Biochemical characterization of effects of plant essential oils on Ralstonia solanacearum by laser Raman spectroscopy.
M. L. PARET (1), L. Green (2), S. K. Sharma (2), A. M. Alvarez (1) (1) Department of Plant and Environmental Protection Sciences, University of Hawaii at Manoa, Honolulu, HI, USA; (2) Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, HI, USA.
Phytopathology 99:S99 .

Abstract: Essential oils of palmarosa, lemongrass and eucalyptus show promise for biological control of bacterial wilt, caused by Ralstonia solanacearum (Rs). Palmarosa and lemongrass oils (0.04, 0.07, 0.14% v/v) had bactericidal effects on Rs whereas eucalyptus oil was bacteriostatic in culture amendment studies. The effects of treatments at different concentrations were evaluated using Raman spectroscopy using a 785 nm near infrared laser. All bactericidal treatments except palmarosa at 0.04% v/v caused reduction in levels of phenylalanine (Phe), tyrosine (Tyr), tryptophan (Trp), nucleic acid bases, carbohydrates, amides and lipids as indicated by reduction in peak height of Phe at 621, 1003 and 1031 cm–1; Tyr at 643, 827, 852, 1158 and 1172 cm–1, Trp at 758 cm–1; nucleic acid bases at 725, 782, 1337 and 1578 cm–1; carbohydrates at 1097 cm–1, amides at 1663 cm–1; and lipids at 1450 and 2932 cm–1 compared to controls. Eucalyptus oil treatments showed less reduction in these peak heights. The bactericidal and bacteriostatic properties of the oils were confirmed by visual analysis with scanning- and transmission- electron microscopy and cell viability studies using epifluorescence microscopy supporting the data obtained by Raman spectroscopy. The Raman studies provide evidence that essential oils degrade cell components, thus confirming their potential use in biological control.