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 200–350 μL L−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 500 mL 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 1 h (μL L−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 702 μL L−1 × h, respectively. At 75% RH, similar inhibition required exposures of 1781, 1274, and 1262 μL L−1 × h, respectively. At 35% RH, O3 toxicity declined markedly, and 99% mortality required 11,410, 10,775, and 7713 μL L−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 2 h later the grapes were exposed to 800–2000 μL L−1 × h of O3. O3 at 800 μL L−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.

Research highlights▶ Ozone gas can kill many harmful microorganisms on fruit such as grapes without leaving chemical residues. ▶ Ozone concentrations to control germination of Botrytis cinerea and Penicillium digitatum spores were determined at three relative humidity levels. ▶ The concentration needed was relatively high and increased as the relative humidity decreased. ▶ Ozone controlled Botrytis cinerea on fresh table grapes and extended shelf life without significant fruit damage.