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Title: CHLORINE CONCENTRATIONS AND THE INOCULATION OF TOMATO FRUIT IN PACKINGHOUSEDUMP TANKS.

Author
item BARTZ, JERRY
item EAYRE, CYNTHIA
item CONCELMO, DIANE
item BRECHT, JEFF
item SARGENT, STEVE

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/26/2001
Publication Date: N/A
Citation: N/A

Interpretive Summary: Chlorine concentrations required for dump tank sanitation in tomato packinghouses is unclear. Current recommendations include an overdose to compensate for chlorine losses to reactions with tomatoes, debris, microorganisms and chemicals. In this report, two postharvest pathogens of tomato fruit were tested to find how well data from spore sensitivity tests predict the efficacy of chlorine in a scale model flume. Chlorine at 25 mg/l (ppm), pH 7.0 and 24C completely inactivated spores of Geotrichum candidum (sour rot) or Rhizopus stolonifer (Rhizopus rot) in less than 60 s in tests conducted in the flume or in a beaker with a stirring bar. G. candidum was slightly more sensitive than R. stolonifer. When wounded fruit were floated in a flume after the addition of spores of either pathogen to water with 30 ppm chlorine at pH 7.0 and 24C, decay after a 6-day was about half of that among fruit treated with inoculum alone. Chlorine concentrations up to 75 ppm failed to prevent sour rot consistently, whereas up to 180 ppm failed to prevent Rhizopus rot consistently. In the model flumes, wounds were likely inoculated by contact with viable spores which were rapidly embedded in the wounds, and thus escaped contact with active chlorine. Thus, when sour rot or Rhizopus rot pathogens are prevalent in tomato fields, chlorine concentrations up to 180 ppm at pH 7.0 and 24C at the packinghouse are not likely to fully protect against postharvest decays. Additional disease control measures should be employed such as the culling of injured fruit and storage of packed fruit at cool temperatures (16 to 20C).

Technical Abstract: Chlorine concentrations that killed spores of Geotrichum candidum (sour rot) or Rhizopus stolonifer (Rhizopus rot) suspended in moving water within 30 to 45 sec did not prevent these fungi from inoculating wounded tomatoes (Lycopersicon esculentum) that were in a water flume containing chlorine and spores. Spores of G. candidum or R. stolonifer at 0.312 to 0.763 spores/ml were mixed with sodium hypochlorite solutions (pH 7.0, 24C) and agitated with a magnetic stirrer or a submersible pump. After designated treatment intervals, samples were spread over acidified potato dextrose agar (APDA) or spotted on APDA containing 0.2% sodium thiosulfate. Free chlorine concentrations of 20 or 25 mg/liter (ppm) were lethal to spores of G. candidum within 30 seconds in vitro tests, spores of R. stolonifer were less sensitive. When wounded tomatoes were placed in a flume and exposed to spores for 1 min, chlorine concentrations of 30 ppm reduced disease incidence during storage at 24C for 6 days to about half of that developin among the inoculated control fruit, which averaged 57% (range 15 to 95%) for R. stolonifer and 38% (range 17 to 58%) for G. candidum. Chlorine concentrations of 75 ppm failed to consistently protect fruit from G. candidum, up to 180 ppm chlorine failed to completely protect fruit from infection by R. stolonifer. Based on observations of movement of a water-soluble dye in wounds on tomato fruit, water entered intercellular spaces below wound surfaces rapidly. The dye in such spaces was unaffected by bathing the wound surface with chlorine, whereas the dye on the surface of the wound was de-colored. Thus, spores suspended in moving water can escape the action of chlorine if they are carried into intercellular spaces by diffusion or by capillary movement of cell sap and water.