Effect of fruit harvest time on citrus canker detection using hyperspectral reflectance imaging

Abstract

The citrus industry has need for effective and efficient approaches to remove fruit with canker before they are shipped to
selective international markets. The objective of this research was to study the effect of fruit harvest time on citrus canker
detection using hyperspectral reflectance imaging. Ruby Red grapefruits with normal surface, canker, and five common peel
diseases including greasy spot, insect damage, melanose, scab, and wind scar were collected during a 7-month harvest period.
Hyperspectral reflectance images were acquired in the wavelength range of 450–930nm. Spectral information divergence (SID)
was used as a discrimination measure to perform statistical comparisons among reflectance spectra of grapefruit samples over
the whole harvest season. The SID values with respect to the mean reflectance spectrum of canker for the 7-month periods were
0.0009, 0.0002, 0.0008, 0.0001, 0.0007, 0.0003, and 0.0004, respectively. Correlation analysis (CA) was used for hyperspectral
band selection. Two-band ratio images using wavelengths of 729 and 834nm selected by CA (R834/R729) gave the maximum absolute
correlation value of 0.811. The mean ratio values for canker were in the range from 1.287 to 1.407, which were higher than
the ratio values for other peel conditions. A simple thresholding and morphological filtering operations were applied to the
two-band ratio images. The classification accuracies were in the range of 93.3–96.7% for each month. The results presented
in this study demonstrated that there is no significant difference among the accuracy for canker detection over the whole
harvest season using the two-band ratio images and threshold based on the spectrum of 7-month average.

KeywordsHyperspectral imaging-Band selection-Image classification-Citrus-Canker-Disease detection-Time effect