Browsing by Subject "Pathogenicity"

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Plasmopara viticola, the downy mildew of grapevine : phenotypic and molecular characterization of single sporangium strains infecting hosts with different resistance levels
(2015) Gómez Zeledón, José Javier; Spring, Otmar
The downy mildew of grapevine, Plasmopara viticola, is one of the most important pathogens in viticulture. Its genetic diversity had been assessed in some previous studies using molecular markers, but the diversity of the infection behavior has not yet been addressed adequately. Therefore, the development of a fast, reliable and uncomplicated assay to screen for pathogen phenotypes on host with different resistance levels was a major task of this work. A leaf disc test was proposed, evaluating sporulation and necrosis produced by the pathogen on Vitis plants with different susceptibility. Using this bioassay, interesting strains were assessed and kept for future studies. The urgent need to work with genetic homogeneous inoculum was shown, because the assays revealed a high phenotypic diversity in isolates collected from the field as a bulk sample. Hence, a cloning technique to obtain single sporangium strains was found useful to avoid working with mixed genotypes. The leaf disc bioassay also allowed screening for fungicide resistance in P. viticola populations. Isolates resistant to dimethomorph and metalaxyl, two important fungicides for oomycetes control, were detected. Higher resistance was associated with fields were the fungicide application was high as well. Some strains were even resistant to doses where the fungicide exhibits phytotoxic activity to grapevine. The approach of characterizing P. viticola pathotypes on different host plants of Vitis vinifera cultivars and Vitis species from North America and Asia revealed a broad spectrum of fully susceptible to completely resistant reactions. This information is of direct practical value in future plant breeding programs, but also provides the chance to select specific host-pathogen combinations to study the mechanisms of resistance or susceptibility. Fluorescence microscopy revealed how the infection progress of highly and lowly virulent strains advance in tolerant and susceptible hosts, and which points of the infection are interesting for future studies. On the molecular level, effectors were investigated to trace their possible involvement in the infection process. It was found that RXLR 1, NLP 1, Elicitin like 2, Glucanase inhibitor 2 and 4 , and 1,3-ß Glucanase 2 are candidates which are upregulated in the earliest infection stages. Following the here established methodology and suggested strategy it should be possible in the future to get a better insight in the mechanisms of infection and resistance of grapevine downy mildew.