You didn't dump a bunch of food in a barrel to reduce weight, and accidentally transfer the fruit as well, did you?
Under what patch did you find the fruit? I've seen a problem in the short lived 1.41 patch that could change the Grey Quisirisi-fruit into just 'fruit'.
You may/should be able to get the fruit back by swapping some inventory files, using the procedure in;
Lost Items - guide and directory of inventory files (written for DD, works for BD)
I'll load an old save before talking to the alchemist to get the inventory files. In the mean time, finish all other quests you have open.
EDIT - Imp plague cure ingredients;
screenshot (27KB)
inventory files (11KB)