The aim of this review was to survey all fungal pathologists with an association with the journal Molecular Plant Pathology and ask them to nominate which fungal pathogens they would place in a 'Top 10' based on scientific/economic importance. The survey generated 495 votes from the international community, and resulted in the generation of a Top 10 fungal plant pathogen list for Molecular Plant Pathology. The Top 10 list includes, in rank order, (1) Magnaporthe oryzae; (2) Botrytis cinerea; (3) Puccinia spp.; (4) Fusarium graminearum; (5) Fusarium oxysporum; (6) Blumeria graminis; (7) Mycosphaerella graminicola; (8) Colletotrichum spp.; (9) Ustilago maydis; (10) Melampsora lini, with honourable mentions for fungi just missing out on the Top 10, including Phakopsora pachyrhizi and Rhizoctonia solani. This article presents a short resume of each fungus in the Top 10 list and its importance, with the intent of initiating discussion and debate amongst the plant mycology community, as well as laying down a bench-mark. It will be interesting to see in future years how perceptions change and what fungi will comprise any future Top 10.

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The Top 10 fungal pathogens in molecular plant pathology

1: Wheat Rusts and the Genetic Bases on Disease Resistance. Introduction. Biology of Pathogen and Host. Host:Pathogen Genetics. Methodologies in Wheat Rust Disease. A Coordinated Strategy for Rust Control. Conclusion. Guide to Gene Descriptions and Illustrations. 2: The Genes for Resistance to Leaf Rust in Wheat and Triticale. Catalogued Leaf Rust Resistance Genes. Temporarily Designated and Miscellaneous Leaf Rust Resistance Genes. 3: The Genes for Resistance to Stem Rust in Wheat and Triticale. Catalogued Leaf Rust Resistance Genes. Temporarily Designated and Miscellaneous Stem Rust Resistance Genes. 4: The Genes for Resistance to Stripe Rust in Wheat and Triticale. Catalogued Stripe Rust Resistance Genes. Temporarily Designated and Miscellaneous Stripe Rust Resistance Genes. References. Appendix I: Species Names Used in the Text, Genomic Formulae and Synonyms. Appendix II: Pathotypes and Accessions (Cultures) Used in Photography.

Wheat Rusts: An Atlas of Resistance Genes

Wild relatives of common wheat, Triticum aestivum, and related species are an important source of disease and pest resistance and several useful traits have been transferred from these species to wheat. C-banding and in situ hybridization analyses are powerful cytological techniques allowing the detection of alien chromatin in wheat. C-banding permits identification of the wheat and alien chromosomes involved in wheat-alien translocations, whereas genomic in situ hybridization analysis allows determination of their size and breakpoint positions. The present review summarizes the available data on wheat-alien transfers conferring resistance to diseases and pests. Ten of the 57 spontaneous and induced wheat-alien translocations were identified as whole arm translocations with the breakpoints within the centromeric regions. The majority of transfers (45) were identified as terminal translocations with distal alien segments translocated to wheat chromosome arms. Only two intercalary wheat-alien transloctions were identified, one induced by radiation treatment with a small segment of rye chromosome 6RL (H25) inserted into the long arm of wheat chromosome 4A, and the other probably induced by homoeologous recombination with a segment derived from the long arm of a group 7 Agropyron elongatum chromosome with Lr19 inserted into the long arm of 7D. The presented information should be useful for further directed chromosome engineering aimed at producing superior germplasm.

Characterization of wheat-alien translocations conferring resistance to diseases and pests: current status

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The Top 10 fungal pathogens in molecular plant pathology: Top 10 fungal pathogens

The development and use of molecular markers for the detection and exploitation of DNA polymorphism is one of the most significant developments in the field of molecular genetics. The presence of various types of molecular markers, and differences in their principles, methodologies, and applications require careful consideration in choosing one or more of such methods. No molecular markers are available yet that fulfill all requirements needed by researchers. According to the kind of study to be undertaken, one can choose among the variety of molecular techniques, each of which combines at least some desirable properties. This article provides detail review for 11 different molecular marker methods: restriction fragment length polymorphism (RFLP), random amplified polymorphic DNA (RAPD), amplified fragment length polymorphism (AFLP), inter-simple sequence repeats (ISSRs), sequence characterized regions (SCARs), sequence tag sites (STSs), cleaved amplified polymorphic sequences (CAPS), microsatellites or simple sequence repeats (SSRs), expressed sequence tags (ESTs), single nucleotide polymorphisms (SNPs), and diversity arrays technology (DArT).

