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SESSION 1 - Discovery - New horizons in plant pathology - page 17 / 65





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The primary inoculum of the fungus Leptosphaeria maculans (blackleg, phoma) are wind-borne ascospores released from pseudothecia on canola stubble. Consequently, the main cultural management strategy to control blackleg in Australia is to allow enough time for all canola stubble to breakdown (three to four years) before the next canola crop is sown. However, anecdotal evidence suggests that short rotations (canola / wheat / canola) have the same severity of blackleg infection compared to long rotations (4 year break between canola crops). To determine if extended crop rotations are warranted, 2- to 42-month old stubble from paddocks in three diverse rainfall environments (ranging from 300 to 650mm annually) in south-eastern Australia was evaluated for the number of ascospores released. Ascospores were discharged from stubble of all ages and environments, however, >97% of ascospores produced by the fungus were from the previous season’s canola stubble, with few released from older stubble. The severity of blackleg infection in canola crops was similar for both short and long rotations. The distance between current canola crops and stubble from the previous season was measured and found to be critical to infection severity.  The overall finding of this research suggests that crop rotation is not as important in reducing disease levels as previously thought and that crop isolation from stubble of the previous season is the critical factor. Although a 500m buffer from canola stubble of the previous season provided the best disease reduction, a distance of greater than 100m significantly reduced disease severity.

Diagnostics for Phytoplasmas, Unculturable Phytopathogenic Mollicutes

Nigel A. Harrison

University of Florida, Plant Pathology Dept., 3205 College Avenue, Fort Lauderdale, FL 33314, USA

Phytoplasmas are wall-less polymorphic bacteria morphologically resembling members of the Mollicutes. These unculturable prokaryotes are regularly observed in phloem sieve tube elements of at least 1,000 plant species worldwide affected by yellows diseases and in the salivary glands of insect vectors transmitting these diseases. Phytoplasmas exist in nature in a dual host system characterized by alternate passage between plant and insect (primarily cicadellid) hosts. Without confirmation of identity through cultural techniques, phytoplasma detection and characterization has traditionally relied upon elucidation of biological properties including symptomatology, plant host range, and vector specificity. Application of DNA-based methods to studies on phytoplasma diseases has led to substantial improvements in detection of these enigmatic endocelluar organisms. Based largely on PCR amplification of evolutionarily conserved genes rRNA and ribosomal protein genes, further RFLP or sequence analysis amplified gene products have also provided a facile means for accurately identifying phytoplasmas and a framework for their classification. Phylogeny of 16S rRNA gene sequences has been adopted as the foundation for a formal taxonomy of these organisms in which phytoplasmas are clearly recognized as a unique genus of Mollicutes comprised of numerous species and related strains of a species. Projects in progress to sequence A+T-rich genomes of phytoplasmas which vary from 530 to 1,350 kb in size, are yielding insights into the minimal gene complement required for parasitism and pathogenicity of plant and insect hosts.  Identification and expression of phytoplasma membrane protein genes for antisera production and strain-specific sequence data for primer typing, are much needed improvements to existing diagnostics and integral to understanding the epidemiological role that strain variation plays in recurrent outbreaks of phytoplasma diseases.

Advances in molecular diagnostics – new solutions for old problems

Rick Mumford, Tom Fisher, Kelvin Hughes, Anna Skelton, Jenny Tomlinson, Kathy Walsh, Ian Barker and Neil Boonham, Central Science Laboratory, Sand Hutton, York, YO41 1LZ, U.K.

In the last decade, developments in molecular (nucleic acid-based) diagnostic methods have lead to significant improvements in the detection of plant pathogens. By using methods such as the polymerase chain reaction (PCR), the range of targets that can now be reliably diagnosed has grown to the extent that there are now extremely few, known pathogens that cannot be identified accurately by using laboratory-based diagnostics.  However, while the detection of pathogens in individual, infected samples is becoming simpler, there are still many scenarios that present a major challenge to diagnosticians and plant pathologists. Amongst these are the detection of pathogens in soil or viruses in their vectors, high-throughput testing and the development of generic methods, that allow samples to be simultaneously screened for large numbers of pathogens.  Another major challenge is to develop robust technologies that avoid the reliance on well-equipped central laboratories; making reliable diagnostics available to pathologists in the field or in less-developed countries. In recent years, much of the research carried out on phytodiagnostics has focussed in these areas and as a result many novel, routine diagnostic tests

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