Antimicrobial activities of plant essential oil vapours against Acidovorax citrulli and Xanthomonas campestris on Cucurbitaceae, Brassicaceae and Solanaceae seeds

AIM

This study was done to develop a seed decontamination treatment for organic seeds against plant pathogens (Acidovorax citrulli and Xanthomonas campestris) using essential oil (EO) vapours without affecting the seeds' germination rate.

METHODS AND RESULTS

By using a diffusion assay and determining minimum inhibitory and lethal concentrations, we screened two EO vapours which were most inhibitory to A. citrulli (cinnamon bark and garlic EO vapours) and X. campestris (onion and garlic EO vapours). After 48 h of exposure to EO vapours at 25°C and 43% or 85% relative humidity (RH), no significant decrease (p > 0.05) in germination rates was observed compared with those of control seeds. It was observed that EO vapour treatment at 25°C and 43% or 85% RH for 48 h caused significant population reductions (p ≤ 0.05) (ca. 0.3-2.6 log colony forming unit/g) compared to those of untreated seeds.

CONCLUSION

Applications of EO vapours showed significant (p ≤ 0.05) antimicrobial effects against A. citrulli and X. campestris on both laboratory mediums and plant seeds without decreasing the germination rate of seeds.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides useful information for the development of natural seed sterilization treatments using EO vapours.

Development of a decontamination method to inactivate Acidovorax citrulli on Cucurbitaceae seeds without loss of seed viability

BACKGROUND

Acidovorax citrulli is a plant pathogen causing bacterial fruit blotch in Cucurbitaceae family. Applying high concentration of disinfectants to seeds containing plant pathogen may substantially decrease the germination rate of seeds. Therefore, it is necessary to develop a hurdle technology which can inactivate plant pathogens without decreasing seed viability. This study was conducted to develop a decontamination method to inactivate the plant pathogen Acidovorax citrulli on Cucurbitaceae seeds by sequential treatments with aqueous chlorine dioxide (ClO₂ ), drying, and dry heat.

RESULTS

The maximum ClO₂ concentration that did not lower germination rates of cucumber, honeydew melon, and watermelon seeds was ca. 100 μg mL^-1 of ClO₂ for 5 min. Optimal incubation conditions for drying seeds that had been treated with aqueous ClO₂ were determined as 25 °C and 43% relative humidity (RH) for 48 h. The maximum dry-heat temperature that did not reduce germination rates of seeds, which had been treated with ClO₂ and dried at 25 °C, was 60 °C at 43% RH for 24 h. When seeds containing A. citrulli (6.4-7.0 log CFU g^-1 ) were treated with aqueous ClO₂ (50 μg mL^-1 , 5 min), dried (25 °C, 43% RH, 24 h), and dry heated (60 °C, 43% RH, 24 h), the pathogen was inactivated to below the detection limit from all three seed types (<-0.5 log CFU g^-1 ).

CONCLUSION

The decontamination conditions to inactivate A. citrullii from Cucurbitaceae seeds without decreasing the seed viability were determined (sequential treatment with ClO₂ [50 μg mL^-1 , 5 min], dried [25 °C, 43% RH, 24 h], and dry heated [60 °C, 43% RH, 24 h]). The results of this study may also be applicable to other plant pathogens on other types of seeds. © 2019 Society of Chemical Industry.

Characterization of Efficiency and Mechanisms of Cold Atmospheric Pressure Plasma Decontamination of Seeds for Sprout Production

The consumption of fresh fruit and vegetable products has strongly increased during the past few decades. However, inherent to all minimally processed products is the short shelf life, and the risk of foodborne diseases, which have been increasingly related to such products in many parts of the world. Because of the favorable conditions for the growth of bacteria during the germination of seeds, sprouts are a frequent source for pathogenic bacteria, thus highlighting the need for seed decontamination to reduce the risk of foodborne illness. Consequently, this study focused on cold atmospheric pressure plasma (CAPP) treatment of artificially inoculated seeds in a diffuse coplanar surface barrier discharge to determine the inactivation efficiency for relevant foodborne pathogens and fungal spores. Plasma treatment of seeds resulted in a highly efficient reduction of microorganisms on the seed surface, while preserving the germination properties of seeds, at least for moderate treatment times. To characterize the mechanisms that contribute to microbial inactivation during plasma treatment, an experimental setup was developed to separate ultraviolet light (UV) and other plasma components. The combination of bacterial viability staining with confocal laser scanning microscopy was used to investigate the impact of ozone and other reactive species on the bacterial cells in comparison to UV. Further characterization of the effect of CAPP on bacterial cells by atomic force microscopy imaging of the same Escherichia coli cells before and after treatment revealed an increase in the surface roughness of treated E. coli cells and a decrease in the average height of the cells, which suggests physical damage to the cell envelope. In conclusion, CAPP shows potential for use as a decontamination technology in the production process of sprouts, which may contribute to food safety and prolonged shelf life of the product.