Recent disease outbreaks caused by (re-)emerging plant pathogens have been associated with expansions in pathogen geographic distribution and increased virulence. For example, in the past two decades’ wheat yellow (stripe) rust, Puccinia striiformis f. sp. tritici, has seen the emergence of new races that are adapted to warmer temperatures, have expanded virulence profiles, and are more aggressive than previous races, leading to wide-scale epidemics. Here, we used field-based genotyping to generate high-resolution data on P. striiformis genetics and carried out global population analysis. We also undertook comparative analysis of the 2014 and 2013 UK populations and assessed the temporal dynamics and host specificity of distinct pathogen genotypes. Our analysis revealed that P. striiformis lineages recently detected in Europe are extremely diverse and in fact similar to globally dispersed populations. In addition, we identified a considerable shift in the UK P. striiformis population structure including the first identification of one infamous race known as Kranich. Next, by establishing the genotype of both the pathogen and host within a single infected field sample, we uncovered evidence for varietal specificity for genetic groups of P. striiformis. Finally, we found potential seasonal specificity for certain genotypes of the pathogen with several lineages identified only in samples collected in late spring and into the summer, whereas one lineage was identified throughout the wheat growing season. Our discovery of which wheat varieties are susceptible to which specific P. striiformis isolates, and when those isolates are prevalent throughout the year, represents a powerful tool for disease management.

BACKGROUND: Reliance on fungicides to manage disease creates selection pressure for the evolution of resistance in fungal and oomycete pathogens. Rust fungi (Pucciniales) are major pathogens of cereals and other crops and have been classified as low-risk for developing resistance to fungicides; no case offield failure of fungicides in a cereal rust diseasehas yet been recorded. Recently, the Asian soybean rust pathogen, Phakopsora pachyrhizi evolved resistance to several fungicide classes, prompting us to screen a large sample of the globally widespread wheat yellow rust pathogen, Puccinia striiformis f. sp. tritici (Pst), for mutations associated with fungicide resistance. RESULTS: We evaluated 363 Pst isolates from Europe, the USA, Ethiopia, Chile, China and New Zealand for mutations in the target genes of demethylase inhibitor (DMI; Cyp51) and succinate dehydrogenase inhibitor (SDHI; SdhB, SdhC and SdhD) fungicides. A high proportion of Pst isolates carrying a Y134F DMI resistance-associated substitution in the Cyp51 gene was found among those from China and New Zealand. A set of geographically diverse Pst isolates was also found to display a substitution in SdhC (I85V) that is homologous to that reported recently in P. pachyrhizi and linked to SDHI resistance. CONCLUSION:The identification of resistance-associated alleles confirms that cereal rusts are not immune to fungicide resistance and that selection for resistance evolution is operating at high levels in certain locations. It highlights the need to adopt fungicide resistance management practices and to monitor cereal rust species for development of resistance.

The quantitative barley leaf rust resistance gene, Rph26, was fine mapped within a H. bulbosum introgression on barley chromosome 1HL. This provides the tools for pyramiding with other resistance genes. A novel quantitative resistance gene, Rph26, effective against barley leaf rust (Puccinia hordei) was introgressed from Hordeum bulbosum into the barley (Hordeum vulgare) cultivar 'Emir'. The effect of Rph26 was to reduce the observed symptoms of leaf rust infection (uredinium number and infection type). In addition, this resistance also increased the fungal latency period and reduced the fungal biomass within infected leaves. The resulting introgression line 200A12, containing Rph26, was backcrossed to its barley parental cultivar 'Emir' to create an F population focused on detecting interspecific recombination within the introgressed segment. A total of 1368 individuals from this F population were genotyped with flanking markers at either end of the 1HL introgression, resulting in the identification of 19 genotypes, which had undergone interspecific recombination within the original introgression. F seeds that were homozygous for the introgressions of reduced size were selected from each F recombinant and were used for subsequent genotyping and phenotyping. Rph26 was genetically mapped to the proximal end of the introgressed segment located at the distal end of chromosome 1HL. Molecular markers closely linked to Rph26 were identified and will enable this disease resistance gene to be combined with other sources of quantitative resistance to maximize the effectiveness and durability of leaf rust resistance in barley breeding. Heterozygous genotypes containing a single copy of Rph26 had an intermediate phenotype when compared with the homozygous resistant and susceptible genotypes, indicating an incompletely dominant inheritance.

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Pseudomonas fluorescens FD6 has been shown to possess many beneficial traits involved in the biocontrol of fungal plant pathogens, such as Botrytis cinerea and Monilinia fructicola. Vfr (virulence factor regulator) a highly conserved global regulator of gram-negative bacteria, such as the human pathogen Pseudomonas aeruginosa, is required for the expression of many important virulence traits. The role of Vfr in the regulation of biocontrol traits, such as the production of antibiotics to control fungal pathogens by antagonistic bacteria, has not been elucidated. This study investigated the effect of a vfr mutant derived from P. fluorescens FD6 to better understand the regulation of some important biocontrol traits associated with the bacterium. Biochemical studies indicated that the production of the antibiotics 2,4-diacetylphloroglucinol, pyrrolnitrin and pyoluteorin, was markedly enhanced in the vfr mutant. The vfr mutation also increased biofilm production, swimming motility and the expression of exopolysaccharide-associated gene (pelA, pslA and pslB) transcripts, but reduced protease production. Wheat rhizosphere and root tip colonization by the vfr mutant was higher than that by the wild type at 7 and 21days after inoculation. These findings demonstrate that Vfr modulates the expression of several key traits and the production of important antibiotics involved in the biocontrol potential of P. fluorescens FD6.