Management of Postharvest Decay of Tomato Fruit with Inorganic Salts and Natural Products.

Ahmed, Firas A.
Tropical Plant Pathology
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Gray mold caused by Botrytis cinerea and soft rot caused by Pectobacterium carotovorum are two of the most devastating pre- and postharvest diseases of tomato worldwide. A survey of fresh-market tomato fruit was conducted to determine the fungal and bacterial pathogens most commonly associated with postharvest disease on Oahu, Hawaii. The survey confirmed that gray mold and soft rot were the most common postharvest diseases of tomato in Hawaii. Pathogenicity tests revealed that 33 of 99 fungal isolates and 10 of 17 bacterial isolates were pathogenic on tomato varieties known as common market, cherry, and grape tomato. Based on fruit assays one fungal isolate, Botrytis cinerea (B03) and one bacterial strain, Pectobacterium carotovorum (BA17), were selected because both produced consistent symptoms and were highly virulent. As consumer-acceptable options for postharvest disease management are inadequate for gray mold and soft rot, experiments were designed to evaluate possible control measures for these two diseases. Effects of eleven natural products on spore germination and mycelial growth of Botrytis cinerea (B03) were evaluated using multi-well microplates and inhibitory assays, respectively. Capsicum chinense cv. Datil, C.annuum cv. Carnival, and an Agrichem Proprietary Formulation (APF) completely inhibited fungal germination at all evaluation times. Treatments with 40% plant extracts increased the generation of intercellular reactive oxygen species and the plasma membranes of fungal conidia were damaged. Fungal spores exposed to 40% plant extracts of C. chinense or C.annuum or 1 ml/L APF showed distinct signs of deterioration, deformation and condensed cytoplasm as observed using transmission electron microscopy. Capsicum chinense and C.annuum applications at 60% completely inhibited lesion development on tomato fruit following immersion for 10 min. An inorganic salt, potassium tetraborate tetrahydrate (B4K2O7.4H2O) (PTB), was evaluated for effects on the growth of P. carotovorum using strain BA17. Complete inhibition of bacterial colony development was achieved by treatment with PTB at 100 mM both at pH 9.2 and after adjustment to pH 7.0. Bactericidal activity was quantified and validated by counting fluorescently-labeled live and dead bacterial cells using flow cytometry, and reconfirmed using qPCR with high-affinity photoreactive DNA binding dye propidium monoazide (PMA). The results of flow cytometry, qPCR, and culturing confirmed that bacterial cells were killed following exposure to PTB at 100 mM. Bacterial cell membranes were damaged following a 5-min treatment and extrusion of cytoplasmic material from bacterial cells was observed using electronic transmission microscopy. Soft rot incidence on inoculated tomato fruit was reduced by dipping infected fruit in PTB at 100 mM for 5 min and no lesions developed following a 10-min treatment. PTB and APF were assessed as a preharvest spray in the greenhouse for reducing gray mold of tomato fruit caused Botrytis cinerea. PTB and APF were applied at three ripening stages: turning, light red, and pink. Both compounds reduced the disease severity of gray mold in greenhouse tomatoes when applied at the turning stage when fruit were stored either at 4 or 25˚C. In this study, recombinase polymerase amplification (RPA) was used for specific and rapid detection of Pectobacterium using lateral flow strips. The assay also included tissue from host plants (tomato and potato) as an internal control to enhance the reliability and accuracy of the assay. The assay readily distinguished Pectobacterium from other bacterial genera and proved to be an efficient assay for identification of the target pathogen in plant tissues. In summary, APF and PTB show promise for reducing gray mold as preharvest applications in the greenhouse tomato. In contrast, PTB is an effective alternative to other bactericides and antibiotics for controlling soft rot disease of tomato as a postharvest practical application.
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