Whilst in a state of madness Jonathan Strange develops an intense loathing of pineapples. These innocent fruits obtrude themselves on his notice even when they are in fact not there - as happens when he takes a little of the tincture of madness and is at once convinced that he is menaced by phantom pineapples on every side. Even his Venetian landlord, who knocks at his door no doubt with some mundane enquiry wholly unrelated to pineapples, appears to Strange to have an entire fruit in his mouth.
It later transpires from a conversation between the Duke of Wellington and Lord Sidmouth that Strange's abhorrence continues such that he casts spells to prevent any pineapples from ever accidentally coming near him. The spells make it impossible for anyone to bring one within his vicinity and this, for the time of his residence in Venice, causes a serious (and occasionally dangerous) hindrance to the trade of Venetian fruitsellers and watermen.
We do not quite know whether it is the taste, smell, texture or mere physical appearance which renders this fruit so disagreeable to Strange. It may even be their emblematic meaning, for in art they denote hospitality, and we must admit that our Hero, trapped within the awful Pillar of Darkness and rendered a madman by his addiction to the Tincture, is at this point less agreeably affable than at any time in his life.
It is a curious coincidence that Henry Woodhope's intended bride Sophronia Watkins also shares Strange's disgust at this fruit. It savours a little of mental instability and does not augur well for the happy pair.