Pythium vs. Phytophthora — What's the Difference?

Pythium is a genus of parasitic oomycetes. They were formerly classified as fungi.

Phytophthora (from Greek φυτόν (phytón), "plant" and φθορά (phthorá), "destruction"; "the plant-destroyer") is a genus of plant-damaging oomycetes (water molds), whose member species are capable of causing enormous economic losses on crops worldwide, as well as environmental damage in natural ecosystems. The cell wall of Phytophthora is made up of cellulose.

Any fungus of the genus Pythium

Any of many parasitic pseudofungi, of the genus Phytophthora, that cause brown rot in plants.

Destructive parasitic fungi causing brown rot in plants