Effects of a Hot Water or Heated Thiabendazole Dip on Postharvest Degreening of Citrus Fruit

Objective: Postharvest degreening treatments are commonly used to improve citrus fruit peel color, especially early in the season. Nevertheless, peel color of degreened fruit is lighter than that of fruit with on-tree maturation. This practice also significantly enhances the incidence and severity of stem-end rot (SER) caused by Lasiodiplodia theobromae, an important postharvest disease of Florida citrus fruits. A postharvest fungicide drench before degreening is commonly used commercially to control this disease. To improve peel color of degreened citrus fruit and reduce the use of fungicides, this study evaluated the effects of a 2 min dip in heated water with or without thiabendazole (TBZ) on color changes, internal qualities, and SER development of citrus fruit. Methods: In 2015, ‘Fallglo' tangerines were harvested on 12 October, navel oranges on 12 October, and grapefruit on 5 November and 3 December, all from groves near Fort Pierce, Florida. After harvest, fruits were immediately transferred to the packinghouse and received the following, 2-min dip treatments: un-dipped (dry control), water at 20◦C (water control), TBZ (1000 mg.kg^-1) at 20◦C, hot water alone at 52◦C, TBZ (1000 mg.kg^-1) at 52◦C, and TBZ (100 mg.kg^-1) at 52◦C. All fruit were then degreened using 5 µL L⁻¹ of ethylene at 29◦C with 90-95% RH for 8 h (‘Fallglo') or 48 h (navel and grapefruit), and then left for an additional 24 hr “airing” time (without ethylene). The fruit were stored under ambient room conditions. Results: Hot water treatments (with or without TBZ) accelerated peel color changes recorded as hue (H◦), a* and a*/b* during storage, with the color turning deeper yellow earlier than fruit treated at 20◦C. Previous research has shown that an additional 24 hr “airing” time promotes further color change in ‘Fallglo', but little additional color change occurs in navel or grapefruit, which was also observed in the current experiments. Grapefruit harvested in December demonstrated a greater response to the hot water treatments than fruit harvested in November. As expected, ethylene degreening accelerated the outbreak of SER of navel oranges and red grapefruit during storage, but 1000 mg.kg^-1 TBZ significantly controlled this disease, with hot water alone or hot water with a lower concentration of TBZ (100 mg.kg^-1) providing similar control effect. Neither TBZ nor hot water treatments had significant effects on fruit quality. Conclusion: The above hot water dip and fungicide treatments show promise for improving color development while reducing postharvest decay under Florida conditions.