Toxicity of ozone gas to conidia of Penicillium digitatum, Penicillium italicum, and Botrytis cinerea and control of gray mold on table grapes

Penicillium digitatum, Penicillium italicum, and Botrytis cinerea attack fresh fruit and cause significant postharvest decay losses. The toxicity of ozone (O₃) gas at different relative humidities to control their conidia was determined. Conidia distributed on cover glasses were exposed to an atmosphere containing 200–350μLL⁻¹ of O₃ gas at 35%, 75%, and 95% relative humidity (RH) at 25°C. O₃ gas was produced by UV light generators and passed through three 500mL solutions of saturated MgCl₂ (35% RH), NaCl (75% RH), or K₂SO₄ (95% RH). O₃ and RH inside the chamber were monitored. O₃ exposures were quantified as concentration × time products adjusted to 1h (μLL⁻¹×h). After exposure to O₃ for varying periods, the conidia were removed from the chamber, placed on potato dextrose agar and their germination observed. Conidia died more rapidly at higher humidity than at lower humidity, and P. digitatum and P. italicum were more resistant to O₃ than B. cinerea. At 95% RH, 99% of the conidia of P. digitatum, P. italicum, and B. cinerea were incapable of germination after O₃ exposures of 817, 732, and 702μLL⁻¹×h, respectively. At 75% RH, similar inhibition required exposures of 1781, 1274, and 1262μLL⁻¹×h, respectively. At 35% RH, O₃ toxicity declined markedly, and 99% mortality required 11,410, 10,775, and 7713μLL⁻¹×h, respectively. These values can be used to select O₃ gas exposures needed to control these conidia. Conidia of B. cinerea were sprayed on to the surface of table grapes and 2h later the grapes were exposed to 800–2000μLL⁻¹×h of O₃. O₃ at 800μLL⁻¹×h or more reduced the incidence of infected berries by 85% and 45% on ‘Autumn Seedless’ and ‘Scarlet Royal’ grapes, respectively. Fumigation with O₃ can control postharvest pathogenic fungi on commodities that tolerate this gas, or it can be applied to disinfect processing equipment and storage rooms when the produce is not present.