Visualisation of downy mildew development in cucumber using thermography
infrared thermography, leaf temperature, fungal infection, transpiration rate
Early stress-induced physiological changes in plants have been mainly detected after destructive sampling followed by biochemical determinations and studies of gene expression. Remote sensing and digital image analysis are methods of acquisition and interpretation of measurements of an object without physical contact between the measuring device and the object. The object can be analysed many times non-invasively and without damage. If physiological changes upon pathogen infection affect the transpiration rate of plants, thermography should provide an elegant way of rapid visualisation due to the negative correlation between transpiration rate and leaf temperature.In this study the effect of a fungal infection on the transpiration rate and on the leaf temperature was investigated by gas exchange measurements and infrared thermography, using cucumber (Cucumis sativus) and the fungus Pseudoperonospora cubensis (Berk. & Curt) as model species.Thermal effects in a cucumber leaf at different stages of pathogenesis of P. cubensis. (a) 3 days after inoculation inoculated area appear as "colder spot" in the thermal image without manifestation of typical symptoms of downy mildew; (b) 5 days after inoculation the colder area has expanded into neighbouring tissue associated with the appearance of chlorotic lesions; (c) 7 days after inoculation area with necrotic cell death were detectable as warmer spots in the thermal image.
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