Efficacy of essential oils in the management of soft rot caused by Pectobacterium aroidearum in lettuce

Lettuce is susceptible to several diseases, especially soft rot caused by bacteria of the genus Pectobacterium. Due to the adaptability of this pathogen and the lack of disease control registered for the crop, the objective of this work was to evaluate the effects of essential oils in the management of soft rot caused by P. aroidearum in lettuce. The study was developed at the Universidade do Estado da Bahia, Juazeiro, BA, Brazil, and the essential oils (EOs) of orange, bergamot, lemongrass, palmarosa, citronella, cloves, tea tree, rosemary, sage, and ginger were used in concentrations of 0.25; 0.5; 0.75 and 1.0% to assess the in vitro growth inhibition of the bacterium. Subsequently, the curative effects of the disease were evaluated by applying the EOs that obtained the best results in vitro in lettuce plants of the susceptible variety "Mônica". The treatments were applied, via spraying, 12 hours after inoculation using the bite method with bacterial suspension. The best in vivo treatment was selected to assess its preventive and curative activity, as well as to find the ideal concentration for reducing epidemiological variables and chromatographic characterization. The EOs of palmarosa, sage, citronella, lemongrass, and cloves (0.25%), and that of sage (0.75%), inhibited bacterial growth in vitro. The EO of salvia showed the best results in vivo, inhibiting the growth of the disease in concentrations of 0.50 and 0.75%, so it was selected for the preventive and curative control tests alone. The preventive treatment was not efficient for the management of soft rot in lettuce, however, from the regression analysis, a concentration of 0.64% of the salvia EO was found as a potential for curative control of this bacteriosis, as it reduces the incidence and severity of the disease. Linalyl acetate and linalool were found in higher concentrations in the chromatographic analysis. These components, probably, conferred the bactericidal capacity of the EO of sage, being potential for the use in the control of P. aroidearum in lettuce.

Essential oils in the management of soft rot of kale in the brazilian semiarid region / Óleos essenciais no manejo da podridão mole em couve no semiárido brasileiro

The aim of this study was to analyze the effect of essential oils on the control of soft rot of kale. Clove essential oil at 0.25%, lemongrass and palmarosa essential oils at 0.5%, melaleuca and orange essential oils at 0.75%, bergamot, rosemary, sage and ginger essential oils at 1% were evaluated for the in vitro inhibition of Pectobacterium carotovorum subsp. brasiliensis (Pcb) and control of soft rot of kale, sprayed 72 hours before or seven hours after inoculation. Clove, citronella, bergamot, rosemary, palmarosa, sage, melaleuca, and lemongrass oils completely inhibited the growth of Pcb. Lemongrass oil (0.5%) caused 0% of disease incidence (INC), providing 100% of disease control in both periods of inoculation. Clove oil (0.25%) showed a lower INC (25%) when applied after inoculation, providing a control percentage of 71.42%. The lemongrass and clove essential oils were analyzed by GC/FID (Gas Chromatography ­ Flame Ionization Detector) and by GC/MS (Gas Chromatography /Mass Spectrometer). The major components were eugenol (91,9%) for clove oil and citral, isometric mixture of neral (34,1%) and geranial (42,9%) for lemongrass oil. The Minimum inhibitory concentration (MIC) of lemongrass, clove oils and their major components (citral and eugenol, respectively) was determined by using a broth macrodilution technique, as well as they were evaluated at different concentrations on the control of soft rot of kale, sprayed according descriptions above. The MIC was 0.03125% for citral, and 0.0625 and 0.125% for lemongrass and clove oils, respectively. Eugenol didn't show MIC. Lemongrass oil at 0.125% (post-inoculation) and citral at 0.125% (pre and post-inoculation) provided the highest percentages of disease control (33.33, 50, and 100%, respectively). Clove oil at 0.125% (post-inoculation) showed better effectiveness than eugenol (0.25%), providing a percentage of disease control of 16.67%. Lemongrass and clove essential oils were the most effective in control of soft rot of kale, suggesting that these oils have a potential to be used as antibacterial agents.

O objetivo do estudo foi avaliar o efeito de óleos essenciais no controle da podridão mole em couve. Os óleos essenciais de cravo a 0,25%, capim-limão e palmarosa a 0,5%, citronela, melaleuca e laranja a 0,75%, bergamota, alecrim, sálvia e gengibre a 1% foram avaliados na inibição in vitro de Pectobacterium carotovorum subsp. brasiliensis (Pcb) e controle da podridão mole em couve, pulverizados 72 horas antes ou sete horas após a inoculação. Os óleos essenciais de cravo, citronela, bergamota, alecrim, palmarosa, sálvia, melaleuca e capim-limão inibiram completamente o crescimento de Pcb. O óleo de capimlimão (0,5%) promoveu 0% de incidência (INC) da doença (percentual de controle de 100%), em ambos os períodos de inoculação. O óleo de cravo (0,25%) proporcionou menor INC (25%) quando aplicado após inoculação (percentual de controle de 71,42%). Os óleos essenciais de capim-limão e cravo foram analisados por GC/FID (cromatografia gasosa/detector por ionização de chama) e por GC/MS (cromatografia gasosa/ espectometria de massas). Os componentes majoritários foram eugenol (91,9%) no óleo de cravo e citral (neral34,1% e geranial- 42,9%) no óleo de capim-limão. A concentração inibitória mínima (CIM) dos óleos essenciais de capim-limão e cravo e de seus componentes majoritários (citral e eugenol, respectivamente) foi determinada por meio da técnica de macrodiluição em caldo, bem como foram avaliados, em diferentes concentrações, no controle da podridão mole em couve, pulverizados conforme descrito acima. A concentração inibitória mínima (CIM) foi de 0,03125% para o citral, e de 0,0625 e 0,125% para os óleos de capim-limão e cravo, respectivamente. O eugenol não apresentou CIM. O óleo de capim-limão a 0,125% (pós-inoculação) e o citral (0,125%), em ambos os períodos de inoculação, proporcionaram os maiores percentuais de controle (33,33; 50 e 100%, respectivamente). O óleo de cravo a 0,125% (pós-inoculação) mostrou maior eficiência que o eugenol (0,25%), promovendo um percentual de controle de 16,67%. Os óleos essenciais de capim-limão e cravo destacaram-se na eficiência de controle da podridão mole em couve, sugerindo que esses óleos têm potencial para serem utilizados como agentes antibacterianos.