The significance of postharvest disinfection for prevention of internal decay of table grapes after storage

• The combination of disinfection with ethanol after harvest and SO2 during storage maintained grapes free of decay for 14 weeks.
• Latent infections were ruled out as a potential cause for internal decay in ‘Scarlotta’ grapes.

Table grapes can accumulate significant amount of dust carrying potential inoculum and this can be a significant problem for late season grapes or in regions with high dust content. Internal decay of table grapes during storage under SO2 atmosphere is expressed as visually intact berries but extensive internal decomposition. One hypothesis is that these symptoms are caused by latent infections of Botrytis cinerea or by another pathogen. Alternatively, SO2 restricts external fungal growth but its initial level is not sufficient to prevent penetration of external natural inoculum into the berry. To test these hypotheses clusters of ‘Scarlotta Seedless' grapes were externally disinfected by dipping in ethanol after harvest and cold-stored in the presence of one or two SO2 sheets for 6, 10, or 14 weeks followed by 3 days at shelf life. After 6 weeks of storage the untreated control grapes contained 60% decay – mainly typical gray mold symptoms – whereas SO2-treated berries displayed internal decay without external symptoms. Ethanol reduced decay to 4.1% and the combination of ethanol treatment and SO2 sheets eliminated the symptoms completely. The effect of the combined treatment was sustained after 10 and 14 weeks of cold storage. Only B. cinerea was recovered from berries that showed internal decay, ruling out potential involvement other pathogens. While the untreated clusters appeared dusty with dull red color, after external disinfection the original red color of the berries was apparent. It can be concluded that external disinfection of grapes after harvest can prevent internal decay under conditions which restrict external fungal growth.

‘Scarlotta seedless’ before and after treatment with ethanol showing the effect on berry appearanceLeft – untreated control’ right – dipping in ethanol (black dots are marks of inoculation sites).Figure optionsDownload as PowerPoint slide