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Sunflower downy mildew caused by Plasmopara halstedii is one of the most potentially important diseases. So far, a complete, major gene resistance (Pl) has been used successfully, but with the appearance of eight races in France since 2000, research on more durable resistance was undertaken. In this study, we present new results concerning the evolution of pathogenicity of P. halstedii under conditions of re-enforced infection and different Pl gene selection pressures. Moreover, we imagine the evolution of virulence and aggressiveness of P. halstedii under a mixture model of sunflower inbred lines carrying the two types of resistance which may enhance durable resistance against it. Examples of host-parasite interactions including the influence of plant mixture models against pathogens are equally presented to understand how the pathogen develops its pathogenicicity.
Components of quantitative resistance in sunflower (Helianthus annuus) to Plasmopara halstedii, the pathogen causing downy mildew, were investigated. Percentage infection, latent period, sporulation density and reduction of hypocotyl length were compared on two sunflower lines showing different levels of quantitative resistance in the field infected with different P. halstedii strains of races 100, 300, 710 and 714 in controlled conditions. The inbred line BT, rather susceptible in the field, showed a higher percentage infection, a higher sporulation density, a shorter latent period and less reduced hypocotyl length than inbred line FU, which showed a greater resistance in the field. The very good resistance of inbred line FU observed in the field was confirmed by the measurements of quantitative resistance criteria described in this study. Percentage infection of FU was 1.4% less than BT, latent period of BT was 12.4% less than FU, sporulation density of FU was 22.3% less than BT and reduced hypocotyl length of BT was 15.3% less than FU. It seems that the criteria such as latent period, sporulation density and reduction of hypocotyl length may be used to measure quantitative resistance in sunflower to P. halstedii.
An extensive survey of downy mildew (Sclerospora graminicola) incidence on pearl millet (Pennisetumglaucum) grown in farmers’ fields was undertaken in 2002 in pearl millet-growing areas of Nigeria – the Sahel, Sudan Savanna, northern Guinea Savanna and southern Guinea Savanna agroecological zones. Downy mildew incidence was assessed on two types of pearl millet “gero” and “maiwa” and in 32 locations in these zones. Incidence in “gero” millet was the highest in Sudan Savanna and the lowest in the Sahel zone. Cropping pattern and seed rate affected incidence of the disease which was highest in the fields with pure stand than mixed stand cropped with legumes..
Grazing preferences of collembolans Ceratophysellasp. 1 were noted. The collembolans feed on the sporangia of Pseudoperonospora cubensisgrowing on plants Cucumis sativusin greenhouses in Esfahan province in Iran. Sporangia of Pseudoperonospora cubensiswere the preferred food of all fungal species noted in the alimental tract of the springtails. All digestive tracts of the springtails contained sporangia of P. cubensis.
The obligate oomycete Sclerospora graminicola (Sacc.) Schroet, is the incitant of downy mildew disease, which is the main constraint in pearl millet production worldwide. Different elicitors from Trichoderma hamatum UOM 13, e.g. mycelial extract and cell wall glucans, were assessed for their resistance elicitation efficiency and the possible underlying mechanisms. Both mycelial extract and cell wall glucans of T. hamatum UOM 13 positively influenced seed quality parameters of pearl millet, significantly enhanced seed germination and seedling vigor in comparison to the untreated control. Seed priming with cell wall glucan elicitors of T. hamatum UOM 13 suppressed downy mildew on susceptible pearl millet seedlings under greenhouse conditions by induction of systemic host resistance. Of the different elicitor delivery methods tested, transplant root dip was more effective than seed treatment and foliar spray. A combination of transplant root dip + seed treatment + foliar spray was significantly more effective than the single delivery methods. The induced resistance corresponded to up regulation of genes of important defense proteins upon pathogen inoculation. Transcripts of genes of defense enzymes glucanase, phenylalanine ammonia lyase, peroxidase and polyphenoloxidase were significantly increased due to the T. hamatum UOM elicitor effect. Expression of hydroxyproline-rich glycoprotein genes, known to play an important role in cell wall cross-linking, were also up regulated in response to T. hamatum UOM cell wall glucan treatment. This study emphasizes the role of T. hamatum UOM as a potential elicitor of downy mildew resistance in pearl millet and presents novel insights into the involvement of important defense proteins mediating such as resistance trigger.
Resistance to downy mildew was tested on resistant cultivars: Brassica oleracea PI246077 and PI231210, a broccoli-cauliflower line from Polish selection, susceptible cauliflower lines and their F₁ and F₂ progenies. Infection degree of the inoculated plants was determined at the cotyledon and 4-5 leaf stages. To estimate score plant infection at the 4-5 leaf stage the Williams scale designed for plants inoculated at the cotyledon stage was modified. Resistance to the Polish fungus isolate proved to be a dominant character. At the cotyledon stage depending on the source of resistance, it was determined by four (in the broccoli-cauliflower line) or three (in PI USA) dominant complementary genes, while at the stage of 4-5 leaves - by a single dominant gene (in PI231210), two (in PI246077) or three (in the broccoli-cauliflower line) additive dominant genes.
This work presents the results concerning the yielding (time of ripening, weight of clusters and berries, fertility of vines, percent of soluble solids, taste of fruits), winter hardiness and susceptibility to fungal diseases (downy mildew, powdery mil­dew, grey mould and excoriose) of 25 selected cultivars for processing. The investiga­tions were carried out in the grapevine collection of the Research Institute of Pomol­ogy and Floriculture in Skierniewice, Poland. This grapevine collection was estab­lished in 1992, and it is consisted of 234 taxons. Assessed genotypes differed consid­erably in terms of the examined traits. Berries ripened from the second half of August ('Reform') until the second week of October ('V 71141', 'Sibera'). In the years 2005­2009, vines of the hybrid 'V 64035' and cultivars 'Seyval' and 'Cayuga White' were most productive. Berries of 'Delaware' were characterized by having the highest level of soluble solids. Vines of interspecific hybrids were less susceptible to frost damage and fungal diseases than cultivars of V. vinifera ('Chasselas Dore', 'Ortega'). Inter­specific hybrids 'Seyval', 'Bianca', 'Sibera', 'Marechal Foch', 'Rondo' and 'Regent' were distinguished as having the best yielding and highest quality of fruit from among the assessed cultivars, for commercial wine making. 'Aurore', 'Delaware', 'Cascade' and 'Golubok' were relatively reliable in yielding, and their grapes may be used for the production of juice and home wines.
Three separate field experiments were conducted in 2001, 2002 and 2003 to determine the efficacy of combining sowing date, seed treatment with metalaxyl and use of host plant resistance for the control of pearl millet downy mildew. Early sowing gave lower disease incidence and higher grain yield than late sowing. The disease was controlled when metalaxyl treated seeds were sown early. The highest disease incidence and the lowest grain yields were obtained when untreated seeds were sown late. Use of resistant pearl millet cultivar along with seed treatment using metalaxyl at the dose of 2 g a.i./kg seeds greatly reduced disease incidence and increased grain yield in comparison with the seed treatment of susceptible cultivar.
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