Antibacterial activity of essential oils on Xanthomonas vesicatoria and control of bacterial spot in tomato

The objective of this work was to evaluate the effects of plant essential oils (EOs) on the growth of Xanthomonas vesicatoria, on bacterial morphology and ultrastructure, and on the severity of tomato bacterial spot. EOs from citronella, clove, cinnamon, lemongrass, eucalyptus, thyme, and tea tree were evaluated in vitro at concentrations of 0.1, 1.0, 10, and 100% in 1.0% powdered milk. The effect of EOs, at 0.1%, on the severity of tomato bacterial spot was evaluated in tomato seedlings under greenhouse conditions. The effects of citronella, lemongrass, clove, and tea tree EOs, at 0.1%, on X. vesicatoria cells were evaluated by transmission electron microscopy. All EOs showed direct toxic effect on the bacteria at a 10%-concentration in vitro. Under greenhouse conditions, the EOs of clove, citronella, tea tree, and lemongrass reduced disease severity. EOs of clove and tea tree, and streptomycin sulfate promoted loss of electron-dense material and alterations in the cytoplasm, whereas EO of tea tree promoted cytoplasm vacuolation, and those of citronella, lemongrass, clove, and tea tree caused damage to the bacterial cell wall. The EOs at a concentration of 0.1% reduce the severity of the disease. Resumo - O objetivo deste trabalho foi avaliar o efeito de oleos essenciais (OEs) no crescimento de Xanthomonas vesicatoria, na morfologia e na ultraestrutura bacteriana, e na severidade da mancha-bacteriana do tomateiro. OEs de citronela, cravo-da-india, canela, capim-limao, eucalipto, tomilho e arvore-de-cha foram avaliados in vitro nas concentracoes de 0,1, 1,0, 10 e 100% em leite em po a 1,0%. O efeito dos OEs, a 0,1%, na severidade da mancha-bacteriana do tomateiro foram avaliados em plantas de tomateiro em casa de vegetacao. Os efeitos dos oleos de citronela, capim-limao, cravo-da-india e arvore-de-cha, a 0,1%, nas celulas de X. vesicatoria foram avaliados por meio de microscopia eletronica de transmissao. Todos os OEs apresentaram efeito toxico direto sobre as bacterias na concentracao de 10% in vitro. Em casa de vegetacao, os OEs de cravo-da-india, citronela, arvore-de-cha e capim-limao reduziram a severidade da doenca. Os OEs de cravo-da-india e arvore-de-cha, e o sulfato de estreptomicina promoveram perda de material eletro-denso e alteracoes no citoplasma, enquanto o OE de arvore-de-cha promoveu vacuolizacao do citoplasma, e os de citronela, capim-limao, cravo-da-india e arvore-de-cha causaram danos a parede celular bacteriana. Os OEs, na concentracao de 0,1%, reduzem a severidade da doenca. Termos para indexacao: controle alternativo de doencas de plantas, ultraestrutura celular, bacteria fitopatogenica.