Marshall, M. 20 Aug 2011.  Mass-extinction fungi could turn on trees again. NewScientist.

“During Earth’s biggest mass extinction 250 million years ago, usually tame soil fungi ran amok, decimating most of the world’s trees. A repeat is possible, if climate change weakens trees too much.

The Permian extinction saw 95% of species wiped out, dwarfing the K/T extinction that ended the dinosaurs’ reign. According to Mark Sephton of Imperial College London, a knock-on effect of the vast volcanic eruptions that triggered the extinction was a global fungal plague”.

Trees were vulnerable to attack by fungi because they were already weakened by to much heat, drought, and acidification from volcanic eruptions.