Óleos essenciais para controle da podridão mole em couve-chinesa

Abstract

Chinese cabbage (Brassica pekinensis L.) is very important in the Brazilian horticulture due to its nutritional value and increasing yielding. In the Zona da Mata and Agreste of Pernambuco State, Brazil, this vegetable is intensely cultivated in the conventional system although its production is limited by several factors including the soft rot disease. It was evaluated the action of essential oils in controlling soft rot in Chinese cabbage and their influence in colorimetry and physicochemical characteristics of this vegetable. Preliminary fitotoxicity tests selected 11 oils. In the greenhouse, plants of cv. Natsume were sprayed with the oils of bergamot, lemon grass, copaiba, eucalyptus citriodora, eucalyptus globulus, fennel, ginger, mint, sweet orange, lemon and clary sage (0.5%) and the antibiotic Mycoshield® (3 g L-1), and inoculated with P. carotovorum subsp. carotovorum (Pcc-c) after 72 h. The disease severity was evaluated every six hours until 48 h, and the final severity (SEV) and area under the disease progress curve (AUDPC) were determined. The 11 oils and Mycoshield® reduced similarly SEV and AUDPreduced SEV in 53.1 and 38.8% and AUDPC in 37.0 and 27.5%, respectively. The oils of bergamot, copaiba, eucalyptus citriodora, mint and sweet orange were selected for further studies and tested for stability of the effectiveness of disease control in relation to three strains of Pcc. The interaction oils x strains was not significant (P≤0.05). The five oils reduced the SEV and AUDPC without differences between themselves or from Mycoshield®, except for copaiba oil that was less effective than the antibiotic in reducing the AUDPC. In the in vitro tests, filter paper discs were soaked in those five oils (0.5%) and Mycoshield® and deposited on culture medium containing Pcc-c. The pathogen was inhibited in vitro only by Mycoshield®. The technique of overlapping plates was used for detection of antibacterial activity of volatile substances at different pHs. Pcc-c growth was inhibited only by the oils of mint and bergamot at pH 7.0, and by the oil of copaiba at pH 8.0. The colorimetry of Chinese cabbage leaves, the ascorbic acid content and the pH of the plants treated with those five oils have not changed compared to the control without Pcc-c. The acidity was elevated by mint oil and total soluble solids (˚Brix) by the oils of sweet orange, eucalyptus citriodora and bergamot.