First author: Department of Plant and Environmental Protection Sciences (PEPS), University of Hawaii at Manoa, Honolulu 96822; second author: Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa; and third author: PEPS, University of Hawaii at Manoa.
Essential oils of palmarosa, lemongrass, and eucalyptus have shown promise as biofumigants for control of the bacterial wilt disease of edible ginger (Zingiber officinale) caused by Ralstonia solanacearum race 4 in previous potting medium studies. Biochemical changes in R. solanacearum cells were evaluated with micro-Raman spectroscopy following treatment with essential oils at different concentrations (0.04, 0.07, and 0.14% [vol/vol] of culture medium) and changes in cell structure were observed using electron microscopy. All treatments except palmarosa oil at 0.04% caused significant reductions in levels of amino acids, purine and pyrimidine bases of nucleic acids, carbohydrates, and lipids, as indicated by significant reduction in Raman peak heights at 621, 1,003, and 1,031 inverse centimeters (cm–1) (phenylalanine); 643, 827, 852, 1,158, and 1,172 cm–1 (tyrosine); 758 cm–1 (tryptophan); 725, 782, 1,337, and 1,578 cm–1 (adenine, cytosine plus uracil, adenine, and adenine plus guanine, respectively); 1,097 cm–1 (carbohydrates); and 1,127, 1,450, and 2,932 cm–1 (lipids) compared with untreated controls. Lemongrass oil treatments were the most effective in degrading cellular components. Scanning electron microscopy of palmarosa and lemongrass-oil-treated cells showed rupture of cell walls and cell debris but no degradation was noted for eucalyptus-oil-treated cells. Palmarosa- and lemongrass-oil-treated cells were positively stained with uranyl acetate when viewed by transmission electron microscopy whereas controls and eucalyptus-oil-treated cells were negatively stained, indicating that the cell membranes were intact. The viability of eucalyptus-oil-treated cells was confirmed by cell culture following treatment. Micro-Raman spectroscopy is a powerful tool which can be further employed to better understand effects of fumigants and other bactericides on bacterial cells.