A Backcross Line of Thatcher Wheat with Adult Plant Leaf Rust Resistance Derived from Duster Wheat has Lr46 and Lr77

Evaluation of the resistance to Chinese predominant races of Puccinia triticina and analysis of effective leaf rust resistance genes in wheat accessions from the U.S. National Plant Germplasm System

Puccinia triticina, which is the causative agent of wheat leaf rust, is widely spread in China and most other wheat-planting countries around the globe. Cultivating resistant wheat cultivars is the most economical, effective, and environmentally friendly method for controlling leaf rust-caused yield damage. Exploring the source of resistance is very important in wheat resistance breeding programs. In order to explore more effective resistance sources for wheat leaf rust, the resistance of 112 wheat accessions introduced from the U.S. National Plant Germplasm System were identified using a mixture of pathogenic isolates of THTT, THTS, PHTT, THJT and THJS which are the most predominant races in China. As a result, all of these accessions showed high resistance at seedling stage, of which, ninety-nine accessions exhibited resistance at adult plant stage. Eleven molecular markers of eight effective leaf rust resistance genes in China were used to screen the 112 accessions. Seven effective leaf rust resistance genes Lr9, Lr19, Lr24, Lr28, Lr29, Lr38 and Lr45 were detected, except Lr47. Twenty-three accessions had only one of those seven effective leaf rust resistance gene. Eleven accessions carried Lr24+Lr38, and 7 accessions carried Lr9+Lr24+Lr38, Lr24+Lr38+Lr45, Lr24+Lr29+Lr38 and Lr19+Lr38+Lr45 respectively. The remaining seventy-one accessions had none of those eight effective leaf rust resistance genes. This study will provide theoretical guidance for rational utilization of these introduted wheat accessions directly or for breeding the resistant wheat cultivars.

Virulence Phenotypes of the Wheat Leaf Rust Pathogen, Puccinia triticina, in the United States from 2018 to 2020

Collections of wheat leaves infected with the leaf rust fungus, Puccinia triticina, were obtained from the southeastern states, the Ohio Valley, the Great Plains, and Washington in 2018, 2019, and 2020 to determine the prevalent virulence phenotypes in the wheat-growing regions of the United States. In the hard red winter wheat region of the southern and mid Great Plains, MNPSD and MPPSD were the two most common phenotypes in 2018 and 2019. In 2020, BBBQD with high virulence to durum wheat was the most common phenotype in the southern Great Plains. In the hard red spring wheat region of the northern Great Plains, MNPSD, MPPSD, MBDSD, and TBBGS were the predominant phenotypes. In the soft red winter wheat region of the southeastern states and Ohio Valley region, MBTNB, MCTNB, and MNPSD were the three most common phenotypes. Collections in Washington had phenotypes LBDSG, LCDSG, LCDJG, and MBDSB that were not found in any other region. Isolates with virulence to the leaf rust resistance (Lr) gene Lr11 were most frequent in the southeastern states and Ohio Valley regions. The frequency of isolates with virulence to the Lr39 gene was highest in the Great Plains region and frequency of isolates with virulence to the Lr21 gene was highest in the northern Great Plains region. Selection of virulence phenotypes by Lr genes in the different market classes of wheat, combined with the effects of clonal reproduction, overwintering in southern regions, and low migration between the Great Plains region and eastern wheat-producing regions, has maintained the different P. triticina populations in the United States.

Genome-Wide Association Mapping Indicates Quantitative Genetic Control of Spot Blotch Resistance in Bread Wheat and the Favorable Effects of Some Spot Blotch Loci on Grain Yield

