Early detection of foot rot on sweet potato based on auto-fluorescence of phytoalexin

Abstract

Sweet potato foot rot is difficult to detect at the pre-symptomatic stage because visible symptoms are minimal or absent, whereas conventional approaches are often destructive or unsuitable for rapid screening. In this study, a UV fluorescence imaging workflow was developed for pre-symptomatic evaluation of sweet potato foot rot based on host-response-associated autofluorescence. The method uses two-band fluorescence acquisition at 460 and 520 nm under 375 nm excitation, followed by pixel-wise and distribution-based image analysis. Rather than relying only on localized bright regions or overall signal intensity, the workflow represents tissue states using multi-channel pixel distributions and exploratory quantitative descriptors. Scatter distributions were used to visualize joint channel behavior, and exploratory single- and dual-band descriptors were used for quantitative comparison between Control and Stage 1 samples. Pre-symptomatic Stage 1 samples, which were macroscopically indistinguishable from Controls, exhibited characteristic distribution features that differed from Control patterns under the same acquisition and preprocessing conditions. Scopoletin injection reproduced Stage 1-like fluorescence distribution patterns, supporting consistency with phytoalexin-associated autofluorescence. The workflow was further applicable to a second sweet potato cultivar without cultivar-specific parameter tuning, and complementary ATR-FTIR measurements supported chemical interpretation. These results establish a distribution-based fluorescence imaging framework for pre-symptomatic sweet potato foot rot evaluation and provide a basis for future automated analysis within sampling-based screening workflows.