Browsing by Subject "Plant disease"

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Effects of elevated atmospheric CO2 concentrations on insects and pathogens of spring wheat (Triticum aestivum L. cv. Triso) and oilseed rape(Brassica napus cv. Campino)
(2012) Oehme, Viktoriya; Fangmeier, Andreas
It is suggested that plants, herbivore insects and pathogens will be affected by rising atmospheric CO2. The working hypothesis of this study was that elevated CO2 will affect plant composition and will thus exert influence on plant-insect interactions by changing the nutritive value for insects feeding on phloem sap. To test this hypothesis, experiments were carried out on wheat and oilseed rape in two different systems: controlled environment chambers (climate chamber system) and an open field exposure system with natural climatic and soil conditions (Mini FACE system). The abundance of detrimental insects from different feeding guilds and plant damage by parasitic organisms were examined in a Mini FACE system, while the consequences of elevated CO2 on aphid performance and potential correlations to phloem sap composition of host plants were observed in controlled environment chambers. The concentrations of amino acids and carbohydrates in the phloem of host plants were analysed by high?performance liquid chromatography (HPLC), using a fluorescence detector for amino acids and the evaporative light scattering detector for carbohydrates. In a Mini-FACE system, phenological development of spring wheat and OSR was not significantly changed due to CO2 enrichment. However, elevated CO2 induced changes in plant chemistry (increased carbon:nitrogen ratio and defensive compounds), which resulted in changes in population densities of some pest species. In order to monitor alterations in insect population density, two different methods were applied: direct counts (method 1) and using of yellow sticky traps (method 2). These methods showed both increases and decreases of insect numbers due to elevated CO2, depending on species and on the period of observation. Concerning plant pathogens, leaves of spring wheat were only slightly and not significantly damaged by Erysiphe graminis, Puccinia striiformis, Puccinia recondita and Septoria tritici during the 2006/2008 years in all treatments. Also the OSR was not significantly damaged by Peronospora parasitica. The frequency and severity of disease infestation on spring wheat and OSR was not significantly impacted by elevated CO2. In controlled-environment chambers, the phenology, above ground biomass and RGR of OSR were not significantly impacted due to elevated CO2. And although the phenology of spring wheat was not influenced by raised CO2, significant increases were observed for plant above ground biomass and RGR. The aphid presence significantly reduced the aboveground biomass and RGR of spring wheat, while no effects due to aphids were observed in OSR. High-CO2 treatment differently impacted the performance of aphids. Slight and non-significant increases due to elevated atmospheric CO2 conditions were observed for the aphid relative developmental stages and intrinsic rates of increase, while the weight and RGR were significantly increased for Rhopalosiphum padi and decreased for Myzus persicae. In order to clear CO2-impacts on the insect performance, phloem sap from host plants was analysed for the composition and concentration of amino acids and carbohydrates. In summary, although the phenological development of spring wheat and OSR was not affected due to elevated CO2, significant changes were found for the concentration of carbohydrates in the phloem sap of spring wheat and individual amino acids in both host plants. These alterations in plant chemistry affected the performance and abundance of herbivore insects.
