Phytomyxea

thumb|right|Life cycle of Plasmodiophora brassicae in [[cabbage.]] 220px|thumb|Clubroot on cauliflower

The Phytomyxea are a class of parasites that are cosmopolitan, obligate biotrophic protist parasites of plants, diatoms, oomycetes and brown algae. They are divided into the orders Plasmodiophorida (ICZN, or Plasmodiophoromycota, ICBN) and Phagomyxida. Plasmodiophorids are best known as pathogens or vectors for viruses of arable crops (e.g. club root in Brassicaceae, powdery scab in potatoes, and rhizomania in beets, especially sugar beets and some spinaches).