Physiologic Specialization of Puccinia triticina on Wheat in the United States in 2004

Race and Virulence Dynamics of Puccinia triticina in China During 2007 to 2021

The challenge of wheat leaf rust on wheat production is a recurring issue. Race identification of Puccinia triticina (Pt) serves as the foundation for preventing and controlling this disease. In a 15-year study, we identified 2,900 isolates of Pt from 20 provinces, cities, or autonomous regions in China during 2007 to 2021 and found 332 virulence phenotypes with 11 predominant phenotypes: PHT (8.3%), THT (5.4%), PHK (4.5%), PHJ (3.7%), THJ (3.6%), SHJ (3.5%), THS (3.3%), FGD (2.9%), THK (2.6%), PHS (2.4%), and PHD (2.0%). The virulence frequency for 40 Lr genes was identified across different years and areas; one major reason for the race dynamics was the attenuation to Lr1 and Lr26, which was more evident in southwest China. Lr9, Lr24, Lr28, Lr38, and Lr42 maintained effectiveness in China, while Lr2c, Lr10, Lr12, Lr14a, Lr14b, Lr22a, Lr33, and Lr36 nearly lost their effectiveness against wheat leaf rust disease. No significant difference was found among predominant phenotypes in different areas (P > 0.1). However, 12 Lr sites were found to have differences in virulence frequencies with values greater than 20% across various locations; furthermore, the lowest and highest virulence values were observed in north China (Area 1) and northwest China (Area 5), respectively. According to phenotype dynamics, PHT, THT, FGD, THK, and PHS are more likely to persist over time. In addition, much attention should be given toward discovering rising combinations of virulent phenotypes.

Multilocus Genotypes of the Wheat Leaf Rust Fungus Puccinia triticina in Worldwide Regions Indicate Past and Current Long-Distance Migration

Many plant pathogenic fungi have a global distribution across diverse ecological zones and agricultural production systems. Puccinia triticina, the wheat leaf rust fungus, is a major pathogen in many wheat production areas of the world. The objective of this research was to determine the genetic relatedness of P. triticina in different worldwide regions. A total of 831 single-uredinial isolates collected from 11 regions were characterized for multilocus genotype at 23 simple sequence repeat loci and for virulence to 20 lines of wheat with single genes for leaf rust resistance. A total of 424 multilocus genotypes and 497 virulence phenotypes were found. All populations had high heterozygosity and significant correlation between virulence and molecular variation, which indicated clonal reproduction. The populations from North America and South America, Central Asia and Russia, and the Middle East and Europe were closely related for multilocus genotypes and many individual isolates from other continental regions were closely related. Twenty-seven multilocus genotypes were found in more than one continental region, and 13 of these had isolates with identical virulence phenotypes. The wide geographic distribution of identical and highly related multilocus genotypes of P. triticina indicated past and more recent migration events facilitated by the spread of clonally produced urediniospores.

Mapping of Leaf Rust Resistance Genes and Molecular Characterization of the 2NS/2AS Translocation in the Wheat Cultivar Jagger

Winter wheat cultivar 'Jagger' was recently found to have an alien chromosomal segment 2NS that has Lr37, a gene conferring resistance against leaf rust caused by Puccinia triticina The objective of this study was to map and characterize the gene(s) for seedling leaf rust resistance in Jagger. The recombinant inbred line (RIL) population of Jagger × '2174' was inoculated with leaf rust pathogen THBJG and BBBDB, and evaluated for infection type (IT) response. A major quantitative trait locus (QTL) for THBJG and BBBDB was coincidently mapped to chromosome arm 2AS, and the QTL accounted for 56.6-66.2% of total phenotypic variation in infection type (IT) response to THBJG, and 72.1-86.9% to BBBDB. The causal gene for resistance to these rust races was mapped to the 2NS segment in Jagger. The 2NS segment was located in a region of approximately 27.8 Mb starting from the telomere of chromosome arm 2AS, based on the sequences of the A genome in tetraploid wheat. The Lr17a gene on chromosome arm 2AS was delimited to 3.1 Mb in the genomic region, which was orthologous to the 2NS segment. Therefore, the Lr37 gene in the 2NS segment can be pyramided with other effective resistance genes, rather than Lr17a in wheat, to improve resistance to rust diseases.

Pathotype Diversity of Phytophthora sojae in Eleven States in the United States

Pathotype diversity of Phytophthora sojae was assessed in 11 states in the United States during 2012 and 2013. Isolates of P. sojae were recovered from 202 fields, either from soil samples using a soybean seedling bioassay or by isolation from symptomatic plants. Each isolate was inoculated directly onto 12 soybean differentials; no Rps gene or Rps 1a, 1b, 1c, 1k, 3a, 3b, 3c, 4, 6, 7, or 8. There were 213 unique virulence pathotypes identified among the 873 isolates collected. None of the Rps genes were effective against all the isolates collected but Rps6 and Rps8 were effective against the majority of isolates collected in the northern regions of the sampled area. Virulence toward Rps1a, 1b, 1c, and 1k ranged from 36 to 100% of isolates collected in each state, while virulence to Rps6 and Rps8 was less than 36 and 10%, respectively. Depending on the state, the effectiveness of Rps3a ranged from totally effective to susceptible to more than 40% of the isolates. Pathotype complexity has increased in populations of P. sojae in the United States, emphasizing the increasing importance of stacked Rps genes in combination with high partial resistance as a means of limiting losses to P. sojae.

