This is book number 11 in the Dragonriders of Pern series.
For generations, the dragonriders had dedicated their lives to fighting Thread, the dreaded spores that periodically rained from the sky to ravage the land. On the backs of their magnificent telepathic dragons they flew to flame the deadly stuff out of the air before it could reach the planet's surface. But the greatest dream of the dragonriders was to find a way to eradicate Thread completely, so that never again would their beloved Pern be threatened with destruction.
Now, for the first time, it looks as if that dream can come true. For when the people of Pern, led by Masterharper Robinton and F'lar and Lessa, Weyr-leader and Weyrwoman of Benden Weyr, excavate the ancient remains of the planet's original settlement, they uncover the colonists' voice-activated artificial intelligence system--which still functions. And the computer has incredible news for them: There is a chance--a good chance--that they can, at long last, annihilate Thread once and for all.
"When McCaffrey's beloved dragons roar and their riders soar upon the beasts' mighty backs...fans of Pern will likely be enthralled." --Publishers Weekly