The Gnomish Plague was an ogre-created pandemic that was intended to blackmail the gnomes to service in Ravagog, in Book 4: Neverseen. The Neverseen, the rogue rebel group, had persuaded King Dimitar to use the ancient plague to disrupt the Lost Cities.
Caused by a microbiotic called Drakostomes, the plague used shadowvapor to affect the gnomes, and the trees of their homes, including Brackendale.
The cure to the drakostomes was Panakes blossoms, a type of ancient, mythical tree whose blossoms had healing powers. Calla, after realizing that a Panakes was a gnome who accepted sacrifice, transformed into one in order to save her fellow gnomes.
When the plague infects trees, the tree trunks become twisted and cracked, their branches sag and droop, and they drop their leaves, which turns mold-green and sallow yellow.