Genome-Wide Association Mapping for Resistance to Leaf and Stripe Rust in Winter-Habit Hexaploid Wheat Landraces

Leaf rust, caused by Puccinia triticina (Pt), and stripe rust, caused by P. striiformis f. sp. tritici (Pst), are destructive foliar diseases of wheat worldwide. Breeding for disease resistance is the preferred strategy of managing both diseases. The continued emergence of new races of Pt and Pst requires a constant search for new sources of resistance. Here we report a genome-wide association analysis of 567 winter wheat (Triticum aestivum) landrace accessions using the Infinium iSelect 9K wheat SNP array to identify loci associated with seedling resistance to five races of Pt (MDCL, MFPS, THBL, TDBG, and TBDJ) and one race of Pst (PSTv-37) frequently found in the Northern Great Plains of the United States. Mixed linear models identified 65 and eight significant markers associated with leaf rust and stripe rust, respectively. Further, we identified 31 and three QTL associated with resistance to Pt and Pst, respectively. Eleven QTL, identified on chromosomes 3A, 4A, 5A, and 6D, are previously unknown for leaf rust resistance in T. aestivum.

Physiologic Specialization of Puccinia triticina on Wheat in the United States in 2005

Collections of Puccinia triticina were obtained from rust-infected wheat leaves by cooperators throughout the United States and from surveys of wheat fields and nurseries in the Great Plains, Ohio River Valley, southeast, California, and Washington State, in order to determine the virulence of the wheat leaf rust population in 2005. Single uredinial isolates (797 in total) were derived from the collections and tested for virulence phenotype on lines of Thatcher wheat that are near-isogenic for leaf rust resistance genes Lr1, Lr2a, Lr2c, Lr3a, Lr9, Lr16, Lr24, Lr26, Lr3ka, Lr11, Lr17a, Lr30, LrB, Lr10, Lr14a, Lr18, Lr21, Lr28, and winter wheat lines with genes Lr41 and Lr42. In the United States in 2005, 72 virulence phenotypes of P. triticina were found. Virulence phenotype TDBGH, selected by virulence to resistance gene Lr24, was the most common phenotype in the United States, and was found throughout the Great Plains region. Virulence phenotype MCDSB with virulence to Lr17a and Lr26 was the second most common phenotype and was found widely in the wheat growing regions of the United States. Virulence phenotype MFPSC, which has virulence to Lr17a, Lr24, and Lr26, was the third most common phenotype, and was found in the Ohio Valley region, the Great Plains, and California. The highly diverse population of P. triticina in the United States will continue to present a challenge for the development of wheat cultivars with effective durable resistance to leaf rust.

Wheat leaf and stem rust in the United States

Leaf rust, caused by Puccinia triticina, is a common and widespread disease of wheat in the US. On an annual basis, over 50 races of the leaf rust fungus are detected. There are at least 5 major groups of genetically distinct P. triticina isolates in the US based on allelic variation at microsatellite loci. Distinct regional race populations of P. triticina are found in the US, due to the widespread use of race-specific leaf rust resistance (Lr) genes in different market classes of wheat. In the south-eastern States where soft red winter wheats are grown, races with virulence to Lr9, Lr11, and Lr18 are predominant. In the southern Great Plains region where hard red winter wheats are grown, races with virulence to genes Lr9, Lr17, Lr24, and Lr26 are common. In the northern Great Plains region where hard red spring wheats are grown, races with virulence to Lr2a and Lr16 are common. Due to the wide dispersal of P. triticina, some races are found in all regions of the US. Highly effective durable resistance to leaf rust has been difficult to achieve due to the high degree of virulence variation in the P. triticina population and the rapid selection of races with virulence to effective Lr genes in wheat cultivars. Hard red spring wheat cultivars with genes Lr16, Lr23, and Lr34 have been highly resistant for more than 10 years in Minnesota and the Dakotas. Stem rust, caused by P. graminis f. sp. tritici, has not been a common disease of wheat in the US since the last major epidemics in the 1950s. The low levels of stem rust infections in the US can be attributed to the increasing use of highly resistant winter and spring wheat cultivars, which has greatly reduced the overall level of stem rust urediniospores. Eradication of the alternate host, Berberis vulgaris, has reduced the number of races and slowed the emergence of new races. Resistance genes Sr2, Sr6, Sr17, Sr24, Sr31, Sr36, and SrTmp are common in the winter wheats. Genes Sr6, Sr9b, Sr11, and Sr17 are common in the spring wheats. Spring wheat cultivars may also have adult plant stem rust resistance derived from cv. Thatcher. Many of the winter and spring wheats are susceptible to the new stem rust race from East Africa; however, cultivars with resistance to this race can be found in each of the major wheat classes.