Cercospora infection in sugar beet leaves: analysis using chlorophyll-fluorescence and thermal imaging

Introduction: A robotised mobile fluorescence imaging system was used to visualise the early stages of the Cercospora fungal infection in attached sugar beet leaves. To further explore the possibilities of fast disease resistance screening, excised leaf pieces were continuously monitored after infection. Materials and Methods: For infection of sugar beet (Beta vulgaris L.) with Cercospora beticola, the upper sides of a leaf from the 4th leaf pair or excised leaf strips were sprayed with a spore solution. A chlorophyll fluorescence imaging system, a thermal camera and a video camera, mounted side-by-side on a gantry (Cartesian XYZ) robot (measuring area 2m x 1m) were positioned sequentially above the same attached leaf region or excised leaf piece to capture the respective co-localized chlorophyll fluorescence emission, thermal and video images. Results and conclusions: At Cercospora infection loci, pre-symptomatic high-intensity spots became first apparent 6 days post infection (dpi) in the chlorophyll fluorescence images. Apart from photosynthetic efficiency also transpiration was affected, as witnessed by the appearance of co-localised, low temperature spots in the thermal image, a few hours after the increase in chlorophyll fluorescence intensity. This is in striking contrast with the decrease in transpiration observed after TMV infection of resistant tobacco (Chaerle et al.,1999). At 6.5 dpi, a central dark spot formed at the center of the white spots in the chlorophyll fluorescence images - a co-localised visible lesion appeared at 7dpi. At the 7dpi timepoint, chlorophyll fluorescence symptoms were maximal. An increase in chlorophyll fluorescence emission, was also observed early during the resistance response of tobacco to TMV, but in this case was rapidly followed by a pronounced intensity decrease. Recognition of differences in the (the kinetics of the) pre-visual response(s) could lead to a rapid differentiation of emerging (biotic) stresses by imaging