Dragonseye by Anne McCaffrey

When the volcanoes rumble and the powerful storms begin brewing on Pern, it means one thing: Thread. For 257 years Pern has been free of the life-destroying Thread, but now the Red Star has reappeared in the sky and soon the deadly Threadfall will follow. In the holds and weyrs across the land, the genetically-engineered dragons of Pern and their human riders begin feverishly training to combat the Thread, for only dragon fire can destroy the silvery invaders. But, incredibly, one Lord Holder refuses to believe the Thread will fall again, and he may endanger the entire planet.