Spot blotch caused by the fungus Bipolaris sorokiniana poses a serious threat to bread wheat production in warm and humid wheat-growing regions of the world. Hence, the major objective of this study was to identify consistent genotyping-by-sequencing (GBS) markers associated with spot blotch resistance using genome-wide association mapping on a large set of 6,736 advanced bread wheat breeding lines from the International Maize and Wheat Improvement Center. These lines were phenotyped as seven panels at Agua Fria, Mexico between the 2013-2014 and 2019-2020 crop cycles. We identified 214 significant spot blotch associated GBS markers in all the panels, among which only 96 were significant in more than one panel, indicating a strong environmental effect on the trait and highlights the need for multiple phenotypic evaluations to identify lines with stable spot blotch resistance. The 96 consistent GBS markers were on chromosomes 1A, 1B, 1D, 2A, 3B, 4A, 5B, 5D, 6B, 7A, 7B, and 7D, including markers possibly linked to the Lr46, Sb1, Sb2 and Sb3 genes. We also report the association of the 2NS translocation from Aegilops ventricosa with spot blotch resistance in some environments. Moreover, the spot blotch favorable alleles at the 2NS translocation and two markers on chromosome 3BS (3B_2280114 and 3B_5601689) were associated with increased grain yield evaluated at several environments in Mexico and India, implying that selection for favorable alleles at these loci could enable simultaneous improvement for high grain yield and spot blotch resistance. Furthermore, a significant relationship between the percentage of favorable alleles in the lines and their spot blotch response was observed, which taken together with the multiple minor effect loci identified to be associated with spot blotch in this study, indicate quantitative genetic control of resistance. Overall, the results presented here have extended our knowledge on the genetic basis of spot blotch resistance in bread wheat and further efforts to improve genetic resistance to the disease are needed for reducing current and future losses under climate change.

Development and deployment of KASP markers for multiple alleles of Lr34 in wheat

Heterogeneous Lr34 genes for leaf rust in winter wheat cultivar 'Duster' and KASP markers for allelic variation in exon 11 and exon 22 of Lr34. Wheat, Triticum aestivum (2n = 6x = 42, AABBDD), is a hexaploid species, and each of three homoeologous genomes A, B, and D should have one copy for a gene in its ancestral form if the gene has no duplication. Previously reported leaf rust resistance gene Lr34 has one copy on the short arm of chromosome 7D in hexaploid wheat, and allelic variation in Lr34 is in intron 4, exon 11, exon 12, or exon 22. In this study, we discovered that Oklahoma hard red winter wheat cultivar 'Duster' (PI 644,016) has two copies of the Lr34 gene, the resistance allele Lr34a and the susceptibility allele Lr34b. Both Lr34a and Lr34b were mapped in the same linkage group on chromosome 7D in a doubled-haploid population generated from a cross between Duster and a winter wheat cultivar 'Billings' which carries the susceptibility allele Lr34c. A chromosomal fragment including Lr34 and at least two neighboring genes on its proximal side but excluding genes on its distal side was duplicated in Duster. The Duster Lr34ab allele was associated with tip necrosis and increased resistance against leaf rust at adult plants in the Duster × Billings DH population tested in the field, demonstrating the function of the Duster Lr34ab allele in wheat. We have developed KASP markers for allelic variation in exon 11 and exon 22 of Lr34 in wheat. These markers can be utilized to accelerate the selection of Lr34 in wheat.

Virulence of Puccinia triticina, the Wheat Leaf Rust Fungus, in the United States in 2017

Samples of wheat leaves infected with the leaf rust fungus, Puccinia triticina, were obtained in 2017 from agricultural experiment station plots, demonstration plots, and farm fields in the Great Plains, the Ohio Valley, the southeastern states, California, and Washington in order to determine the prevalent virulence phenotypes present in the United States. A total of 65 virulence phenotypes were identified among the 469 single uredinial isolates that were tested on 20 near-isogenic lines of Thatcher wheat that differ for leaf rust resistance genes. Virulence phenotypes MBTNB at 11.3% of the overall population, and MCTNB at 7.0%, were the first and third most common phenotypes. Both phenotypes were found mostly in the southeastern states and Ohio Valley region. Phenotype TFTSB at 10.9% was the second most common phenotype and was found mostly in southern Texas. Virulence to leaf rust resistance gene Lr39, which is present in hard red winter wheat cultivars, was highest in the Great Plains region. Virulence to Lr11 and Lr18, which are present in soft red winter wheat cultivars, was highest in the southeastern states and Ohio Valley region. Virulence to Lr21, which is present in hard red spring wheat cultivars, was highest in the northern Great Plains region. The predominate P. triticina phenotypes from the soft red winter wheat regions of the southeastern states and Ohio Valley area differed from those in the hard red winter and hard red spring wheat areas of the Great Plains region. Collections from Washington had unique virulence phenotypes that had not been previously detected.