Monitoring of Alternaria alternata infection in postharvest green pepper fruit during storage by multicolor fluorescence and chlorophyll fluorescence

Determining the index for monitoring the fungal disease severity of postharvest green pepper during storage is still a challenge to be solved in research to avoid food loss, linked to the economical loss. The aim of the present study was to evaluate the potentiality of fluorescence characteristics for the determination of disease index. In this study, postharvest green peppers were inoculated with Alternaria alternaria and stored at ambient temperature, the lesion diameter, ergosterol content, and chlorophyll fluorescence parameters were determined. The green pepper fruit was also extracted with ethanol and water, and the traditional excitation wavelengths at 254 nm and 365 nm were used to screen optimal emission wavelength, and corresponding fluorescence intensities were determined. Ergosterol content was found to be correlated with lesion diameter. It was observed that the number of emission peaks of the multi-color fluorescence of ethanol extract from peppers were more than those of water extract. Using both excitation wavelengths at 254 nm and 365 nm yielded 3–4 types of visible fluorescence, in contrast to 1–2 types obtained with a single excitation wavelength. The ethanol extract of peppers after column chromatography with an excitation wavelength of 225 nm, one fluorescence in the ultraviolet region and four visible fluorescence including red, cyan, purple, and blue were obtained. Red and blue fluorescence might be chlorophyll fluorescence and flavonoid fluorescence, respectively, while cyan and purple fluorescence were composite colors of multiple fluorescence. The fluorescence intensity with excitation and emission matrix (EEM) 225 nm/324 nm, electron transport rate (ETR), photochemical quenching (qP) showed high correlations with lesion diameter, of which correlation coefficients were 0.99^**, −0.98^** and −0.98^**, respectively. Finally, it was suggested that the fluorescence intensity with EEM 225 nm/324 nm, ETR and qP would be potential to be probes for fungal disease index in postharvest green peppers.