The planet of the plant-like Phylosians, nearly destroyed by a disease brought by Stavos Keniclius, explored in The Infinite Vulcan.
Phylos is the homeworld of the Phylosians, plant-like beings led by Agmar. They were once a thriving people, but a disease carried by the human scientist Stavos Keniclius nearly wiped them out, and the survivors depend on him.
In The Infinite Vulcan, the Enterprise arrives on a survey mission, and Sulu is stung by a plant called a retlaw. The Phylosians help him, then their master kidnaps Spock. Once Keniclius's plan is abandoned, his clone and a giant Spock clone stay to restore the Phylosian civilization.