Quantifying the ultraviolet-induced fluorescence intensity in green mould lesions of diverse citrus varieties: Towards automated detection of citrus decay in postharvest

Abstract

Citrus fungal infections developing during fruit storage and transportation can cause significant economic losses after harvest. The most important is caused by the fungus Penicillium digitatum, which infects the fruit through rind wounds and causes a rot lesion. The symptoms of decay are difficult to notice by the human eye in the initial stages of decay development because the colour of the lesion is very similar to that of the healthy rind. One method to detect this disease early is to illuminate the fruit with ultraviolet (UV) light since the disease causes visible fluorescence. Manual inspection exposes the workers to UV light, which is dangerous for their skin and eyes. An alternative is to use artificial vision systems. But not all varieties show the same level of fluorescence, and even some do not produce this phenomenon, making it challenging to create effective automatic detection systems based on image analysis. This work has studied and determined the fluorescence level of 104 varieties of oranges and mandarins using hyperspectral and colour imaging. The samples were inoculated with spores of the P. digitatum in controlled conditions. Images were captured exposing the fruit under UV light (380 nm) using a colour camera and a hyperspectral imaging system. The fluorescence level of each variety was measured using three colour coordinates and the hyperspectral images. Best correlations between the spectral and the colour-based systems were achieved using the green (G) colour coordinate of the RGB colour space (R2 =0.85). Navel and common oranges emit the most fluorescence, while 16 varieties (mostly blood oranges and other mandarins) have very low or undetectable fluorescence.