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An overview of molecular marker methods for plants

Three hundred and fifty three Triticum accessions, several also classified as Aegilops and comprising 13 diploid, tetraploid or hexaploid species, were screened for seedling and adult-plant resistance to Puccinia triticina Eriks. using a mixture of pathotypes UVPrt2, 3, 9 and 13. Seedlings were spray-inoculated with a suspension of freshly collected urediospores in distilled water containing Tween 20® seven days after planting. Infection types (ITs) were scored 10 days post-inoculation (d.p.i.). Fully expanded flag leaves were inoculated and ITs and leaf rust severity were scored 16 d.p.i. One hundred and eighty two of the accessions were resistant to moderately resistant in the adult stage, whereas 126 were resistant or moderately resistant as seedlings to the pathotype mixture. Hypersensitive adult-plant resistance was particularly apparent in lines of T. timopheevii, T. sharonense, T. longissimum, T. searsii and T. turgidum. In T. turgidum, which comprised 272 accessions, approximately 44% of the adult plants were resistant to moderately resistant compared to 28% of the seedlings. The expression of these adult-plant resistances varied between hypersensitive flecking of flag leaves, and small pustules commonly associated with chlorosis and/or necrosis of leaf tissue. Partial resistance, expressed by small pustules without any apparent chlorosis, was observed in species such as T. tauschii, T. turgidum ssp. durum and T. turgidum ssp. pyramidale.

Detection of adult-plant resistance to Puccinia triticina in a collection of wild Triticum species

The Lr62/Yr42 translocation comprises mostly alien chromatin and retains only the very distal end of wheat chromosome arm 6AL, including the telomere. The large amount of foreign chromatin prohibits commercial use of the resistance and an attempt was therefore made to exchange some of the introgressed chromatin for wheat chromatin. Plants heterozygous for the translocation, but lacking the Ph1 locus were testcrossed with Chinese Spring nullisomic 6A tetrasomic 6B or nullisomic 6A tetrasomic 6D plants. Resistant (Lr62) testcross F1 progeny were evaluated for the presence of three marker loci and the data used to do a three-point genetic mapping analysis. Forty one recombinants were identified and characterised with further markers. The data revealed that Lr62/Yr42 occurs towards the distal end of 6AS and that the 6AS telomeres of the wheat and translocation chromosomes were homoeologous. While the wheat (Chinese Spring ph1b mutant) and translocated chromosomes 6A shared sufficient homoeology to allow for regular allosyndetic recombination, there were also major structural differences between them, including a duplication (marked by Xgwm334) and translocation (marked by Xsopw7). The duplicated region occurred on chromosome 6A of the CS ph1b mutant and probably resulted from its inherent genomic instability. The structural differences caused irregular meiotic pairing and complex segregation data that were difficult to interpret. It was, however, possible to explain the majority of recombination products in keeping with the expected low frequency of allosyndetic recombination, and to identify the most promising recombinants. These retained both Lr62 and Yr42 within a comparatively small region of foreign DNA at the 6AS telomere.

Modification of the Aegilops neglecta - common wheat Lr62/Yr42 translocation through allosyndetic pairing induction.

Near isogenic lines differing for the presence or absence of group 3 wheat x rye translocations were developed in three wheat varieties. These included a non-compensating 3AS.3RS translocation and two compensating translocations with constitutions 3AL.3RS and 3BL.3RS. Results of a yield trial suggest that each translocation is associated with a sizeable yield penalty and is unlikely to be of use in breeding in its present form.

An evaluation of three Sr27-carrying wheat X rye translocations.

An attempt was made to determine whether the salt tolerance genes of Thinopyrum distichum would be expressed in the presence of the rye genomes and if it would be possible to improve and utilize its hybrid with rye as a salt-tolerant fodder crop. Th. distichum florets were pollinated with diploid and tetraploid rye but hybrids were obtained only with tetraploid rye. The F 1 had 28 chromosomes and expressed high levels of salt tolerance. Fertility could not be induced through colchicine treatment but the plants produced some apomictic seeds during the summer months. The F 2 had 20-22 chromosomes and retained two complete or near-complete rye genomes. The F 2 plants responded better to colchicine treatment, however, the doubled hybrids remained male sterile. In summer, one F 2 plant produced many apomictic seeds with chromosome numbers ranging from 14 to 16 (primarily rye chromosomes). The mechanism that caused the preferential loss of Thinopyrum chromosomes in each apomictic cycle remains unknown. The F 3 phenotypically resembled rye, had primarily reductional male meioses and produced seemingly functional pollen, but were self-sterile.

Hybridization of Thinopyrum distichum and Secale cereale

A relationship was observed between seedling growth rate and seedling emergence under conditions of soil capping. Coleoptile and root growth rates were strongly influenced by the origin of the seed sample and the genotype involved, whereas genotype × environment interactions were small and not significant. A test-cross analysis revealed the presence of strong epistatic effects for the characters coleoptile growth rate and coleoptile diameter. Both additive and dominance variation contributed strongly to the total genetic variation for coleoptile growth rate, while the - dominance appeared to be ambidirectional. Genetic variation for coleoptile diameter was found to be predominantly additive. Whereas both traits were expected to respond well to selection, no consistent correlations with simple plant characteristics could be established, a situation that may limit the utilization of the observed genetic differences through indirect selection.

G. F. Marais

Papers

Detection of adult-plant resistance to Puccinia triticina in a collection of wild Triticum species

Modification of the Aegilops neglecta - common wheat Lr62/Yr42 translocation through allosyndetic pairing induction.

An evaluation of three Sr27-carrying wheat X rye translocations.

Hybridization of Thinopyrum distichum and Secale cereale

Initial seedling growth rates in a semi-dwarf spring wheat collection.