Abstract
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.
Highlights
Diseases of tomato (Solanum lycopersicum L.) are responsible for significant yield losses
The objective of this work was to evaluate the effect of plant essential oils on the growth of X. vesicatoria, on bacterial morphology and ultrastructure, and on the severity of tomato bacterial spot
None of the essential oils inhibited the growth of X. vesicatoria in vitro, at the concentration of 0.1% (Table 1)
Summary
Diseases of tomato (Solanum lycopersicum L.) are responsible for significant yield losses. Bacterial spot, caused by a number of species of bacteria of the genus Xanthomonas (Dowson) (Jones et al, 2000), is among the most important of these diseases. Several studies have shown that it is possible to use essential oils to control plant diseases (Pereira et al, 2011). Medicinal plants contain substances that are able to play an important role in plant‐pathogen interaction, by activating plant defense mechanisms (Schwan‐Estrada et al, 2003; Lucas et al, 2012). Antifungal substances, similar to fungicides, may be present in those plants, acting directly on the pathogens (Rosato et al, 2007). Medice et al (2007) observed that essential oils from eucalyptus [Corymbia citriodora (Hook.) K.D.Hill & L.A.S. Johnson], thyme (Thymus vulgaris L.), neem Juss.), and citronella [Cymbopogon nardus (L.) Rendle.] completely inhibited the germination of urediniospores of Asian soybean rust
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