Application of Fourier Transform Infrared Spectroscopy for Detection of Bacterial Fruit Blotch Disease

Bacterial fruit blotch (BFB), caused by Acidovorax citrulli affects the production of cucurbits worldwide, and causes substantial economic losses. Since cucurbit seeds are the most important source of inoculum for BFB outbreaks, seed health testing is an important component of disease management. Currently, there are no nondestructive assays that are sensitive enough to reliably detect A. citrulli-infected seeds. Fourier transform infrared spectroscopy (FTIR) may provide the sensitivity necessary to detect A. citrulli-infected seeds. Attenuated total reflectance (ATR) FTIR was evaluated for the detection of watermelon seeds infected with A. citrulli by pistil inoculation. A. citrulli cells produced a unique signature at a detection limit of approximately 105 cfu/mL. Infected, dry watermelon seeds whose embryos were infected with A. citrulli produced a different spectral profile compared to non-infected seeds. Spectral subtractions between infected and non-infected seeds suggest the potential for indirect detection of A. citrulli by altered ester C-O absorbance bands. Principal component analysis (PCA) of seeds infected with bacterial concentrations ranging from 0.001 – 0.1 OD demonstrated potential for multivariate detection of infected seeds at intermediate contamination levels (0.01 OD) relative to non-infected seeds. This separation was driven by high loading of ester C-O absorbance at frequencies ranging from 1120 - 1000 cm-1, though absorbances identified in pure A. citrulli culture were not observed. These results suggest that FTIR can be used to nondestructively detect seeds infected with moderate levels (105 cfu/mL) of A. citrulli infection.