Submitted to: Postharvest Biology and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/25/2007
Publication Date: 8/21/2007
Citation: Smilanick, J.L., Mansour, M., Mlikota, G.F., Sorenson, D. 2007. Control of citrus postharvest green mold and sour rot by potassium sorbate combined with heat and fungicides. Postharvest Biology and Technology.
Interpretive Summary: Decay of citrus fruit during storage and marketing is a serious problem for citrus fruit producers worldwide, and its ususally controlled by applications of fungicides. Fungicide residues are avoided by some consumers, they can be costly, and fungi have developed resistance to some of them. In our work, we found a common food preservative, potassium sorbate, could replace them although it was somewhat less effective, or it could be added to the fungicides to improve their performance. Using sorbate reduced fungicide residues, because adequate effectiveness to stop fruit decay could be accomplished with very low rates of the fungicides if they were combined with potassium sorbate. Potassium sorbate could control fungicide-resistant isolates of the rot fungi. Potassium sorbate is commonly added to soft drinks, snack foods, and bakery products. Although not approved for use on certified organic products, it is excempt from residue tolerances.
Technical Abstract: Potassium sorbate (KS), a common food preservative, was evaluated to control postharvest decay of citrus fruit. Significant advantages of KS over the commonly used sodium bicarbonate, which similarly improved fungicide performance, are the relatively low salt concentration of KS, the absence of sodium, its lower pH, so disposal of used KS solutions would raise fewer regulatory issues. The influence of KS concentration and pH (pH 4 to 9) on the germination of spores of Penicillium digitatum was determined alone or in combination with four postharvest citrus fungicides, imazalil (IMZ), thiabendazole (TBZ), pyrimethanil, and fludioxonil. The EC95 concentrations of KS to inhibit spore germination were lowest from pH 4 to 6. To control green mold on inoculated fruit, KS was compatible with these fungicides and consistently improved their performance. KS alone or fungicide-KS solutions were more effective when heated. The combination of KS and sodium bicarbonate was only moderately better than either alone. Green mold caused by an isolate of P. digitatum resistant to IMZ and TBZ was effectively controlled when KS was added to a heated IMZ or TBZ solutions. Heat, but not KS, increased residues of all of the fungicides in oranges. Sour rot, caused by Geotrichum citri-aurantii, was reduced from 94.5% among control lemons, to 49.1 and 47.2%, respectively, by 30 s immersion in KS or sodium bicarbonate at 1% wt/vol at 25C, and to 37.0 and 15.7%, respectively, when these solutions were at 50C.