Use of sensor technologies to estimate and assess the effect of various plant diseases on crop growth and development
(2008) Gröll, Kerstin; Claupein, Wilhelm
The topic of this study was ?Use of sensor technologies to estimate and assess the effect of various plant diseases on crop growth and development?. The background of the investigation can be seen in the challenge of developing a sensor system for the site-specific identification of plant diseases. The most widely used practice in disease control is still to spray fungicides uniformly over fields at different times during the vegetation period. However, most diseases are not distributed uniformly across a field, but occur in patches. During the early stage of epidemics large areas of the field are disease free. Excessive use of fungicides increases costs and can increase fungicides residue levels on agricultural products. As there is an increasing pressure to reduce their use by targeting fungicide spraying only on those places in the field where they are needed, the challenge is to provide farmers the the appropriate technological solutions. A simple and cost-effective optical device, based on the measurement of canopy reflectance in several wavebands, would allow disease patches to be identified and thus controlled. The implementation of these reflectance measurement data into crop growth models would allow for the development of site-specific decision rules whether to spray or not to spray. The specific objectives of the Ph.D. thesis were to: develop and test reflectance measurements as a possible technology to identify reflectance signatures of various plant diseases; develop suitable sets of calibrations that can be used for the identification and quantification of plant diseases; test different sensor systems at different spatial resolutions for their ability to identify plant diseases; develop a strategy to use plant disease information gained from sensor measurements as input dataset for the simulation of wheat growth under disease pressure in CERES-Wheat. In greenhouse experiments at the University of Hohenheim and in field experiments at the experimental station ?Ihinger Hof? of the University of Hohenheim the influence of the diseases powdery mildew, septoria leaf blotch and wheat eyespot on the reflectance of winter wheat was analyzed. To measure the reflectance of the plants three different sensor systems were used. Plant reflectance was measured with a digital camera (LEICA S1 PRO, LEICA Kamera AG, Solms, Germany) at leaf scale (0.5 cm²) and with the spectroradiometer Field Spec® Hand Held (ASD, Inc. Boulder, CO, USA) (0.5 m²) and the Yara N-Sensor in the field-scan modus (12 m²) 2 m above the canopy. The diseases powdery mildew, septoria leaf blotch and wheat eyespot have been analyzed. In a first approach it was tested if it is possible to detect plant diseases using reflectance measurements. The greenhouse studies showed that powdery mildew could be identified especially in the visible wavelength range. Also a correlation between powdery mildew pustules and reflectance changes was possible. Powdery mildew is a leaf disease and changes could directly be detected by a sensor system (Chapter 5). Out of this the second approach was to analyze if a stem disease that cannot directly be detected could be identified using a sensor system. The influence of wheat eyespot was investigated in a field experiment with winter wheat. The results showed that wheat eyespot could not be detected with the digital camera and the spectroradiometer. The problem was the low infection level and the distance between the measuring place and the infection place (Chapter 6). In a next step common vegetation indices were tested for their ability to identify plant diseases. Different vegetation indices were selected out of the literature to detect powdery mildew and septoria leaf blotch in the field using a spectroradiometer. Results indicated that the common vegetation index REIP was able to detect powdery mildew at an infection level of 7 %. With the common vegetation indices septoria leaf blotch could be detected only at a late infection level of 13.7 %. Out of this the new vegetation index DII was developed, which was able to detect septoria leaf blotch at an early infection level of 4 % (Chapter 7). Not only the place of infection but also the spatial resolution seems to play an important role in the identification of plant diseases. In a further approach different sensor systems with different spatial resolutions were tested in a field experiment for the identification of septoria leaf blotch. The results showed in general that septoria leaf blotch could be identified especially in the infrared wavelength range compared to powdery mildew that could especially identified in the visible wavelength range. The results showed further that the lower the spatial resolution , the more difficult it gets to identify plant diseases site-specifically. With a spatial resolution of 0.5 cm² a identification and quantification was possible. With a spatial resolution of 0.5 m² only a identification was possible and with a spatial resolution of 12 m² not identification and quantification was possible. That might be because of the resulting mixture of healthy and diseased plants (Chapter 8). The last step of this work was then to show how reflectance measurements could be implemented into crop growth models to calculate decisions whether to spray or not to spray fungicides on a site-specific level. Summarizing, the overall results of this study indicated that an identification of plant diseases was possible under certain conditions. An identification was possible if the infection place was also the measuring place and if a sensor system was used with a high spatial resolution. The results also showed that it was possible in a certain way to differ between biotroph and necrotroph plant diseases. For a holistic farming concept it is necessary in the future that reflectance measurements are integrated in a crop growth model to give farmers a decision tool that decides whether the infection is critical enough to spray or not.