Evaluation of disinfectants to control contamination of citrus degreening rooms with conidia, 2008

Authors: Smilanick, Joseph; SORENSON, DAVID - Fruit Growers Supply

Submitted to: Plant Disease Management Reports
Publication Type: Research Notes
Publication Acceptance Date: 7/15/2009
Publication Date: 8/18/2009
Citation: Smilanick, J.L., Sorenson, D. 2009. Evaluation of disinfectants to control contamination of citrus degreening rooms with conidia, 2008. Plant Disease Management Reports. 3:V143.

Interpretive Summary: Citrus fruit rot after harvest unless actions are taken to retard this process; one of the approaches used to reduce these losses is to eliminate the spore contamination of packinghouses. In this work, we evaluated a variety of sanitizers and compared them in effectiveness to formaldehyde, an older product employed for this purpose that has some worker safety and environmental issues associated with its use. This work showed that the newer sanitizers could significantly and usefully reduced this contamination, although all were inferior in effectiveness to formaldehyde used for this purpose.

Technical Abstract: Early season oranges are harvested and treated with ethylene gas for two or more days in humidified ‘degreening’ rooms at 20C to accelerate the degradation of chlorophyll to enhance the orange color of the fruit rind. Decay of the fruit during this process by P. digitatum is common and between cycles of degreening disinfection of the room is needed to minimize the inoculum of this pathogen. Formaldehyde fumigation has long been applied for this purpose, but regulatory issues have made it necessary to find an alternative to it. This trial was conducted in a commercial citrus fruit ‘degreening’ room in Exeter, CA. The rooms were 6.1 m in height, 6.1 m in width, and 18.3 m in length, with plywood walls and ceiling and a concrete floor. Air circulation was provided in each room by three fans of two horsepower each that operated continuously throughout the tests. Isolate PD90 of P. digitatum was cultured for 2 wk at 20C on potato dextrose agar (PDA) and a dense suspension of conidia was prepared adding a small volume of 0.1% wt/vol Triton TX-100 to the plates and rubbing the colony surface with a glass rod. The dense conidial suspension was brushed onto one side of each of 24 craft wood sticks, 10 cm in length and 0.5 cm wide, and they were dried in air one day before use. About 1 hr before the sanitizers were applied, 20 craft wood sticks were attached throughout a room where the sanitizers were to be applied, and four were placed in an identical untreated room. The rooms were humidified for about 1 hr before the sanitizers were applied and the relative humidity was about 85 to 90% during each test. The sanitizers were placed inside the reservoir of the fogger and water was added to a final volume of 113.6 L. The treatments applied were: 1) formalin solution (formaldehyde 37% methanol 15%; Sigma-Aldrich Chemical Co., St, Louis, MO); 2) Storox (hydrogen dioxide or hydrogen peroxide 27%; BioSafe Systems, Hartford, CT); 3) Agrizone (mineral oxychloride oxidizer 10-20%, Previse Manufacturing, Inc. Corpus Christi TX); 4) Sani-T-10 (quaternary ammonium; mixture of n-alkyl dimethyl benzyl ammonium chloride and n-alkyl dimethyl ethylbenzyl ammonium chloride, 5% each, Spartan Chemical Co., Inc., Maumee, OH); 5) Chemchlor (sodium hypochorite 12.5% Brenntag Pacific, Inc. Santa Fe Springs, CA); 6) isopropyl alcohol (isopropyl alcohol, 99.9% a.i., Sigma Chemical Co., Chicago IL); 7) JetOxide (hydrogen peroxide 26.5% and peroxyacetic acid 4.9%, JET Harvest Solutions, Longwood, FL); 8) HDH peroxy (peroxyacetic acid 6% and hydrogen peroxide 30%, HDH Agri Products, LLC, Tavares, FL); and 9) ethyl alcohol (ethyl alcohol 95%, Sigma-Aldrich Chemical Co., St, Louis, MO). The room was closed and the fogger was operated for about 3 hr until all of the solution was dispensed. After 24 hr, the room was opened and the craft wood sticks were retrieved. Each was placed in a sterile test tube, a small volume of 0.1% wt/vol Triton TX-100 was added, it was vortexed briefly, and a 20 uL volume containing 200 to 500 conidia was placed on PDA. After 18 hr at 20C, the number of germinated conidia was counted at 200X with a compound microscope. The germination percentage did not increase with longer incubation. Formalin solution was the most effective sanitizer, while Storox and Agrizone were the most effective alternatives to it. JetOxide was applied at a relatively low rate compared to the other materials. Chlorine was moderately effective at this relatively high rate (the total chlorine content was 2000 mg/L). Ethyl and isopropyl alcohols and Sani-T-10 were not effective, even when used at very high rates.