Essential oils and thermal treatment in the postharvest control of green mold in orange

ABSTRACT Green mold is the primary postharvest disease of citrus fruits. Alternative products and processes for plant disease control have been increasingly required. The aim of this study was to evaluate the antifungal activity of essential oils on P. digitatum in oranges, alone or in combination with thermal treatment. Thus, an isolate of the fungus was subjected, in vitro, to different concentrations of oils of cinnamon, lemongrass and palmarose, in PDA culture medium; the antifungal activity of the volatile compounds of oils was also evaluated. Furthermore, a study was conducted to verify the effect of oils on inoculated oranges, in a protective and curative mode. Oranges were inoculated with 10 uL conidial suspension (105 conidia mL-1), two incubation periods (4 h before or 24 h after treatments). The treatments with essential oils that were sprayed were: control, cinnamon, lemongrass and palmarose, at 0.5 and 1.0 g L-1, plus Tween20. Another test with cinnamon oil, to determine the best dose, was conducted by using 0.0; 0.12; 0.25; 0.5 and 1.0 g L-1 and a blend of cinnamon (0.12 g L-1) and lemongrass (0.12 g L-1). Storage was at 25 °C and 80% relative humidity (RH) for up to 6 days. Subsequently, a test was conducted in oranges inoculated 4 h before treatments: control; cinamon (0.12 g L-1); thermotherapy (60 °C for 20 s); thermotherapy + cinnamon oil; and imazalil (1000 mg L-1). The fruits were stored at 10 °C/85% RH for 6 days plus 3 days at room conditions. In vitro, cinnamon oil was most fungitoxic for P. digitatum, totally inhibiting the mycelial growth index at concentrations higher than 0.5 g L-1 by contact and significantly reducing it by its volatile compounds. For inoculated oranges, cinnamon oil was more effective as curative and lemon grass as protective. In the screening test, the dose of 0.12 g L-1 cinnamon oil showed better result compared to higher doses and to the blend of cinnamon and lemon grass. The thermotherapy combination followed by spraying of cinnamon oil significantly reduced green mold development (AACPD) in fruits (40.5%) stored under refrigeration; however, the fungicide imazalil provided a more effective control (97%) during prolonged storage.