Browsing by Subject "Xanthomonas"

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Effiziente Verfahren für die Züchtung neuer Erdbeersorten (Fragaria x ananassa Duch.)
(2015) Bestfleisch, Markus; Wünsche, Jens Norbert
In this thesis, different approaches aimed on efficient breeding of new strawberry cultivars were investigated that comprise horticultural traits such as earliness and yield potential as well as resistance traits against two of the most important pathogens in strawberry cultivation, Botrytis cinerea and Xanthomonas fragariae. High yield potential and an extended ripening period in combination with disease resistance form the basis for economic success and contribute to a competitive and sustainable strawberry cultivation in Germany. In a traditional diallel cross breeding experiment, a set of 13 strawberry cultivars were crossed in a reciprocal way. The crossings resulted in a total of 144 F1-populations which were evaluated in a field trial over two years with regards to their horticultural traits in order to investigate the general and specific combining abilities of the parental cultivars concerning the traits earliness and marketable yield. Within this thesis, a statistical model was developed to explain the components of genetic variance and the breeding value of the parental cultivars based on their calculated combining abilities. It was demonstrated that there is no reciprocal effect on the progeny and it is practically irrelevant whether a cultivar is used as mother or father in the crossing experiment. The genetic variance in the breeding experiment is mainly based on the general combining ability (GCA) of the parental cultivars (additive effects). Specific and reciprocal combining abilities (non-additive effects) appeared less relevant. These findings are taken into account for the further development of the breeding approach which has also a main focus on resistance towards the grey mold disease and the angular leaf spot disease. The grey mold disease, caused by the necrotrophic fungus Botrytis cinerea Pers. [teleomorph Botryotinia fuckeliana (de Bary) Whetzel], is the most important fungal disease in strawberries and requires frequent applications of chemical plant protection products. The angular leafspot disease, caused by Xanthomonas fragariae Kennedy & King, is the most important bacterial disease in strawberry cultivation worldwide and there are currently no plant protection products available within the EU for an effective control of the disease. In a successful control strategy, the cultivation of resistant cultivars is of fundamental importance and the results of this thesis will contribute to the development of a targeted breeding approach. Initially, resistance tests were established for both diseases. Reproducible results concerning the resistance characteristics of different cultivars were achieved in artificial inoculation experiments and adapted evaluation scales. With this approach, more than 100 strawberry cultivars, wild-types and breeding clones from the collection of the German Fruit Gene Bank of the JKI in Dresden-Pillnitz were evaluated in three years of testing. Concerning B. cinerea, a total of five genotypes were identified as partially resistant. In parallel, six partially resistant genotypes were found towards X. fragariae. No completely resistant genotyped were identified until now. Additionally to the resistance test against X. fragariae, the systemic dispersal of the bacteria within the plant was further investigated with molecular-biological methods. On the one hand, bacterial DNA was detected by a sensitive nested-PCR method in different plant tissues of inoculated plants from partially resistant and susceptible cultivars at different time points. Additionally, a GFP-tagged virulent X. fragariae strain was produced and used for inoculation experiments in different strawberry genotypes as well. The systemic dispersal of the bacteria was visualized under the fluorescent microscope and the results were confirmed by the nested-PCR detection method. Already three days after the inoculation, X. fragariae spreads systemically throughout the entire host plant and can be detected in all plant tissue samples. The bacteria were detected in all plant parts of partially resistant cultivars after three days post-inoculation as well. The results from the resistance tests lead to direct recommendations for the choice of cultivars on the one hand and on the other hand, the identified partially resistant genotypes can be used in further targeted breeding approaches. The results of this thesis show new ways and strategies for the improvement of strawberry breeding programs. They increase the success for targeted breeding of new cultivars for strawberry cultivation in Germany in order to maintain a competitive and sustainable strawberry production in the future.
Neu auftretende bakterielle Blattfleckenerreger an Radies und Entwicklung eines Resistenztests als Grundlage für die Züchtung resistenter Sorten
(2014) Scholze, Inka S.; Vögele, Ralf
Radish is one of the most important vegetable crops in Rhineland Palatinate. During the last decades, an increase in bacterial leaf spots on radish has been observed. Although the bulb is unaffected, leaf spot symptoms lead to a decline in sales and profits as German consumers prefer marketable red radish bunches including the freshness-indicating foliage. So far, preventive methods for the control of bacterial pathogens on radish were limited to irrigation strategies (for example drip irrigation) or field hygiene. These methods are, however, often difficult to implement and their effect predominantly insufficient. Resistant or tolerant breeds would provide a solution, however, breeding companies have been lacking the necessary information regarding the causative agents and their biology and epidemiology needed for the development of such breeds. Previous examinations of infested plants and seed lots suggest different pseudomonads and Xanthomonas campestris as possible agents. The main objective of this study was therefore to identify the relevant pathogens causing leaf-spot symptoms on radish. Furthermore, another objective was to clarify the infection and growth requirements of bacterial pathogens on hosts for the development of a screening method for resistance to leaf spot pathogens on radish. On account of increasing compensation claims from breeding companies by farmers, the role of seed transmissibility was also to be determined. during a three year observation, bacterial pathogens from plants and seed lots were isolated and characterized. The characterization of unknown bacterial strains was performed by a combination of physiological and molecular methods. Molecular characterization methods such as 16s rDNA sequence analysis and MLST (multilocus sequence typing) were needed to complete the designation of P. syringae pathovars. In addition, virulence assays clarified the importance of single pathovars. Hence bacterial leaf spot pathogens could be determined as P. syringae pv. maculicola, P. viridiflava, P. cannabina pv. alisalensis and X. campestris whereas the former two species were the most abundant in Rhineland Palatinate. Infection trials on radish plants concluded that the different pathogens induced different typical symptom characteristics. To evaluate the influence of environmental factors such as humidity and temperature, it was demonstrated that inoculations with pseudomonads and a leaf spot inducing X. campestris strain caused successful infection and symptom development under a wide temperature range. Humidity was shown to be the most influential factor limiting infestation intensity with P. viridiflava displaying a higher demand for humid conditions than P. syringae pv. maculicola and X. campestris. All three types induced a higher disease infestation on radish plants in high humidity conditions (~94% rH) contrary to a lower infestation in dry conditions (~63% rH). In a first test series, plants inoculated with P. viridiflava were almost symptom free under dry conditions, whereas P. syringae and X. campestris were still able to induce leaf spots. Based on the results of infections assays, it was possible to develop a screening method on radish plants for resistance to the leaf spot inducing bacterial pathogen P. syringae pv. maculicola. The screening method was based on the spray-inoculation of radish leaves fixed in water filled orchid tubes, incubated under high humidity in a climate chamber at 24 °C day/10 °C night. Determination of resistance was performed 7 to 10 days past inoculation by rating disease intensity of infested leaves. Whereas tests could be performed by inoculations with P. viridiflava and P. cannabina as well as P. syringae, the latter proved to be most suitable for the screening method.