmc175 confirmed the presence of this gene in a high proportion of Australian cultivars that showed stem rust resistance in Kenya. These markers can be used for marker-assisted pyramiding of Sr28 with other stem rust resistance genes

Inheritance of resistance to Ug99 stem rust pathogen in selected barley lines

Stem rust (Puccinia graminis tritici) race Ug99 is a devastating disease of wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.). Barley is the world’s fourth most important cereal crop after wheat, maize (Zea mays L.) and rice (Oryza sativa L.). The seedlings of F2 populations were inoculated 7–10 d after planting and when the first leaf was fully emerged (Feekes stage 1). The observed resistant (R):susceptible (S) ratio  f the F2 populations HKBL-1385-13 × 08-UT-86 (χ2 0.05 = 0.480, P ≥ 0.488), HKBL-1385-13 × 09-N2-19 (χ2 0.05 = 0.641, P ≥ 0.424), ND2649 × 09-AB-78 (χ2 0.05 = 1.05, P ≥ 0.309) and ND25882 × 09-AB-78 (χ2 0.05 = 3.599, P ≥ 0.058) exhibited Mendelian segregation of 3R:1S suggesting that the resistance is conferred by major genes. In addition, F2 populations derived from the crosses Karne × 09-N2-19 (χ2 0.05 = 1.343, P ≥ 0.246), Nguzo × 09-AB-78 (χ2 0.05 = 0.429, P ≥ 0.512), ND26249 × 09-N2-52 (χ2 0.05 = 2.576, P ≥ 0.111) and ND25882 × 09-AB-78 (χ2 0.05 = 0.980, P ≥ 0.322) conformed to a phenotypic ratio of 9:7 indicating that there are genes modifying the resistance to Ug99. The results of this study suggest that there are valuable major genes for stem rust resistance that could be used to improve susceptible barley and wheat germplasm.Keywords: barley, Mendelian, stem rust, Ug9