On the night of the 29th of September, the Devil is said to spit on blackberries.
Some versions of the tale say it’s to curse them because when he was kicked out of heaven, he landed in a prickly patch of blackberries. Other versions say it’s to claim them as his own.
Consequently, all blackberries must be gathered by this date. It’s also, conveniently, usually the time past which they have gone mouldy, anyway. The mould that will settle on them this late in the year is the grey botrytis cinerea mould.
Because they looked so awful, they were said to be covered with the Devil’s “spits.”
Peters, Rick. Seasonal food: blackberries. Manchester: The Guardian. 9 September 2010.