Expert-based thresholds allow the segmentation of various stages of the symptom development. Two weeks-old bean plants cv. Flavert were inoculated with either Xff CFBP4834-R (1.106 CFU ml_1) or mock. This leaflet inoculated with Xff CFBP4834-R was sampled on bean P.vulgaris cv. Flavert at 11 dai. A: visible image obtained by scanning. Necrosis is clearly visible on the left marge of the leaflet surrounded by wilted tissues. B: Fv/Fm image obtained by chlorophyll fluorescence imaging. The three stages of the symptom development, i.e. necrotic, wilted and impacted tissues, were segmented respectively with the thresholds 0.25 ≤ Fv/Fm, 0.25 < Fv/Fm ≤ 0.45 and 0.45 < Fv/Fm ≤ 0.6. Black areas represent non-selected pixels with the threshold. After the segmentation step, the proportion of pixels in each segment may be quantified.