Effect of Heat Treatment Uniformity on the Control of Botrytis cinerea on Harvested Tomato
Surface-sterilized breaker-stage tomato (Lycopersicon esculentum Mill. cv. DRW 453) fruit were randomly divided into seven lots: four lots' fruit were fully exposed to circulating air at 23°C (control), 36°C, 38°C, or 39°C, respectively, whereas the remaining three were treated in a custom-designed twin-chambered insulated forced-air unit that allowed half (bisected along stem- to blossom-scar plane) of each tomato to be exposed to 39°C air at the same time the other half was exposed to forced air at 23°C, 36°C, or 39°C in respective experimental runs. Following the 23 h exposure to different temperatures, fruit were cooled by room temperature forced-air for 2 h, and then inoculated with week-old Botrytis cinerea Pers mycelium agar plugs. After 24 h storage at 23°C, inoculated tomatoes were stored at 13°C for 48 h before the plugs were removed; the fruit then remained under the same storage conditions for a further 72 h. To assess the effect of the heat treatment on B. cinerea development, the occurrence of hypersensitive response (HR), of tissue breakdown, lesion diameter and the abundance of mycelia (4-point subjective scale) were assessed on the 6th day after inoculation. A non-parametric one-way procedure followed by ANOVA and Duncan's multiple range test was conducted to investigate the effect of treatment conditions on the parameters assessed. The single-temperature heat treatment(s) most effective in limiting pathogen development varied according to the parameter measured: 38°C for HR; 36°C, 38°C, or 39°C for tissue breakdown and mycelium abundance; and 38°C or 39°C for lesion size. The ideal heat treatment effect was only obtained if the material to be treated was exposed to a particular temperature for a designed duration. Within the temperature range investigated, bilateral differences in temperature across the fruit significantly affected disease control: decreasing temperature differences significantly improved the uniformity for disease control. The importance of improving the uniformity of heating media among treated fruits and around each individual fruit was addressed.