Residue levels, persistence and effectiveness of imazalil against a resistant strain of penicillium digitatum when applied in combination with heat and sodium bicarbonate.

Green and blue molds, respectively caused by Penicillium digitatum Sacc., and P. italicum Wehmer, are the most important postharvest diseases of citrus fruit Postharvest management of these pathogens is mainly based on the application of thiabendazole (TBZ) or imazalil (IMZ) fungicides. However, their intensive and prolonged use has led to the selection of TBZ- IMZ-resistant strains of these pathogens and to a reduction of TBZ and IMZ effectiveness to control postharvest decay. However, while TBZ may become completely ineffective against TBZ-resistant strains of P. digitatum, reduction of IMZ efficacy is only partial, and an effective control of decay can still be achieved by increasing its concentration, heating the treatment-solution and/or combining IMZ with sodium bicarbonate (SBC) or other food additives or natural salts. In this study, 'Desiderio' and 'Nova' mandarins were inoculated with spores of a sensitive strain of P. digitatum to IMZ and TBZ (PDs) or with a strain of P. digitatum with double resistance to both fungicides (PDr) and immersed in IMZ or TBZ emulsions at increasing concentrations up to 1000 mg/L or in IMZ (25, 200 or 400 mg/L), SBC (0.5, 1 or 2%) or IMZ + SBC emulsions either at 20 or 40 degrees C. IMZ was superior to TBZ to control decay of 'Desiderio' mandarins incited by PDs and was also effective to control decay in fruit inoculated with PDr, while TBZ even at the highest rate was completely ineffective. In 'Desiderio' mandarins inoculated with PDs, a complete control of decay was achieved with 25 mg/L IMZ but in fruit inoculated with PDr, 25 mg/L IMZ were ineffective to control decay despite in combination with SBC at 2% a synergistic effect was detected. In contrast, a good control of decay was achieved with 400 mg/L IMZ. In 'Nova' mandarins after 1 week of incubation at 20 degrees C decay incidence in fruit dipped in 400 mg/L at 20 degrees C or 200 mg/L IMZ at 40 degrees C was almost completely inhibited, while the addition of SBC at 0.5, 1 or 2% did not improve treatments performance in fruit inoculated with PDs. However, when 'Nova' mandarins were inoculated with PDr, SBC showed a modest but significant control of decay and in combination with IMZ either at 400 mg/L and 20 degrees C or 200 mg/L and 40 degrees C, significantly improved decay control. SBC did not affect IMZ residue load in 'Valencia' oranges, whereas dipping the fruit in 400 mg/L IMZ at 20 degrees C produced similar IMZ residue load as dips at 200 mg/L IMZ at 40 degrees C. In all cases, residue levels of IMZ never exceeded 2 mg/kg, which is about 40% of the maximum residue limits (MRLs) allowed in European countries. Thus, despite the selection of IMZ-resistant strains of P. digitatum, IMZ continues to be highly effective to control green mold of citrus fruit at concentrations leaving on fruit surface residue levels below the MRLs.