| Ralstonia solanacearum (Rs) race 4 causes bacterial wilt of edible ginger in the tropics and sub-tropics. Fumigation of Rs-infested soils with methyl bromide has been the most successful disease management tool for decades but is being phased out due to its detrimental effects on the ozone layer. The current study focused on understanding the host range and biology of Rs in Hawaii, evaluating serological and molecular methods of detection from environmental samples, identification of bioindicator plants, and testing of plant essential oils as alternative biofumigants of Rs. In host-range studies, 12 of the 14 tested ginger species wilted due to Rs. The pathogen survived for 90-180 days in the soil. Culturing on modified SMSA medium and testing with Immunostrips were the most sensitive methods for detection. Tissue-cultured ginger, "micro-sized" red ginger (Alpinia purpurata ) and "micro-sized" spiral ginger (A. purpurata) enhanced Rs populations 100--1000 fold and are effective bioindicators of Rs. Palmarosa (Cymbopogon martini), lemongrass ( C. citratus) and eucalyptus (Eucalyptus globulus) oils were investigated for their effects on Rs. Three levels; 0.04, 0.07 and 0.14% v/v of oils were evaluated by in vitro and pot studies. In vitro studies indicated that palmarosa and lemongrass oils had bactericidal and eucalyptus oil had bacteriostatic properties. Bacterial viability was analyzed by epifluorescence microscopy following staining with Syto9 and propidium iodide; 95-100% cell death were observed when treated with palmarosa and lemongrass oils at all concentrations and eucalyptus oil at 0.14% v/v. Rs was not detected in infested pots after treatment with palmarosa oil at all concentrations and lemongrass oil at 0.07% and above. None of the treatments reduced the growth or yield of ginger. Structural variations in Rs cells due to the treatments were observed with Scanning and Transmission Electron Microscopy; palmarosa and lemongrass oil treated cells were ruptured; but no cell rupture was noted in eucalyptus oil treated cells. The effects of the treatments on Rs cells were evaluated with micro-Raman spectroscopy. Lemongrass oil was the most effective treatment as indicated by significant reduction in levels of amino acids, nucleic acid bases, carbohydrates, amides and lipids. |
