The protective role of Pseudomonas spp. PCI2 on post harvest deterioration of tomato caused by Alternaria alternata

Abstract

The high moisture content and water-soluble nutrients in tomato fruits make them perishable and susceptible to a number of fungal pathogens that cause postharvest rots. The Alternaria genus, widely distributed in nature, includes pathogenic species that can infect field crops or cause significant post-harvest damage, behaving as a facultative pathogen that is favored by stress, maturity, and senescence of the host. In the tomato fruit, the conidia of the fungus germinate and penetrate through wounds and the infection remains latent until maturity when tissues weaken. A. alternata infection is visualized as dark brown to black, smooth, slightly sunken, firm-textured lesions that can reach several centimeters in diameter. The application of fungicides is a common strategy used in an attempt to minimize post-harvest losses; however, this practice has caused environmental problems due to its residual toxicity, stimulated the appearance of strains resistant to active principles, and generated concern for human and animal safety. A sustainable alternative is the development of products based on biological control agents. Among them, bacteria of the Pseudomonas genus have been reported as efficient against various fungi. In this work, the effectiveness of Pseudomonas sp. PCI2 in suppressing diseases caused by A. alternata in tomato fruits was evaluated. To establish the antagonistic activity of the bacterial strain against the fungal strain, commercially ripe tomato fruits were superficially disinfected by immersion in a suspension of 2% sodium hypochlorite for 2 min, rinsed with sterile distilled water, and then they were dried by a stream of sterile air in a laminar flow chamber. Incision wounds were made on the fruits, 3 mm deep and 3 mm in diameter in the equatorial region. Immediately, 20 µL of an aqueous suspension of Pseudomonas sp. PCI2 were applied to each of the wounds, evaluating different concentrations (109 , 107 , and 105 CFU/mL). Three hours later, 15 µL of a suspension containing 104conidia/mL of A. alternata was applied to each wound. The fruits were kept in a chamber at 20ºC and 95% humidity for 7 days in plastic containers protected with plastic wrap. Inoculation with the Pseudomonas sp. PCI2 strain showed a significant decrease in the symptoms of the disease, with an average reduction of 50% in the area of the lesions, a result that was observed with all bacterial concentrations, suggesting the use of the lowest. Therefore, this microorganism can be considered a promising tool in the biological control of A. alternata and suitable for the design of an effective strategy for the conservation of fruits.