Table grapes are a popular and valuable crop, yet they are constantly at risk of developing grey mould caused by the fungus Botrytis. This is widely considered the most important postharvest disease affecting the storage and commercialisation of table grapes worldwide (Zoffoli and Lahorre, 2011).
Sulphur dioxide held the key
For over 80 years, the traditional way to arrest the development of grey mould has been to pre-treat the grapes with sulphur dioxide (SO2) post-harvest. After pre-treatment, the fruit is then placed in cardboard boxes together with SO2 pads before being loaded into the shipping container.
The SO2 pads contain metabisulphite, which reacts to moisture. When grapes respire water, it triggers a chemical reaction that causes the pads to release sulphur dioxide, thereby preventing the grapes from being affected by mould during transportation. The combination of pretreatment with SO2 and the use of SO2 pads during transportation has so far been the only reliable solution.
“Grey mould is an important postharvest disease in fruit, including table grapes. Traditionally, sulphur dioxide (SO2) has been extensively used both at harvest and during transit and storage as an effective control measure. However, there are some restrictions on the use of SO2 that make alternative control methods desirable”, explained Dr Bruno Defilippi Bruzzone, Head of the Postharvest Unit at the Institute for Agricultural Research (INIA), La Platina Experimental Research Centre, Chile.
Effective but problematic
Although the SO2-based technique has proved its effectiveness over decades as a means to control and limit the growth of grey mould, it is not without its problems. Sulphur dioxide is a harsh chemical that attacks certain metals and the rubber materials commonly used in container production.
Another important area of concern is maintaining the status of organic fruits. Once a fruit has been treated with chemicals of any sort, it loses its valuable organic designation. This naturally includes exposure to SO2. However, this is by no means the end of the story.