1USDA-ARS, Fresno, CA, USA; 2Sunkist Growers, Lindsay, CA, USA

Background and objectives
Green mould and sour rot of citrus, caused by Penicillium digitatum and Geotrichum citri-aurantii, respectively, are economically important post-harvest diseases of citrus worldwide. The primary infection courts of these fungi are wounds on fruit inflicted during harvest and subsequent handling. Eradication of these incipient infections is required to achieve acceptable levels of control. Green mould is controlled by the fungicides ortho-phenyl phenate, imazalil and thiabendazole; there are no registered fungicides to control sour rot in the USA. New methods are needed, because the green mould pathogen developed resistance to these chemicals, and regulatory issues and public concerns about health risks of ingesting fungicide residues threaten fungicide use in the future. We evaluated a combination of heat and chemical control, employing compounds that have well-studied environmental and animal toxicological properties and extensive precedents as additives or natural components in foods. By selecting these compounds, we hope to facilitate their approval by minimizing health, environmental and disposal issues. A process that meets these criteria is the immersion of fruit in heated solutions of sodium carbonate or bicarbonate, a practice described about 70 years ago. Although it worked well, disposal of the solutions after use is restricted in some locations because of the high pH and sodium content. Therefore, we evaluated lime-sulfur solution for this purpose.

Materials and methods
Recommended methods to evaluate citrus post-harvest fungicides were used [1]. Lemons and oranges were selected by hand from field bins after harvest, before any commercial post-harvest treatments were applied. Before each experiment, the fruit were washed with water, inoculated and randomized. P. digitatum or G. citri-aurantii were cultured on potato dextrose agar at 20C for 1 week. Spores were rubbed from the agar surface with a glass rod after a small volume of sterile water containing 0.05% Triton X-100 was added. The spore suspension was passed through two layers of cheesecloth and adjusted to one million spores per ml. The inoculum solution of G. citri-aurantii contained 10 g/ml cyclohexamide to increase infections [1]. Each fruit was inoculated once by dipping a stainless-steel rod in the spore solution and making a puncture 1 mm wide by 2 mm deep. The wounds penetrated the albedo tissue but not the juice sacs and simulated natural inoculation. After inoculation, the fruit were incubated for 24 h at 20C. Fruit were treated by 1-6 min immersion in 3-6% (w/v) lime-sulfur solution (29% calcium polysulfide), 3% sodium carbonate, or a mixture of 4% borax and 2% boric acid, or sprayed with 2000 g/ml imazalil to run-off, then stored for 3 weeks at 10C after treatment, and the numbers of infected fruit recorded.

Results and conclusions
The incidence of post-harvest green mould of lemons and oranges was reduced from 100 and 90%, respectively, among inoculated control fruit, to 0.7 and 3.0% by immersion of the fruit for 3 min in 3% lime-sulfur solution at 43C. Heating of the solution from 27 to 43C improved its effectiveness, particularly on oranges. The treatment was equal or slightly superior in effectiveness to imazalil or solutions of sodium carbonate or borax/boric acid in repeated tests. Sour rot of lemons was reduced from 64 to 35% by immersion for 3 min in 3% lime-sulfur at 43C. The risk of injury to the fruit was low. Among five navel orange varieties and one lemon variety, no rind injuries were observed after immersion for 6 min in 6% lime-sulfur at 49C. A significant advantage of lime-sulfur solution is that because it is applied to soil as a conditioner, disposal of spent solutions to land or sewers may be approved more easily compared to other compounds that are not approved for soil application.

1. Eckert JW, Brown GE, 1986. Evaluation of postharvest treatments for citrus fruits. In KD Hickey, ed., Methods for Evaluating Pesticides for Control of Plant Pathogens. American Phytopathological Society, St Paul, Minnesota, pp. 92-97.