The effect of temperature, exposure time and pH on imazalil residue loading and green mould control on citrus through dip application

Imazalil (IMZ) is the most relied upon fungicide for use against citrus green mould, caused by Penicillium digitatum. In South Africa, the IMZ sulphate formulation is used as dip application in packhouses. Previous studies showed that IMZ efficacy and residue loading of this formulation is highly affected by pH in the application solution. This study investigated the effect of pH, exposure time and temperature on residue loading, green mould control and sporulation inhibition when citrus fruit were dipped in IMZ sulphate. Clementine mandarins, lemons and navel oranges were dipped in a 500mgL−1 solution of IMZ sulphate for 15, 45 or 90s, at a temperature of 23, 35 or 45°C, and at pH levels of 3 or 6. At pH 3, similar residues were loaded on fruit regardless of fruit type, temperature or exposure time (1.18–1.51mgkg−1). At pH 6, both temperature and exposure time influenced residue loading: at 23°C, higher residue levels were observed than at pH 3 (2.03–2.73mgkg−1, increasing with longer exposure time); at 35°C and 45°C, residue levels increased significantly with 15s, 30s and 45s exposure time (3.34, 5.11, 7.24mgkg−1 and 5.31, 9.65 and 13.10mgkg−1, respectively). Clementine mandarin fruit loaded higher residues at pH 6 (6.96mgkg−1) than lemons and navels (5.20 and 4.82mgkg−1, respectively). The MRL was frequently exceeded at longer exposure times. Green mould control was lower at pH 3 and was influenced by temperature and exposure time (92.9–97.4% and 42.8–45.1% for lemons and Clementine mandarins, respectively). Better control was observed at pH 6 regardless of temperature and exposure time (96.9–98.7% and 39.3–57.9% for lemons and Clementine mandarins, respectively). On navel oranges, increased exposure time led to increased green mould control (84.7–92.5%). Sporulation incidence was lower at pH 6 (12.5–28.8%, 19.1–39.2% and 4.6% for lemons, Clementine mandarins and navel oranges, respectively) than at pH 3 treatments (72.7–89.9%, 35.8–91.4% and 77.2% for lemons, Clementine mandarins and navel oranges, respectively). The results show that temperature, pH and exposure time are important parameters in dip application using IMZ sulphate formulation with significant effects on green mould control, more so than residue loading alone.