Submitted to: Plant Disease Management Reports
Publication Type: Research notes
Publication Acceptance Date: 3/1/2012
Publication Date: N/A
Citation: Interpretive Summary: Citrus fruit will rot after harvest and most common solution to control rot under commercial conditions is to apply fungicides, and these can make the fruit more difficult to export or market domestically to some buyers. We applied a variety of treatments that avoided fungicides and did not leave chemical residues in the fruit and found some of these approaches were effective enough to merit consideration for commercial use by growers. This research provided insights into new approaches to increase the storage life of fresh citrus fruit use that could be adopted as a practical management tool for citrus growers.
Technical Abstract: Significant losses can occur after the harvest during the storage and marketing of citrus fruit in California due to green and blue molds, caused by P. digitatum and P. italicum, respectively. Currently, both diseases are controlled by application of the fungicides imazalil, sodium ortho-phenyl phenate, pyrimethanil, or thiabendazole. Alternative methods are needed because the widespread use of these chemicals in packinghouses has led to the proliferation of resistant strains of the pathogens, fungicide residues comprise marketing issues for some buyers, and the discharge of used solutions from packinghouses has come under increasing regulatory scrutiny. The aim of this test was to determine the efficacy of several generic compounds to control green mold and blue mold. Most are either exempt from or would have minimal residue and other regulatory requirements before they could be used. Freshly harvested, light green lemons cv. Eureka were randomized and then inoculated 24 hours before treatment by dipping a stainless steel rod with a 1 mm wide by 2 mm in length tip into a suspension containing 106 conidia/ml of each pathogen and making a single puncture near the blossom end (blue mold) or stem end (green mold) on each fruit. P. digitatum and P. italicum isolates were cultured for two weeks at 25C on potato dextrose agar. The temperature of the fruit at the time of inoculation and subsequent storage was 20C. Five replicates of 20 fruit each were prepared for each treatment. The fruit were immersed for 60 seconds in a 2% (vol/vol) solution of H2O2 (30% a.i., hydrogen peroxide, Brenntag Pacific, Inc., Fresno, CA) containing 6 mM copper sulfate (Sigma-Aldrich Chemical Co., St, Louis, MO), either alone or followed by a second treatment of 60 seconds immersion in (2% a.i., wt/vol) solutions of potassium phosphite (54% a.i., Helena-Prophyt, Helena Chemical Co., Collierville, TN), potassium sorbate (99% a.i., Fruit Growers Supply, Exeter, CA), or sodium bicarbonate (99% a.i., Dwight and Church, Inc., Princeton, NJ).The fruit were not rinsed after treatment and the solution volume of each treatment was 20 liters. The temperature of the solutions was 25C or 50C (±1C). For comparison purposes, fruit were also immersed for 15 seconds in a 50C solution of 200 mg/L imazalil (Deccozil, 22.5% a.i., DECCO US Post-Harvest, Inc., Monrovia, CA), which simulates a common commercial packinghouse treatment. After treatment, the fruit were stored at 20C and the number of infections counted after 7 days. Both blue mold and green mold were controlled effectively by the combination of treatments of hydrogen peroxide followed by potassium phosphite, even when both of the solutions were 25C. Applied alone, potassium sorbate and potassium phosphite were superior to hydrogen peroxide or sodium bicarbonate. Heating them improved their effectiveness. Hydrogen peroxide treatment followed by a second treatment with potassium sorbate or sodium bicarbonate was moderately effective at 25C and improved markedly at 50C. Imazalil completely controlled both diseases. Hydrogen peroxide injured the rind of some fruit when used at 50C, but not 25C. The sequence of hydrogen peroxide followed by potassium phosphite was the most promising treatment, primarily because the control of both diseases occurred without heating the solutions, which entails considerable expense and increases risk of injury to the fruit, they do not deposit residues in the fruit of concern, and disposal of used solutions containing these compounds in relatively easy to do, except copper sulfate would may be an issue in some locations and require approved disposal or reclamation.