Synchronous fluorescence spectroscopy for early diagnosis of citrus canker in citrus species.

The current study presents the application of fluorescence emission spectroscopy and synchronous fluorescence spectroscopy (SFS) coupled with chemometrics in diagnosing citrus canker disease in citrus species. Leaf samples were collected from random plants of grapefruit and Mexican lime species from an orchard. Fluorescence spectroscopy demonstrated the disease severity by predicting an increase in the blue–green spectral region and a decline in chlorophyll bands in the citrus leaves. As a result of the disease severity, characteristic changes in the SFS spectrum were observed. Noticeably, SFS provided additional information on the occurrence of significant bands of leaf fluorophores representing various biochemical changes in leaves at different disease stages. The current study led to the inference that a higher intensity of α-tocopherol and gallic acid is found in healthy and asymptomatic leaves; in contrast the band intensity of flavonoids, caffeic acid, tannins, flavins and chlorogenic acid increases in diseased samples, proportionally to the disease severity. Principal component analysis showed a clear separation between healthy and diseased samples. It is important to note that the asymptomatic samples were clustered with the symptomatic samples or remained as a separate group, thus revealing disease initiation, an observation which is impossible to make through visual examination. The results indicate that the spectral signatures obtained through front-face fluorescence spectroscopy as well as through SFS of citrus leaves could be used as fingerprints from the detection viewpoint. [ABSTRACT FROM AUTHOR]