A terrible ailment which manifests itself upon those who were born human, but who are becoming Gods.
Fortunately rare, but not so rare that the symptoms are not known: the sufferer's feet cause even bare floorboards to think it is Spring, Summer and Autumn and to burst into fruitful life.
In Pyramids, Pteppicymon XXVIII found this happening to him in Ankh-Morpork on the morning when he qualified as an Assassin. Even loaves of bread burst into corn as his feet passed them. All because, in distant Djelibeybi, his father had died and the Godhood of Pharoah was seeking the heir...
In Wintersmith, Tiffany Aching caused onions to grow in winter: only a sharp word from Granny Weatherwax stopped Nanny Ogg from repeating the experiment with carrot seeds.
Magrat Garlick, when forced by circumstances to use magic to open a stubborn castle door, used a spell with a similar effect to cause the oak planks to hallucinate that they were still part of a living, growing, tree. This earned her a rare and unforced compliment from Granny Weatherwax, which speaks volumes for this being an incredibly hard effect to manage by magic alone. Indeed, Granny frankly said she'd have gone about it a different way - she would have found it easier to ignore the wood of the door, and go in via the stonework surrounding it, by prodding the stone's memories of a time when it was all hot and runny... so if even Granny Weatherwax herself would find this difficult to do via magic, powerful forces indeed are at work here.