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Improved Control of Green Mold of Citrus with Imazalil in Warm Water Compared with Its Use in Wax

The effectiveness of imazalil for the control of citrus green mold (caused by Penicillium digitatum) improved significantly when fruit were treated with heated aqueous solutions of the fungicide as compared with the current commercial practice of spraying wax containing imazalil on fruit. When applied at less than 500 μg·ml^-1 in solutions heated to 37.8°C, control of postharvest green mold of citrus was significantly superior to applications of 4,200 μg·ml^-1 imazalil in wax sprayed on fruit at ambient temperatures. The improvement in imazalil efficacy was obtained with a decrease in fungicide residues on the fruit. Residues of about 3.5 μg·g^-1 imazalil deposited by the application of imazalil in wax reduced the incidence of green mold on lemons from 94.4% among untreated controls to 15.1%, whereas an equal residue deposited by passing fruit through heated aqueous imazalil reduced green mold incidence to 1.3%. Similar differences were found in tests with oranges. Residues of 2 and 3.5 μg·g^-1 imazalil were needed to control the sporulation of P. digitatum on oranges and lemons, respectively. The mode of application of imazalil did not influence control of sporulation. The influence of immersion time, imazalil concentration, and solution temperature on imazalil residues on oranges and lemons was determined in tests using commercial packing equipment, and a model that describes residue deposition was developed. Residues after a 30- or 60-s treatment in heated aqueous imazalil were sufficient to control sporulation, but residues after 15-s treatments were too low and required an additional application of 1,070 μg·ml^-1 imazalil in wax to deposit an amount of imazalil sufficient to control sporulation. An imazalil-resistant isolate of P. digitatum was significantly controlled by heated aqueous imazalil. The incidence of green mold of navel oranges was reduced from 98.8 to 17.4% by treatment in 410 μg·ml^-1 imazalil at 40.6°C for 90 s. However, control of the resistant isolate required imazalil residues on the fruit of 7.9 μg·g^-1, which is within the U.S. tolerance of 10 μg·g^-1 but above the 5 μg·g^-1 tolerance of some countries that import citrus fruit from the United States.