Significant losses can occur after the harvest during storage and marketing of citrus fruit due to green mould, caused by Penicillium digitatum, blue mould, caused by Penicillium italicum, and sour rot, caused by Geotrichum citri-aurantii. In California, applications of sodium bicarbonate, imazalil (IMZ), thiabendazole (TBZ), pyrethanil (PYR), fludioxonil (FLUD), and sodium ο-phenylphenate are used to control green and blue moulds of citrus.
Sodium bicarbonate partially controls green mould, blue mould, and sour rot of citrus fruit, however its addition to IMZ, TBZ, and PYR improves their performance. The sodium bicarbonate addition is useful to reduce rates of fungicides used, furthermore it can be used combined with other treatments, such as biological control or hot water, to improve their performance.
However, disposal of sodium bicarbonate raises regulatory issues in some locations, because the solution has a high electrical conductivity and pH, and it contains sodium, which make disposal of the used solution difficult. For this reason, it should be interesting to evaluate compounds that could improve the fungicides performance, as sodium bicarbonate does, and whose disposal should be environment-friendly.
Sorbic acid and its water-soluble salts, especially potassium sorbate (KS) are common food preservatives. Sorbates have a wide spectrum of antimicrobial action, however their efficacy depends on species and strains differences, extent of contamination, type and composition of substrate, concentration and pH of sorbate, presence of other additives, temperature of storage, and type of packaging.
Studies on sorbates have demonstrated that low sorbate levels can synergistically act with heat to inactivate yeasts, moulds, and bacteria, and can improve the efficacy of some fungicides, such as IMZ and TBZ. Potassium sorbate does not contain sodium, has a lower pH than sodium bicarbonate, and it is effective as a fungicide additive at only 0.5%.
The objectives of the study on citrus fruit conducted by Smilanick et al. (2008) were:
- To determine the effectiveness of KS alone to control citrus green mould and sour rot. Solutions contained 0, 0.5, 2.0, 3.0% KS at native pH.
- To evaluate the KS influence on the performance of the citrus postharvest fungicides IMZ, TBZ, PYR, and FLUD.
- To determine if the KS (concentration was fixed at 0.5%) addition to the fungicides solution would affect residue levels of these fungicides in treated fruit.
- To quantify the effect of heating on the efficacy of the treatments.
- To evaluate the influence of the interval between inoculation and treatments application on the performance of these fungicides when mixed with KS.
KS at concentrations of 0.05 to 2.0% inhibited the germination of conidia of P.digitatum, and its inhibitory activity was higher at lower pH. The inhibitory activity of KS was similar from pH 4 to 6, and it was about 3-and 10-fold less toxic at pH 7 and 8, respectively. Results showed that a 2% aqueous solution of KS, applied as a dip, effectively reduced postharvest decay of several citrus fruit. KS alone was more effective to control green mould and sour rot when it was heated to 50°C. When KS was added to fungicides solutions, it significantly increase their effectiveness without increasing the residues of any of the fungicides in fruits.
KS particularly improved the IMZ effectiveness. The benefit of combing TBZ with KS resulted more modest than that obtained with IMZ. However, the effectiveness of both fungicides was enhanced by heating the solution. Combing PYR or FLUD with KS significantly improved their performance to control green mould, in this case the influence of heat on their performance resulted modest. Often, the increase in effectiveness caused by KS was most apparent when the solutions were heated. The residues and effectiveness of IMZ, TBZ, PYR, and FLUD are enhanced by heating. Thus, fungicides rates applied with heat should be reduced compared to the rates used at ambient temperature to avoid excessive fungicide residues in the fruit.
In general, when treatments are opportunely applied, the control of these postharvest diseases improves. The control of green mold was better when the treatments were applied 12 h after inoculation than when they were applied 36 h after inoculation.
Sorbates residues in citrus fruit have not been studied. Sorbates leave a persistent antimicrobial residue on the fruit that sodium bicarbonate does not. In addition, the sorbate residue could reduce the risk that the fruit carry human pathogens of food safety concern or plant pathogens of quarantine concern. However, KS residues may be an issue to some buyers. In USA, KS has been recently listed as a “minimum risk pesticide” by the US EPA, furthermore the USDA National organic Program does not classify sorbate as an allowed substance while allows sodium bicarbonate to be used.
Concluding, the present work has showed that potassium sorbate is a valid alternative to sodium bicarbonate. KS is similar to sodium bicarbonate in effectiveness to control green mold or sour rot, can improve the performance of fungicides, and has fewer environmental issues associated with its disposal.
Original study. Smilanick JL, Mansour MF, Mlikota Gabler F, Sorenson D. "Control of citrus postharvest green mould and sour rot by potassium sorbate combined with heat and fungicides", 2008, Postharvest Biology and Technology, page. 226-238. For more details: http://ddr.nal.usda.gov/bitstream/10113/17905/1/IND44020583.pdf