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The Phytomyxea are a group of that are parasites of plants. A more common name for them is the plasmodiophorids, but this does not always include Phagomyxa. They typically develop within plant cells, causing the infected tissue to grow into a gall or scab. Important diseases caused by phytomyxeans include club root in cabbage and its relatives, and powdery scab in potatoes. These are caused by species of Plasmodiophora and Spongospora, respectively.

The vegetative form is a multinucleate cell, called a plasmodium. This ultimately divides to form new spores, which are released when the host's cells burst. Both resting spores and motile zoospores, which generally have two smooth flagella, are produced at different stages. Within the plasmodium, dividing nuclei have a distinctive cross-like appearance.

Plasmodiophorids are traditionally considered slime moulds, because of the plasmodial stage. Thus they are often classified as fungi,and given names such as the Plasmodiophoromycota. However, genetic and ultrastructural studies indicate they belong to a diverse group of protists called the Cercozoa, or are closely related to them.